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Regional Climate Projections

Filed under: — rasmus @ 27 August 2007

Regional Climate Projections in the IPCC AR4

glasses How does anthropogenic global warming (AGW) affect me? The answer to this question will perhaps be one of the most relevant concerns in the future, and is discussed in chapter 11 of the IPCC assessment report 4 (AR4) working group 1 (WG1) (the chapter also has some supplementary material). The problem of obtaining regional information from GCMs is not trivial, and has been discussed in a previous post here at RC and the IPCC third assessment report (TAR) also provided a good background on this topic.

The climate projections presented in the IPCC AR4 are from the latest set of coordinated GCM simulations, archived at the Program for Climate Model Diagnosis and Intercomparison (PCMDI). This is the most important new information that AR4 contains concerning the future projections. These climate model simulations (the multi-model data set, or just ‘MMD’) are often referred to as the AR4 simulations, but they are now officially being referred to as CMIP3.

One of the most challenging and uncertain aspects of present-day climate research is associated with the prediction of a regional response to a global forcing. Although the science of regional climate projections has progressed significantly since last IPCC report, slight displacement in circulation characteristics, systematic errors in energy/moisture transport, coarse representation of ocean currents/processes, crude parameterisation of sub-grid- and land surface processes, and overly simplified topography used in present-day climate models, make accurate and detailed analysis difficult.

I think that the authors of chapter 11 over-all have done a very thorough job, although there are a few points which I believe could be improved. Chapter 11 of the IPCC AR4 working group I (WGI) divides the world into different continents or types of regions (e.g. ‘Small islands’ and ‘Polar regions’), and then discusses these separately. It provides a nice overview of the key climate characteristics for each region. Each section also provides a short round up of the evaluations of the performance of the climate models, discussing their weaknesses in terms of reproducing regional and local climate characteristics.

Africa Evaluations of the GCMs show that they still have significant systematic errors in and around Africa, with excessive rainfall in the south, a spurious southward displacement of the Atlantic inter-tropical convergence zone (ITCZ), and insufficient upwelling in the seas off the western coast.

The report asserts that the extent to which regional models can represent the local climate is unclear and that the limitation of empirical downscaling is not fully understood. It is nevertheless believed that land surface feedbacks have a strong effect on the regional climate characteristics.

For the future scenarios, the median value of the MMD GCM simulations (SRES A1b) yields a warming of 3-4C between 1980-1999 and 2080-2099 for the four different seasons (~1.5 times the global mean response). The GCMs project an increase in the precipitation in the tropics (ITCZ)/East Africa and a decrease in north and south (subtropics).

Europe and the Mediterranean.
Europe In general, chapter 11 states that the most rapid warming is expected during winter in Northern Europe and during summer in southern Europe. The projections also suggest that the mean precipitation will also increase in the north, but decrease in the south. The inter-annual temperature variations is expected to increase as well.

A more detailed picture was drawn on the results from a research project called PRUDENCE, which represents a small number of TAR GCMs. The time was too short for finishing new dynamical downscaling on the MMD.

The PRUDENCE results, however, are more appropriate for exploring uncertainties associated with the regionalisation, rather than providing new scenarios for the future, since the downscaled results was based on a small selection of the GCMs from TAR.

Therefore, I was surprised to see such an extensive representation of the PRUDENCE project in this chapter, compared to other projects such as STARDEX and ENSEMBLES. (One explanation could be that the STARDEX results are used more in WGII, although apparently not cited. The results from ENSEMBLES are not yet published, and besides STARDEX is mentioned twice in section 11.10.)

Glomfjord precipitation

There are some results for Europe presented in chapter 11 of IPCC AR4 which I find strange: In Figure 11.6 the RCAO/ECHAM4 from the PRUDENCE project yields an increase in precipitation up to 70%(!) along the west coast of mid-Norway. Much of this is probably due to an enhanced on-shore wind due to a systematic lowering of the sea level pressure in the Barents Sea, and an associated orographic forcing of rain.

The 1961-90 annual total precipitation measured at the rain gauge at Glomfjord (66.8100N/13.9813E; 39 m.a.s.l.) is 2069 mm/year, and a 70% increase will therefore imply an increase to 3500mm/year (Left figure) which in my opinion is unrealistic . Apart from a sudden jump in the early part of the Glomfjord record, there are no clear and prominent trends in the historical time series (Figure left). The low values in the early part are questionable as the neighbouring station series do not exhibit similar jumps/breaks and is probably a result of a relocation of the rain gauges.

An increase of annual rainfall exceeding 1000mm would imply either that evaporation from the Norwegian Sea area must increase dramatically, or the moisture convergence must increase significantly since the water must come from somewhere. However, the whole region is already a wet region (as indicated by the annual rainfall totals) in the way of the storm tracks.

There are large local variations here (see grey curves in left Figure for nearby stations) and Glomfjord is a locations with high annual rainfall compared to other sites in the same area, but even a 70% increase of the rainfall with annual totals exceeding 1000mm at nearby sites (adjacent valleys etc) is quite substantial.

However, one may ask whether the rainfall at Glomfjord may change at a different rate to that of its surroundings. This question can only be addressed with empirical-statistical downscaling (ESD) at present, as RCMs clearly cannot resolve the spatial scales required.

To be fair, another PRUDENCE scenario presented in the same figure, but based on the HadAM3H model rather than the ECHAM4, suggests an upper limit for precipitation increase over northern Europe of 20% over northern Sweden.

Asia One of the key climate characteristics of Asia is the southeast Monsoon system. Chapter 11 suggest that the circulation associated with the Monsoon may slow down, but the moisture in the air may increase. However, while the general seasonal migration of rain is simulated by most climate models, the representation of the observed monsoon maximum rainfall along the west coast of India, northern parts of Bay of Bengal and north India is poor in many models (probably because of too coarse spatial resolution in GCMs).

Satellite view of the Himalayas The GCMs also most likely have significant problems describing the precipitation over Tibet, due to large small-scale spatial geographical features and distorted albedo feedbacks. The net effect may therefore be an increase in the rainfall associated with the Monsoon.

The Asian climate is also influenced by ENSO, but uncertainties in how ENSO will be affected by AGW cascades to the Asian climate.

There are, however, indications that heat waves will become more frequent and more intense. Furthermore, the MMD models suggest a decrease in the December-February precipitation and an increase in the remaining months. The models also project more intense rainfall over large areas in the future.

North America.
North America The general picture is that the GCMs provide realistic representation of the mean SLP and T(2m) over North America, but that they tend to over-estimate the rainfall over the western and northern parts.

The MMD results project strongest winter-time warming in the north and summer-time warming in the southwest USA. The annual mean precipitation is, according to AR4, likely to increase in the north and decrease in southwest.

A stronger warming over land than over sea may possibly affect the sub-tropical high-pressure system off the west coast, but there are large knowledge gaps associated with this aspect.

The projections are associated with a number of uncertainties concerning dynamical features such as ENSO, the storm track system (the GCMs indicate a pole-ward shift, an increase in the number of strong cyclones and a reduction in the medium strength storms poleward of 70N & Canada), the polar vortex (the GCMs suggest an intensification), the Great Plains low-level jet, the North American Monsoon system, ocean circulation and the future evolution in the snow-extent and sea-ice. Some of these phenomena are not well-represented by the GCMs, as their spatial resolution is too coarse. The same goes for tropical cyclones (hurricanes), for which the frequency, intensity and track-statistics remain uncertain.

A number of RCM-based studies provide further regional details (North American Regional Climate Change Assessment Program). Despite improvements, AR4 also states that RCM simulations are sensitive to the choice of domain, the parameterisation of moist convection processes (representation of clouds and precipitation), and that there are biases in the RCM results when GCM are provided as boundary conditions rather than re-analyses.

