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The IPCC report on extreme climate and weather events

Filed under: — rasmus @ 19 November 2011

The IPCC recently released the policy-maker’s summary (SREX-SPM) on extreme weather and climate events. The background for this report is a larger report that is due to be published in the near future, and one gets a taste of this in the ‘wordle‘ figure below. By the way, the phrase ‘ET’ in this context does not refer ‘extra-terrestrial’, and ‘AL’ is not a person, but these refer to the way of citing many scholars: ‘et al.

Fig. 1. The text analysis according to

The fact that the summary is released before the main report is bound to cause some confusion, and has lead to a number of false allegations in the past, such as the main report being written to suit the conclusions of the summary. This is not the case, but I personally think that the IPCC handles the release of these reports in a strange way.

The main report has already been written, but there are some fine details that need to be approved by the member states before it is finalized. My understanding is that the whole process will be open and transparent, and that the previous drafts and review comments will be available in time. Those who already have read the main report are not supposed to cite it before it’s out.

I must also confess that one of the aspects that I’m most curious about concerns tropical cyclones (TCs). Hence, these phenomena was one of the things I looked at first. Here are some quotes:

Average tropical cyclone maximum wind speed is likely to increase, although increases may not occur in all ocean basins. It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged.

The message from the summary of policy-makers is therefore that it is likely [66-100% probability] that there will be fewer or same number but more intense tropical cyclones (including tropical storms, hurricanes, and typhoons) in the future. This conclusion is not new, however, as it was also the concusion of the AR4, as well as the most recent WMO consensus statement on tropical storms.

A combination of stronger tropical cyclone maximum winds but fewer tropical cyclones is nevertheless quite interesting. My feeling is that this statement is still a bit premature, as it surely is based on projections made with global climate models (GCMs). The tropical cyclones are represented differently in the GCMs compared to real world measurements, where the wind speed changes continuously in space.

The message from the SREX-SPM is similar to that of a 2010 study from Nature Geoscience, for which