Furthermore, most RCM simulations have been made for time slices that are too short to provide a proper statistical sample for studying natural variability. There are no references to ESD for North America in the AR4 chapter except for in the discussion on the projections for the snow.

Latin America.
Latin America AR4 states that the annual precipitation is likely to decrease in most of Central America and southern Andes. However, there may be pronounced local effects from the mountains, and changes in the atmospheric circulation may result in large local variations.

The projections of the seasonal mean rainfall statistics for eg the Amazon forest are highly uncertain. One of the greatest sources of uncertainty is associated with how the character of ENSO may change, and there are large inter-model differences within the MMD as to how ENSO will be affected by AGW. Furthermore, most GCMs have small signal-to-noise ratio over most of Amazonia. Feedbacks from land use and land cover (including carbon cycle and dynamic vegetation) are not well-represented in most of the models.

Tropical cyclones also increase the uncertainty for Central America, and in some regions the tropical storms can contribute a significant fraction to the rainfall statistics. However, there has been little research on climate extremes and projection of these in Latin America.

According to AR4, deficiencies in the MMD models have a serious impact on the representation of local low-latitude climates, and the models tend to simulate ITCZs which are too weak and displaced too far to the south. Hence the rainfall over the Amazon basin tends to be under-estimated in the GCMs, and conversely over-estimated along the Andes and northeastern Brazil.

There are few RCM-simulations for Latin America, and those which have been performed have been constrained by short simulation lengths. The RCM results tend to be degraded when the boundary conditions are taken from GCMs rather than re-analyses. There is, surprisingly, no reference to ESD-based studies from Latin America, despite ESD being much cheaper to carry out and length of time interval being not an issue (I’ll comment on this below).

Australia & New Zealand.
Australia The projections for Australia and New Zealand suggest weaker warming in the south and increased frequency of high daily temperatures. The precipitations will, according to MMD, decrease in most of Australia, and it is likely that there will be more drought conditions in southern Australia in the future as a result of a poleward shift in the westerlies and storm track.

The MMD projections for the monsoon rainfall show large inter-model differences, and the model projections for the future rainfall over northern Australia are therefore considered to be very uncertain.

Little has been done to asses the MMD skill over Australia and New Zealand, although analysis suggest that the MMD models in general have a systematic low-pressure bias near 50S (hence a southward displacement of the mid-latitude westerlies). The simulated seas around Australia has a slight warm bias too, and most models simulate too much rainfall in the north and too little on the east coast of Australia.

The quality of the simulated variability is also reported to be strongly affected by the choice of land-surface model.

The projections of changes is the extreme temperatures for Australia and New Zealand has followed a simple approach where the range of variations has been assumed to be constant while the mean has been adjusted according the the GCMs, thus shifting the entire statistical distribution. The justification for this approach is that the effect on changes in the range of short-term variations has been found to be small compared with changes in the mean.

Analysis for rainfall extremes suggest that the return period for extreme rainfall episodes may halve in late 21st century, even where the average level to some extent is diminishing. AR4 anticipates an increase in the tropical cyclone (TC) intensities, although there is no clear trends in frequency or location.

Furthermore, TCs are influenced by ENSO, for which there are no clear indications for the future behaviour. AR4 also states that there may be up to 10% increases in the wind over northern Australia.

AR4 states that downscaled MMD-based projections are not yet available for New Zealand, but such results are very much in need because of strong effects from the mountains on the local climate. High-resolution regional modeling for Australia is also based on TAR or ‘recent ‘runs with the global CSIRO glimate model. A few ESD studies by Timbal and others suggest good performance at representing climatic means, variability and extremes of station temperature and rainfall.

Polar regions.
The Arctic The Arctic is very likely to warm at a higher rate than the global mean (Polar amplification), and the precipitation is expected to increase while the sea-ice will be reduced. The Antarctic is also expected to become warmer albeit more moderate, and the precipitation is projected to increase too.

However, the understanding of the polar climate is still incomplete, and large endeavors such as the IPY hope to address issues such as lack of decent observations, clouds, boundary layer processes, ocean currents, and sea ice.

AR4 states that all atmospheric models have incomplete parameterisations of polar cloud microphysics and ice crystal precipitation, however, the general improvement of the GCMs since TAR in terms of resolution, sea-ice modelling and cloud-radiation representation has provided improved simulations (assessed against re-analysis, as observations are sparse).

Part of the discussion about the Polar regions in AR4 relies on the ACIA report (see here , here and here for previous posts) in addition to the MMD results, but there are also some references to RCM studies (none to ESD-based work, although some ESD-analysis is embedded in the ACIA report). There has been done very little work on polar climate extremes and the projected changes to the cryosphere is discussed in AR4 chapter 10.

The models do in general provide a reasonable description of the temperature in the Arctic, with the exception of a 6-8C cold bias in the Barents Sea, due to the over-estimation of the sea-ice extent (the lack of sea-ice in the Barents Sea can be explained by ocean surface currents that are not well represented in GCMs). The MMD models suggest the winter-time NAO/NAM may become increasingly more positive towards the end of the century.

One burning question is whether is the response of the ice sheet will be enhanced calving/ice stream flow/basal melt. More snow will accumulate in Antarctica, as this ‘removes’ water from the oceans that otherwise would contribute to a global sea level rise. The precipitation in Antarctica, and hence snowfall, is projected to increase, thus partly offsetting the sea-level rise.

Small islands.
Islands AR4 concludes that sea levels will continue to rise, although the magnitude of the sea level rise is not expected to uniform. Furthermore, large inter-model differences make regional sea level projections more uncertain. AR4 states that the Caribbean islands in the vicinity of the Greater Antilles and Mauritius (JJA) will likely face drier summer-time conditions in the future whereas those in northern Indian Ocean are believed to become wetter.

Most GCMs do not have sufficiently high spatial resolution to represent many of the islands, therefore the GCM projections really describe changes over ocean surfaces rather than land. Very little work has been done in downscaling the GCMs for the individual islands. However, there have been some ESD work for the Caribbean islands (AIACC).

Two sentences stating that ‘The time to reach a discernible signal is relatively short (Table 11.1)’ is a bit confusing as the discussion was referring to the 2080-2099 period (southern Pacific). After having conferred with the supplementary material, I think this means that the clearest climate change signal is seen during the first months of the year. Conversely when stating ‘The time to reach a discernible signal’ for the northern Pacific, probably refers to the last months of the year.

Other challenges involve incomplete understanding of some climatic processes, such as the midsummer drought in the Caribbean and the ocean-atmosphere interaction in the Indian Ocean.

Many of the islands are affected by tropical cyclones, for which the GCMs are unable to resolve and the trends are uncertain – or at least controversial.

Empirical-statistical or Dynamical downscaling?
AR4 chapter 11 makes little reference to empirical-statistical downscaling (ESD) work in the main section concerning the future projections, but puts most of the emphasis on RCMs and GCMs. It is only in Assessment of Regional Climate Projection Methods section (11.10) which puts ESD more into focus.

I would have thought that this lack of balance may not be IPCC’s fault, as it may be an unfortunate fact that there are few ESD studies around, but according to AR4 ‘Research on [E]SD has shown an extensive growth in application…’. If it were really the case that ESD studies were scarce, then it would be somewhat surprising, as ESD is quick and cheap (here, here, and here), and could have been applied to the more recent MMD, as opposed to the RCMs forced with the older generation GCMs.

ESD provides diagnostics (R2-statistics, spatial predictor patterns, etc.) which can be used to assess both the skill of the GCMs and the downscaling. I would also have thought that ESD is an interesting climate research tool for countries with limited computer resources, and it would make sense to apply ESD to for climate research and impact studies to meet growing concerns about local impacts of an AGW.

Comparisons between RCMs and ESD tend to show that they have similar skills, and the third assessment report (TAR) stated that ‘It is concluded that statistical downscaling techniques are a viable complement to process-based dynamical modelling in many cases, and will remain so in the future.’ This is re-phrased in AR4 as ‘The conclusions of the TAR that [E]SD methods and RCMs are comparable for simulating current climate still holds’.

So why were there so few references to ESD work in AR4?

I think that one reason is that ESD often may unjustifiably be seen as inferior to RCMs. What is often forgotten is that the same warnings also go for the parameterisation schemes embedded in the GCMs and RCMs, as they too are statistical models with the same theoretical weaknesses as the ESD models. AR4 does, however, acknowledge this: ‘The main draw backs of dynamical models are … and that in future climates the parameterization schemes they use to represent sub-grid scale processes may be operating outside the range for which they were designed’. However, we may be on a slippery slope with parameterisation in RCMs and GCMs, as the errors feed back to the calculations for the whole system.

Moreover, a common statement heard even among people who do ESD, is that the dynamical downscaling is ‘dynamical consistent’ (should not be confused with ‘coherent’). I question this statement, as there are issues of gravity wave drag schemes and filtering of undesirable wave modes, parameterisation schemes (often not the same in the GCM and the RCM), discretisation of continuous functions, and the conservation associated with time stepping, or lateral and lower boundaries (many RCMs ignore air-sea coupling).

RCMs do for sure have some biases,yet they are also very useful (similar models are used for weather forecasting). The point is that there are uncertainty associated with dynamical downscaling as well as ESD. Therefore it’s so useful to have two completely independent approaches for studying regional and local climate aspects, such as ESD and RCMs. One should not lock onto only one if there are several good approaches. The two approaches should be viewed as two complementary and independent tools for research, and both ought to be used in order to get a better understanding of the uncertainties associated with the regionalisation and more firm estimates on the projected changes.

Thus, the advantage of using different methods has unfortunately not been taken advantage of in chapter 11 of AR4 WGI to the full extent, which would have made the results even stronger. When this points was brought up during the review, the response from the authors of the chapter was:

This subsection focuses on projections of climate change, not on methodologies to derive the projections. ESD results are takan into account in our assessment but most of the available studies are too site- or area-specific for a discussion within the limited space that is available.

So perhaps there should be more coordinated ESD efforts for the future?

On a different note, the fact that the comments and the response are available on-line demonstrates that IPCC process is transparent indeed, showing that, on the whole, the authors have done a very decent job. The IPCC AR4 is the state-of-the-art consensus on climate change. One thing which could improve the process could also be to initiate more real-time dialog and discussion between the reviewers and the authors, in additional to the adopted one-way approach comment feed.

One section in IPCC AR4 chapter 11 is devoted to a discussion on uncertainties, ranging from how to estimate the probability distributions to the introduction of additional uncertainties through downscaling. For instance, different sets of solutions are obtained for the temperature change statistics, depending on the approach chosen. The AR4 seems to convey the message that approaches based on the performance in reproducing past trends may introduce further uncertainties associated with (a) models not able to reproduce the historical (linear) evolution, and (b) the relationship between the observed and predicted trend rates may change in the future.

Sometimes the uncertainty has been dealt with by weighting different models according to their biases and convergence to the projected ensemble mean. The latter seems to focus more on the ‘mainstream’ runs.

AR4 stresses that multi-model ensembles only explore a limited range of the uncertainty. This is an important reminder for those interpreting the results. Furthermore, it is acknowledged that trends in large-area and grid-box projections are often very different from local trends within the area.

262 Responses to “Regional Climate Projections”

  1. 51
    dhogaza says:

    Kininmonth is proposing that Hansen has missed something.

    Hansen? Try “a very, very large number of climate scientists including a bunch of physicists”.

  2. 52
    Chris Dudley says:

    More has come up in the blogospher about Dr. Hansen’s motives centered around an error in a Globe and Mail article about sea level rise. The error, a misreading of Hansen et al. (2007) Phil. Trans. R. Soc. A 365 1925, is that 25 meters of sea level rise might be expected by the end of this century. Ad hominem on outlets like americanthinker are fairly extreme. I thought you might want to address this. My take can be found at the Real Energy blog.

    You all are doing great! Keep it up!


  3. 53
    Chris S says:

    James, (Post 25) Thanks for the clarification. That whole part really had me dumbfounded? I thought there were certain areas of the world that were not “implemented” into the models because of lack of data. Your clarifying helps me understand this better.

    And so, in return, to Chuck, I am completely aware of the scientific progress, and that (for me) goes without saying. I was under the impression that geological features weren’t well incorporated with the models, which made “predicting future climates” a bit unrealistic to me, especially under a AGW forcing scenario.

    So I stand corrected, I’d rather see technology build us much better supercomputers in the future (which will eventually come) to help imrpove the models, rather then say they should be incorporated better. From the sound of it, the issue seems miniscule enough that an overal “trend” can definitly be found then, just not precisly, which is proof of the process that Chuck noted in post 43. Thanks for the info guys! (love this site).

    In response to your question Chuck, about what criteria would I use, I would tend to think that the best way to “accurately” incorporate them is when we have a “fully developed” map of the entire planet, from the north pole to the south pole, from the top of the mountains to the bottom of the deepest oceans. Granted, that’s a massive project (which I did read the project was in the process of being started. Can’t remeber the name of the organization doing so, but I sent an article via email to my geology proffessor about this, in which she noted it’s a massive project and would help in all fields of the geosciences, especially Climatology.) but once you would have that along with computers powerful enough to process this information, then you would have a model that was much more effiecent in terms of predictions am I correct? Am I on the right path with this?

  4. 54
    bjc says:

    One last time, how do you validate precipitation predictions when the GISS data appears to run through 1995 and the CRUT data runs through 2000? Are there are data sets available?

  5. 55
    Patrick 027 says:

    Re 50 – PART I:
    solar radiation = mainly shorter than 4 microns ~= SW radiation
    terrestrial radiation = mainly longer than 4 microns ~= LW radiation

    SW radiation absorption is distributed, a majority is at the surface (or within some distance underneath the surface, as in the ocean), the rest is distributed in the atmosphere, some heats the upper atmosphere.

    LW radiation is emitted thermally by materials as a function of their radiative properties, as a function of wavelength, and as a function of temperature, rising with increasing temperature. A limiting value exists for the total and at any given wavelength of the intensity and flux per unit area of LW radiation that can be emitted at a given temperature. A real object can emit a fraction of that limiting value between 0 and 1, this is it’s emissivity. At any given wavelength, if a material can emit LW radiation thermally, it can also absorb – the absorptivity is equal to the emissivity over a given layer thickness, unabsorbed radiation will pass through to the next layer unless scattering or reflection occurs, both of which are relatively unimportant for LW radiation.

    If a layer recieves more heat than it loses, it heats up (phase changes may occur, composition might change via chemical reactions, but otherwise the temperature rises; the reverse if a layer loses more than it gains).

    The temperature profile from the surface to the top of the atmosphere adjusts until the divergence of the LW flux matches the convergence of the SW flux. This results in a hot surface, temperature declining through the troposphere to some height, then rising again to the top of the stratosphere, falling again in the mesosphere, and rising again in the thermosphere (it is my understanding that without the ozone layer absorbing UV rays (portion of SW flux), the stratosphere and mesosphere divisions would not exist, so the troposphere would end where the thermosphere begins; the thermosphere exists because of absorption of a very small portion of the SW flux (the very shortest wavelengths) in the very very small mass uppermost atmosphere.

    Such a pure radiative equilibrium, however, leaves the lowermost portion of the atmosphere unstable to convective processes. Thus convection occurs, and there is a convective adjustment to the above temperature profile. Convection tends to maintain a lapse rate (rate of temperature decline with height) near a moist adiabat (due to latent heat release during ascent and cooling – otherwise the cooling would follow a dry adiabat, where temperature declines faster with height).

  6. 56
    Patrick 027 says:

    Re 50 – PART II:

    Altering the greenhouse effect is changing the LW radiative properties. Doing so without changing the temperature profile of the surface and atmosphere results in imbalances where none existed before. The temperature profile must change in order to restore balance. This is setting aside feedbacks that may further alter the greenhouse effect (water vapor, clouds) or change the amount and distribution of SW absorption (water vapor, clouds, snow and ice, ozone, etc.)- such changes will again require changes in the temperature profile to balance the energy fluxes. (A layer will heat up or cool off until SW flux convergence is balanced by LW flux divergence + convective flux divergence, where convection is only nonzero in the troposphere).

    Because of the way convection works, the surface and the levels of the troposphere warm up or cool off together. (Temperature dependence of the moist adiabats affects this, so that in the tropics, the mid to upper troposphere warms up more than the surface and lower troposphere; however, in polar regions the greatest warming tends to be at the surface, and in winter, for reasons I’m skipping over).

    Within the atmosphere, LW radiation is going up and down, being emitted from all layers and the surface (in varying amounts depending on temperature, and clouds and water vapor and ozone). Some upward emission can go directly to space and is not accompanied by any significant downward emission; the exchange is one-way, and this is a cooling that balances all of the SW absorption within the atmosphere and surface. The distribution of direct radiation to space from within the atmosphere is concentrated upward at any given wavelength when measured along units of optical depth. Increasing the LW opacity (increasing the greenhouse effect) increases the total optical depth and redistributes the radiation to space upward, with less coming from the lower atmosphere if the optical depth was already sizable, and regardless of that, with less coming from the surface. Most emission to space comes from within the troposphere, so as the distribution of radiation to space moves upward with less from the surface, it is distributed at colder temperatures, so less radiation goes to space. This means that, if the LW flux to space had been in balance with the SW flux being absorbed, then there is now an imbalance, with more energy entering the climate system than leaving. The temperature distribution must change as a result – warming occurs until the temperature distribution ‘catches up’ to the distribution of radiation to space; the surface and troposphere both warm up in this process. (it is not the change in LW radiation at the surface which is primarily responsible for surface warming – the surface and lower troposphere must warm, and the climate changes until LW radiation, SW radiation, and convection on average tend to balance each other. Actually, I think the instantaneous radiative forcing (decrease in net upward LW flux) for a doubling of CO2 is greater at the tropopause than at the surface – this implies an instantaneous effect of decreased convection, and this decreased convection communicates the tropopause forcing downward. Once balance is achieved, however, there may be increased convection, if the increased LW opacity of the troposphere slows the net upward LW flux within the troposphere and if this effect is not entirely cancelled or reversed by increased LW flux by higher temperatures; I’ve read that at high enough temperature and relative humidity, water vapor feedback acts to reduce the net LW flux from the surface).

    In addition, while the total LW radiation to space was initially reduced, that from the upper atmosphere (above the troposphere) increases. Thus the upper atmosphere tends to cool until the fluxes are again balanced. In the process, the tropopause (top of the troposphere) rises a little, so the tropopause gets thicker at the expense of the upper atmosphere. Note that this implies that at least the deepest convection gets deeper.

    There is also LW flux exchanges within the atmosphere-surface system from hot layers to cold layers, depending on how well the layers can ‘see each other’ at a given wavelength. Increasing the opacity tends to increase the exchange at shorter distances (the net flow from hot to cold will decrease) but decrease the exchange at longer distances (the net flow from hot to cold will increase), with the distinction shifting to shorter distances as opacity increases. But any increase in LW opacity from any starting point will decrease how well the surface can be seen from any layer of atmosphere. As with shifting the distribution of radiation to space upward, the distribution of radiation from the troposphere and surface to the upper atmosphere will be shifted upward to colder parts, so the upper atmosphere recieves less LW radiation from below (at least until the temperature profile adjusts). The radiation from the upper atmosphere to the troposphere and below will shift downwards, where it is also colder (the base of the stratosphere). Etc…

    The net LW flux upward is the upward LW flux minus the downward LW flux. The flux convergence or divergence is a rate of change of flux with height and corresponds to a heating or cooling rate. The net SW flux convergence distribution does not match the net LW flux divergence distribution within the troposphere; this is allowed in equilibrium because of convection. The net convective flux must transport energy between the two. The amount of convection may change as a result of changing the greenhouse effect but it does not negate surface warming; surface and lower tropospheric warming may be smaller than mid-to-upper tropospheric warming in the tropics because of the temperature dependence of moist adiabatic convection, but this is taken into account in models.

    The above, except for a few parts, largely describes the situation for a single column of surface-atmosphere as if under globally averaged conditions; in such a case, there would be no convection at all above the troposphere and the troposphere itself would be at a moist adiabatic lapse rate. As conditions vary across the surface of the Earth, equilibrium, even averaged over diurnal cycles or over the year, will not be met locally just by vertical fluxes, but horizontal transports of energy will, over years, balance this, and the global averages of vertical fluxes will nearly balance when averaged over some number of years, except when the climate is changing. There will be surface albedo, water vapor, circulation and cloud feedbacks, so both SW and LW fluxes as well as convection is altered, … none of which supports Kininmonth’s conclusion of such a reduced climate sensitivity (I don’t think Hansen or the IPCC is missing anything that Kininmonth brings up).

  7. 57
    Patrick 027 says:

    Re 50 PART III:

    Oh, on greenhouse components/agents (gasses and clouds) having a cooling effect:

    Yes, that’s one way to put it, but it can be misleading. By blocking a portion of the LW flux from below, greenhouse components must assume the role of radiative cooling to space, but they are generally cooler than what was below (at least within the troposphere, where most radiation to space is from), and so the LW flux is less than otherwise. Increasing the LW opacity decreases the LW flux to space… (See above).

    But also, the LW flux divergence corresponding to cooling to space + LW flux divergences and convergences due to exchanges within the atmosphere = net LW flux divergence profile – it balances the SW flux convergence in the upper atmosphere (at least in a column model that is under globally averaged conditions with no horizontal gradients, etc.), but within the troposphere, convection also plays a role. LW flux divergence + SW flux convergence leads to a net overall radiative divergence within the troposphere, which is balanced by convergence of convective fluxes, which come from the divergence of convection at the surface, which balances radiative flux convergence at the surface, which is the net downward radiaton at the surface, as all radiative transfer ceases at some depth. The net downward radiative flux is the downward SW flux – the net upward LW flux, the last of which is the LW flux from the surface – the downward LW flux to the surface from the atmosphere.

  8. 58
    Patrick 027 says:

    In part II of Re 50, I wrote: “This is setting aside feedbacks that may further alter the greenhouse effect (water vapor, clouds) or change the amount and distribution of SW absorption (water vapor, clouds, snow and ice, ozone, etc.)”

    That would seem to imply that ozone does not play a role in the greenhouse effect; but it does.

  9. 59
    Rod B says:

    Hank (49) says, “…Rod, what does the PCMDI site lack?…” If you’re referring to the lead post of this thread (???), proper and acceptable emphasis of the caveats.

  10. 60
    Joseph O'Sullivan says:

    This was a great post. I would like to see more reviews of other parts of the IPCC AR4. Its a great resource for busy laymen like myself who are eager to learn more about the report.

  11. 61
    Chuck Booth says:

    RRe # 53 Chris S. “Am I on the right path with this?”

    Sure, but you are living in a dream world. How long will that take – 5 years? 10 years? 20 years? In waiting for the ideal supercomputing capacity and perfect information to plug into the models, we might well miss a critical window of opportunity to forestall some of the more serious consequences of AGW.

    As I tell my students, there is no such thing as a perfect experiment (or,in this case, a perfect GCM)- you do the best you can with the resources and knowledge you have available at the moment. In science, he/she who hesitates is lost. And in this case, society might lose, as well.

  12. 62
    Preben S says:

    Re #33
    Citing William Kininmonth “the heat content of the atmosphere is miniscule – equivalent to the top 30 cm of the ocean surface.

    When I make a calculation of this, based on simple physics, I get that the heat capacity of the air is equivalent to 170 cm of the ocean surface. The distribution of the temperature over the mass of air is of course not uniform, but I cannot see how the difference can be so big.
    Can somebody point out what is wrong here?

  13. 63
    Walt Bennett says:

    Re: #55, #56


    Thank you for that post. I am still working on what it means in layman’s terms, but would it be correct to say that more heat is transferred when there is greater evaporation, and that this trapped heat is then (not sure what the correct word is here) transferred by certain processes, such that heat which originates in the tropics finds its way to other regions? In other words, is it correct to say that the entire globe is a heat-producing entity, and the climate system (wind, rain) distributes this heat?

    If that is a correct picture, then what I gain from your response to Kininmonth is that higher evaporation must mean that the surface has warmed; this additional warmth is then transferred in all directions. LW escapes at higher elevations (less density), and the area between that new “boundary” and the surface becomes warmer.

    Is that an accurate layman’s summation? Is it correctly stated in scientific terms?

  14. 64
    Mitch Golden says:

    Here’s something I don’t understand:

    Furthermore, large inter-model differences make regional sea level projections more uncertain.

    Isn’t the sea level pretty constant across the globe? Is there some reason to expect that whatever differences do exist will be changed by climate change?

  15. 65
    Robert says:

    Re #64

    There are global warming related factors that can cause local changes in sea level.

    One is that when ice melts and is redistributed around the globe, the Earth’s gravitational field is changed. Areas near where the ice melted have the sea level go down (or rise less) while the other side of the globe gets larger than average rises.

    Another is that when water is less dense or in regions of low atmospheric pressure, it floats higher than dense water or water in areas of high pressure.

    Lastly, wind can pile up water or spread it out. Over the course of thousands of miles of fetch, this can add up to several feet of height.

    Since global warming can change all of these factors, one must put them into the models and one cannot assume that the sea level rise will be uniform.

  16. 66
    Ray Ladbury says:

    Mitch. Short answer: NO. Sea level is determined not just by gravity, but also by Earth’s rotation, prevailing winds, etc. That was one of the marvels of the Panama canal–it took ships through locks to bring it to the proper level of the Atlantic and Pacific oceans.

  17. 67
    PWS says:

    I don’t know much about the models, but it seems to me that the potential for one of the biggest disruptions in climate comes from changes in ocean currents and surface temperatures (which I have heard played a large role in starting and stopping ice ages). Has anybody factored in how the disappearance of the arctic sea ice cover in summer would change ocean circulation patterns, or is this a wild card which could invalidate all the predictions listed above?

  18. 68
    Michael says:

    ‘we might well miss a critical window of opportunity to forestall some of the more serious consequences of AGW’

    Forestall how? Leading scientists have said that all of our efforts to reduce CO2 emissions won’t make a bit of difference. I don’t think you consider the practicality of what is proposed.

  19. 69
    Falafulu Fisi says:

    Jim Crabtree said…
    I think it would be great to have a discussion on this site of statistical models vs dynamic models.

    Good call, but I wondered how many of the members here in RealClimate who have a clue about dynamical systems modeling ? Climate is dynamic and therefore more discussion of climate dynamics is more appropriate.

  20. 70
    Timothy Chase says:

    Falafulu Fisi (#68) wrote:

    Good call, but I wondered how many of the members here in RealClimate who have a clue about dynamical systems modeling ? Climate is dynamic and therefore more discussion of climate dynamics is more appropriate.

    Well, one point that gets mentioned quite often is the fact that the climate models themselves are grounded in the actual physics. They aren’t statistical but based upon the principles of physics such as the equations describing radiation emission, fluid flow and partial pressures.

    And as far as climate dynamics is concerned, it is quite often the case that in one way or another the distinction will be made between the actual path of the weather through phase space and the evolution of the climate system as an attractor in which the weather state is embedded. The weather is largely chaotic (e.g., how hot will it be on 4 Jul 2039?), whereas the climate attractor is rarely chaotic (e.g., summers in Boston will tend to be so many degrees warmer in the 2050s than they were in the 1990s).

    I would hate to have to count how many times it has been necessary to make either of these two points over the past year. Recurring themes.

  21. 71
    Jim Eager says:

    Re 68 Michael: “Forestall how? Leading scientists have said that all of our efforts to reduce CO2 emissions won’t make a bit of difference.”

    We won’t make a bit of difference for the warming that is already in the pipeline as the climate seeks a new equilibrium caused by higher levels of CO2. The aim is to forestall even further warming by halting additional increases in atmospheric CO2 before some of the natural feedbacks take hold in earnest, at which point nothing we do will matter. The latter is not yet inevitable, unless we continue with business as usual, of course.

  22. 72
    bjc says:

    Many thanks for the link to the Norwegian site. I have registered after having to learn 7 or 8 words in Norwegian – hopefully I will get access tomorrow.

  23. 73
    Timothy Chase says:

    Michael (#68) wrote:

    Forestall how? Leading scientists have said that all of our efforts to reduce CO2 emissions won’t make a bit of difference. I don’t think you consider the practicality of what is proposed.

    Given the rate at which we are accumulating carbon dioxide and the nature of the extended process through which the climate achieves radiative equilibrium, the evolution of the climate system is largely indifferent to our CO2 emissions until about 2040-50. After that the paths which we could take begin to diverge, and the more time that passes the greater the divergence.

    For example:

    Under the higher-emissions scenario, winters in the Northeast could warm by 8 F to 12 F and summers by 6 F to 14 F above historic levels by late this century. But under the lower-emission scenario, temperatures during Northeast winters are projected to warm only 5 F to 8 F above historic levels by late-century, and summers by just 3 F to 7 F.

    Global Warming Will Hit U.S. Northeast Hard Unless Action Taken Now;
    Long-term Severity Depends On Near-term Choices, Scientists Say
    July 11, 2007

    However, it is also worthwhile to keep in mind that the effects of black carbon are far more immediate than those of carbon dioxide. Reducing black carbon emissions could make a big difference with respect to the glaciers and greatly reduce the water shortages we are expecting under BAU in the latter part of this century and prolong the point at which the arctic becomes ice free during the summer as well reduce the ice-free seasons we will see in the future.

  24. 74
    FP says:

    What is this bull about Dr. Klaus-Martin Schulte doing a study of papers that proves a change in consensus. I knew there was an older study that overwhelmingly supported it. How could it change so much? Are these 528 papers legit or were they planted by the deniers and an overzealous writing campaign? Or are they just accurate and the papers that don’t endorse the theory don’t do so simply because that theory is beyond the scope of the specific study?

    What is going on?

  25. 75
    FP says:

    What is this bull about Dr. Klaus-Martin Schulte doing a study of papers that proves a change in consensus. I knew there was an older study that overwhelmingly supported it. How could it change so much? Are these 528 papers legit or were they planted by the deniers and an overzealous writing campaign? Or are they just accurate and the papers that don’t endorse the theory don’t do so simply because that theory is beyond the scope of the specific study?

    What is going on?

  26. 76
    Hank Roberts says:


    “US Senate Committee on Environment and Public Works–Minority … Medical researcher Dr. Klaus-Martin Schulte recently updated this research. … The results have been submitted to the journal Energy and Environment, …”

  27. 77
    dhogaza says:

    What is this bull about Dr. Klaus-Martin Schulte doing a study of papers that proves a change in consensus.

    It’s bull, indeed.

    Tim Lambert at Deltoid already has a debunk up.

  28. 78
    Lynn Vincentnathan says:

    #68 & “‘we might well miss a critical window of opportunity to forestall some of the more serious consequences of AGW’

    Forestall how? Leading scientists have said that all of our efforts to reduce CO2 emissions won’t make a bit of difference. I don’t think you consider the practicality of what is proposed.”

    Yes, it is possible that the critical window is closed and we are now in for a very devastating time. But it seems to me that even then our efforts to reduce our GHGs could help, even if a little. A person dying of thirst (our great great grandchild, perhaps) could really use that ounce of water — even tho it seems as nothing to us.

    And surely by arguing about whether or not the critical window is now closed and doing nothing to help the future victims (our own progeny) will all the more ensure that we will pass that runaway tipping point of no return.

    And it is eminently practical to reduce our GHGs through energy/resource conservation/efficiency. The father of practicality, Ben Franklin, used to say things like “a penny saved is a penny earned.” Are we so profligate and degenerate that we’re unable to understand what our fathers, mothers, and grandparents have taught us?

    Can someone explain to me how reducing GHGs costs money. It seems to me that increasing our GHGs entails greater expenditures.

  29. 79
    Falafulu Fisi says:


    [Response: I leave this edited post up solely to make a point. Describing Bayesian inference as ‘blind faith’, calling other posters’ points ‘daft’ and using rhetorical condescension to belittle the commenters here is just not going to fly. You can make all the scientific points you want, but a certain modicum of respect for everyone else is required. Play by the rules or play elsewhere. – gavin]

  30. 80
    Michael says:

    The cost of reducing GHG’s is more apparent in developing countries such as China and India. As industry and population grow, and the average person moves away from poverty – toward using medical services, transportation, recreation, and land use is increased for food production, GHG levels increase dramatically. We can see it happening now, and they are gaining rapidly on the United States. The question: how do you avoid multiple US GHG scenarios across the globe without hindering the rise of millions from poverty?

  31. 81
    Tim McDermott says:

    “The question: how do you avoid multiple US GHG scenarios across the globe without hindering the rise of millions from poverty?”

    Why on earth do you think that carbon-based energy is going to lift the third world from poverty? The price of oil is only going to go up, and coal is likely to follow quickly. Wind and solar are going to be cheaper and more effective in the truly poor parts of the world, because renewable sources of energy do not depend on massive infrastructure investments. They only require incremental investments that start paying off right away.

  32. 82
    Falafulu Fisi says:

    Ok, I appologise to Timothy Chase for the daft comment. But, I stood by my comment regarding blind faith in statistics. Gavin, I specialize in the development in these algorithms including Bayesian Belief Network. In fact that is actually what they are, since you can never know for sure the causal relationships between the inputs & the outputs.

    Don’t get very defensive here Gavin, that is exactly what the definition of blind-faith is, no functional relationships between the inputs & the outputs.

    [Response: You misunderstand me. I am not intervening in whatever point you are trying to make, I am just trying to ensure a reasonable discussion. – gavin]

  33. 83
    Patrick 027 says:

    Re 50 correction:

    ORIGINAL: “There is also LW flux exchanges within the atmosphere-surface system from hot layers to cold layers, depending on how well the layers can ’see each other’ at a given wavelength. Increasing the opacity tends to increase the exchange at shorter distances (the net flow from hot to cold will decrease) but decrease the exchange at longer distances (the net flow from hot to cold will increase),”

    CORRECTION: swap the words ‘increase’ and ‘decrease’ in the paranthetical phrases near the end of the above.

  34. 84
    Chris Dudley says:

    I’ve come across another place where a sea level rise of 25 meters in a century is attributed Hansen et al. (2007). Does anyone know if a statement like this has ever been made by the group? I could see this rate occuring if the final rise is on the order of 100 meters but I don’t see how deglaciation can go faster that the temperature rise which is moderated by the ocean mixing time scale. Thanks for the help.

    [Response: The last time the planet was 3 deg warmer than now was the Pliocene and sea level was 25 meters higher. But that is an equilibrium statement, has no information on timescales and certainly isn’t a near term forecast. But it should give pause to anyone who thinks that adapting to a 3 deg C+ climate change will be easy. – gavin]

  35. 85
    Patrick 027 says:

    The correction in my comment 83 was for the beginning portion of the third-to-last paragraph in my comment 56.

    On that note, this matter (rate of radiative exchange among different layers) can get a bit complicated when looking at different wavelengths.

    The effect of increasing LW opacity that is easier to grasp is that the source of LW radiation coming up at the tropopause (top of the troposphere) from below is shifted upward – away from the warmer surface and upward within the troposphere, where it is colder – therefore leads to the troposphere ‘looking colder and dimmer’ (less LW flux coming up from below at the tropopause) from above, causing it to be in a radiative disequilibrium where more LW + SW radiative energy is going down than the LW energy coming back up (reduced somewhat by changes in downward LW radiation at the tropopause from the upper atmosphere – mainly the stratosphere) that can only be restored (setting aside feedbacks for the moment) by warming the troposphere and surface (they must warm together because of convection).

    If the troposphere is relatively transparent at some LW wavelength initially, then increasing LW opacity initially may increase the radiation from all levels within the troposphere that reaches the tropopause; however, this still comes about with blocking radiation from the surface, which is warmer than the average of the troposphere, so the total LW flux upward at the tropopause from the surface and troposphere combined is still reduced.

    At wavelengths where the troposphere is more opaque initially, then increasing the LW opacity further will enhance the emission from the upper troposphere but at the expense of blocking brighter radiation both from the warmer lower troposphere and the warmer surface.

    At wavelengths where the troposphere is extremely opaque, then most of the LW radiation from below is from a rather short distance, thus further increases in opacity will have little direct effect on LW flux coming up from the troposphere. (One might imagine that the direct effect at such wavelengths could be a cooling of the troposphere and surface as the more rapid (per unit mass depth of atmosphere) changes in temperature in the upper atmosphere, particularly the mid-stratosphere, allow further increases in opacity to have an effect (saturation LW opacity is relative to vertical temperature gradient), and in that case, it would be reducing the downward LW radiation into the troposphere; HOWEVER, the base of the stratosphere tends to be more isothermal (as far as I know) – anyway, the temperature variation at the tropopause will not generally go immediately from slower temperature decline with height to an even faster temperature rise in the base of the stratosphere – and that means that as opacity gets so great in the troposphere that further increases have little effect on upward LW flux at the tropopause, there is also little effect at the tropopause on the downward LW flux as well, and thus little net effect.)

    (Note that in the last four paragraphs I have mostly been discussing a total upward LW flux – distinct from the net upward flux that is the upward LW flux minus the downward LW flux.)

    Any given greenhouse gas will have a significant effect over a range of wavelengths (some ranges larger than others). At high enough concentrations, there will be some wavelengths where the opacity is so great that there is little effect on net LW flux at the tropopause, but even when this occurs, there will still generally be a range of wavelengths where further increases in opacity do matter. (So some people claim that the effect of CO2 is already saturated and additional amounts won’t matter – well, that’s only for some wavelengths (and even at those wavelengths, there would still be an effect on the upper atmosphere, etc… although that may be a cooling effect that could eventually feedback to the troposphere in other wavelengths as a cooling effect, but for CO2 at least, the entirety of the upper atmospheric cooling only reduces the net effect at the tropopause by a little amount) – also, they may be thinking of saturation in terms of blocking radiation directly from the surface to space – this can occur at intermediate opacity where further increases still have a significant effect on radiation from the troposphere.)

    Okay, that was a bit of a tangent.


    Re 63: I think you’re on the right track; I would try to clarify it this way:

    The sun heats the climate system directly; some of that heating is distributed among layers of the atmosphere (via water vapor (in the troposphere), ozone (in the upper atmosphere), clouds (they are not perfect reflectors), other gasses (even some other greenhouse gasses can absorb a little solar radiation, but in relatively small amounts – their effect in the LW is dominant, hence they are still called greenhouse gasses. That goes for water vapor, too.). But a majority is at the surface or just underneath it (in the oceans). Some of that heat goes into raising temperature – that’s sensible heat. Some goes into phase changes – such as evaporation of water vapor from wet surfaces – that’s latent heat. Some LW radiation is emitted directly to space from where SW absorption takes place, but some is not. The remaining heat can get back to space by muliple photon emissions and absorptions (LW exchanges among layers of the atmosphere, the net energy transfer necessarily being from warmer layers to colder layers, until radiation directly to space can occur from one of those colder layers). Within the troposphere, convection also transports heat, and in the net energy transfer rate, it is a majority of upward heat transfer at the surface, globally averaged. Some convection at the surface is of sensible heat. Some is of latent heat, which is converted to sensible heat upon condensation (and freezing, where that occurs). Some of that sensible heat is taken up again as evaporation and melting can occur within the atmosphere, but eventually, precipitation of water from the atmosphere to the surface occurs, and this leaves some once latent heat behind in the air as now sensible heat. Convection can transports this heat upward, to various levels of the troposphere by various amounts, and in various amounts, LW radiation can remove that heat and take it upward and eventually to space. In the vertical, convection goes from warmer levels below to cooler levels higher up, and air cools as it rises and expands, so convection takes heat down a temperature gradient, as does LW radiation. Convection also moves heat horizontally (horizontal transports are refered to as advection), and as a result, LW emission from the atmosphere to space is more evenly distributed from pole to equator than solar heating.

    So increasing the greenhouse effect reduces direct LW emission to space, and at any given wavelength, increases the number of levels LW radiation must be transfered to before escaping to space. Convection cannot go directly to space, so while convection may change, the surface and air still has to warm up within the troposphere in order for the LW emission to space to go back to what it was. And they will warm up because they will be recieving more heat then they lose until equilibrium is restored. They heat up together because they are coupled via convection – if only the top heats up first, this reduces convective and radiant heat loss from below, and so that heats up as well, etc.

    At higher temperatures, more water vapor can be held in the air at a given relative humidity. I think evaporation also tends to occur faster (right?) because the same wind blowing over a warmer wet surface will pick up a greater amount of moisture per unit air, and so at equilibrium, there will also be more precipitation, while the reservoir of water vapor in the atmosphere will tend to be greater. That is a positive feedback because water vapor is also a greenhouse gas (though it also has an effect on SW radiation as well).

    And then, there is enhanced warming near the surface in the polar regions due to loss of snow and ice, which increases solar heating. And then there are changes in circulation and cloud cover, etc…

  36. 86
    PHE says:

    Lynn (78). You are so pessimistic. Given that the average quality of life and life expectancy has never been better throughout the history of mankind – and getting better – the current trend is not bad. We (in the developed world) are doing this at the same time as doing a pretty good job of reducing air pollution, water pollution, etc and learning how to better manage our impact on our natural environment (but of course still have a long way to go). If we allow the non-developed world to develop, there is a good prospect they will follow suit, while learning from our own mistakes. We can not have zero impact on the environment unless we choose to live as Australian aborigines before Europeans arrived. Unless we want this, living on Earth has to be a balance between looking after the environment and maintaining a good quality of life and comfort for ourselves. The fact you are using a computer is proof that you at least partly agree with this.

  37. 87
    Ray Ladbury says:

    PHE, I would say there was more cause for optimism about our species if we had demonstrated the ability to appreciate the risks associated with threats like climate change rather than falling prey to wishful thinking. As yet, there is little evidence of this on a mass scale. Indeed, I am seeing considerable backlash against even the words “sustainability” and “environmentalism”. To date I see no evidence that these large human brains we have give us any survival advantage over colonies of yeast or bacteria. We are still fouling our nests to the point where it will no longer sustain us.

  38. 88
    NeilT says:

    I have a question about tipping points.

    Given that tipping points have recently been stated as “wildcard” accelerators of GW, do we know how to identify them?

    As an example, complete loss of the Arctic Ice in Summer is stated as a significant tipping point.

    To me it would seem to be a 3 phase thing:
    1. Significant thinning of the ice allows a breakup of the ice and heat to penetrate to more and more areas of the sea, casing additional warming
    2. A sudden redcution in both summer and winter ice allows exponential warming to cause unparalleled ice loss
    3. A very quick switch from Ice field to Ice free happens over a period of 3-5 years due to the first two phases.

    If that is the case, we have passed phase1 and 2 and will go into phase3 over the next few years.

    Given that my scenario above is correct I have to ask:

    1. Do we understand what casuse the tipping points in the first place
    2. Do we know how to recognise them as they are happening
    3. Can we insert them into the models at ther right time to correctly forecast the “rapid change” events we observe which might give the public more faith in the models having some basis in their day to day reality.

    After all it would appear to me that unless we understand and can identify these tipping points clearly, then the models are going to be hopelessly optimistic.

    Hindsight is a wonderful thing. It appears to me that we are, this year, 50% of the way to the Summer Arctic Ice tipping point. Should we have been able to recognise and predict this?

  39. 89
    Nick Gotts says:

    RE #86 [Given that the average quality of life and life expectancy has never been better throughout the history of mankind – and getting better – the current trend is not bad. We (in the developed world) are doing this at the same time as doing a pretty good job of reducing air pollution, water pollution, etc]
    While the claim in your first sentence is true, there are very serious grounds for doubting it will continue: climate change itself, groundwater depletion and contamination, soil erosion and salination, deforestation, spread of invasive species. All of these are well-established trends difficult to reverse, but their worst effects come only after long periods of cumulative damage, and have mostly not yet been felt. Rich countries have reduced certain kinds of pollution – those with obvious short-term and local effects – but much of this reduction has been achieved by exporting polluting industrial processes to poor countries; and for greenhouse gases, which do not have significant local effects, performance is way below what is needed to avoid dangerous climate change. Looking after the environment is perhaps the most essential component in maintaining a good quality of life and comfort for ourselves if we look beyond the short term – and we are manifestly failing to do so. Looking after the environment does not mean having no effect on it – it means ensuring that we do not do the ecological systems we depend on irreparable damage.

    Incidentally, the Australian aborigines had profound effects on their environment, mainly through the use of fire. Take a look at Tim Flannery’s “The Future Eaters”.

  40. 90
    Magnus A says:

    #11: Or the water will freexe again. That is possible and how the glaciers grows.

  41. 91
    Magnus A says:

    #84: Gavins answer: No, the last time it was that hot wasn’t at least 2 million years ago. It was 120 000 years ago, at the last interglacier period when it was 5 degree warmer. See Eske Willerslev, here:

    [Response: No. The ‘5 deg’ is actually ‘3-5 deg C’ and is an estimate for Greenland only. Northern hemisphere as a whole was maybe 1 deg C warmer, and globally, it’s not clear if it was warmer at all. Orbital forcing would suggest warmer NH summer, but cooler tropics and stable SH. Plus, even with that sea level rise was 4-6 meters higher! – gavin]

  42. 92
    Chris Dudley says:

    Re #84


    Thanks. To me, reading Hansen et al. (2007) what you are saying is pretty clearly there in the text. But, some people are misinterpreting I think.
    It might be worth an article here to discuss contraints on the possible rate
    of sea level rise that incoporates the use of the late spring insolation anomaly width. Maybe an expanded fig. 3 from Hansen et al. that also shows the timing mismatch with the early spring and summer curves which is given in table 1. would be something to include as well. An expanded scale for Termination I might help too.


  43. 93
    Michael says:

    View #1:

    We are still fouling our nests to the point where it will no longer sustain us.

    It’s a pessimistic view, not invalid, but only one way of looking at it. And it’s not new, but a view repeated over and over through our history. Unfortunately the only solution to this view is the eradication of the human species.

    View #2:

    The human race is nothing short of miraculous. Around six billion and counting, and our worst impact is to raise CO2 levels (a naturally occurring atmospheric component); it is amazing to me that we have not had a worse impact. Evolution has finally produced a species with the future potential to save the planet from catastrophes such as GW, asteroid impacts, disease outbreaks, etc. A massive amount of time, energy and money is being spent on GW, and a solution will be found.

    It is also amazing that on a blog such as RC you see the comment:
    ‘I would say there was more cause for optimism about our species if we had demonstrated the ability to appreciate the risks associated with threats like climate change rather than falling prey to wishful thinking.’

  44. 94
    Aaron Lewis says:

    Re 90
    Glaciers grow when their temperature significantly below 0 C. If there is liquid water present, then their temperature is 0C. When ice is at 0C you can watch it deform with your naked eye when stress is applied to the ice. In warmer conditions (0C), ice climbers often see significant deformation of the ice holding their body weight in a matter of two or three minutes. This can lend a certain urgency to climbing on. Ice is interesting stuff.

    If the base of a glacier is near 0C, and snow falls on it, the additional weight will tend to cause the ice at the base to be extruded horizontally. This is how you get rivers of ice.

  45. 95
    James says:

    Re #80: [The question: how do you avoid multiple US GHG scenarios across the globe without hindering the rise of millions from poverty?]

    Maybe you’re asking the wrong question. A better one might be to consider whether it is better for those millions to continue living in “poverty”, or to die from the side effects of their attempt to secure “wealth”.

    I put the two terms in quotes because I believe the world needs to do some serious thinking about those two concepts. For instance, some of your millions are probably people living in the context of their cultures, much as their ancestors did. Because they don’t have a western lifestyle with access to lots of consumer goods, you adjudge them “poor” and seemingly think having them rise out of that poverty is important enough to risk severely damaging the ecosystem that supports us all. You’ll forgive me if I think your priorities need adjusting.

  46. 96
    Michael says:

    You sit in front of a computer knowing there is a hospital down the road, you have clothes on your back, one of your kids was not just killed by a roving band of looters, you’re not dying of aids with no treatment available, your not in some filthy immigration camp struggling day to day just to have your ill wife and kids die off one by one. And you talk of ‘western lifestyle’ as if it’s some kind of superfluous extravagancy. That’s fine and very easy of you to say, but don’t jump all over people showing true compassion for the poverty stricken.

  47. 97
    Ray Ladbury says:

    James, In my travels in India, Africa, China and Latin America, I have encountered many who were poor by any definition you care to use. I would contend that those without access to adequate healthcare, nutrition or education could be termed poor. I would argue that if infrastructure is inadequate to supply access to these commodities for the majority of citizens, then the regions/nations where this is true might be called poor. People will always do whatever they can to escape poverty–and this includes burning coal or charcoal or wood or dung. It is not a question of development OR the environment. It is a question of how we achieve development while sustaining a livable environment. If we emphasize raising living standards at the expense of the environment, we will fail because the environment will deteriorate to the point where all living standards decrease. On the other hand if we try to preserve the environment for the privileged at the expense of alleviating poverty, the poor will do what they must to survive, with disastrous consequences for the global environment.
    Development need not lead inevitably to consumerism. It must improve opportunity. The poor are not an obstacle to preserving the environment. They are a resource whose creativity we will need to perserve the environment.

  48. 98
    Holly Stick says:

    Michael wrote in #96: “…And you talk of ‘western lifestyle’ as if it’s some kind of superfluous extravagancy. That’s fine and very easy of you to say, but don’t jump all over people showing true compassion for the poverty stricken.”

    Don’t you realise how wasteful we are in the west? We do live a sumptuous, extravagant lifestyle, which we achieved mostly by keeping other people poor. That dire poverty over there was largely caused by us over here. Not very compassionate.

    Michael, when you hear someone say: “The poor have no bread!: don’t reply: “Then let them eat cake!” It is not helpful, it shows a distressing lack of compassion, and it demonstrates that one is completely out of touch with reality.

  49. 99
    Andrew Sipocz says:

    Re #84 and Gavin’s response: Hansen quotes Siddall’s Red Sea work quite a bit. The State of Florida recently commissioned a sea level reconstruction for the Gulf of Mexico (Balsillie and Donoghue 2004) that correlated other records with Siddall’s work. Balsillie concludes there were three periods in the last 6,000 years where the sea level was about a meter above today’s level in the Gulf. In the past it was thought a possible past high stand would have been a one time deal caused by isostatic rebound of the ocean floor, but this work makes it sound like the recent sea level (last 6,000 years) bounced up and down a few meters (above and below today’s level) over the matter of a few hundred years due to changes in global temperature of one degree C or so and nothing above today’s current global temp. This would seem to lend credence to the idea that the polar ice sheets could respond quickly to relatively small global temperature changes. I’d love to see updates on research as they become available, but I want to make the point that most geologists/biologists I speak to here on the Gulf coast believe that sea level has been relatively stable for 6,000 years now and doesn’t respond much at all to anything other than a full scale glaciation or deglaciation and then on the matter of millenia, not centuries. They believe Siddall and others citing a Holocene “bumps” are incorrect. I don’t know who to believe.

    [Response: As far as I can tell, there is no coherent evidence for global bumps of a meter or more during the Holocene. So my instinct would not be to believe Balsillie and Donoghue results are global. But I’m not very familiar with that paper or the rest of that literature, though I have seen plenty of long term SLR reconstructions that don’t show any such thing. – gavin]

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    Sarah Wise says:

    This post is the first I accessed on RealScience. I applaud all of the author’s efforts and am really happy to see the excellent discussions going on.

    I think there is a mismatch, however, between some of the science-specific postings on RealScience and the stated mission of RealScience (as seen at “Welcome to RealScience”) — to reach journalists and the public. I am a fledgling climate science educator, and even I got frustrated with the use of undefined acronyms and jargon in this piece. And it was really long — I almost gave up before finding the “more” link that took me to the answer to the initial question (“what will happen regionally”).

    To keep your target audience paying attention, authors will continually have to write at their level. That doesn’t mean dumbing down, it means distilling the essence of your message.

    Perhaps there needs to be some direction of lay/teacher viewers to the best/shortest/most accessible pieces and allow the longer, more technical pieces occupy a separate space?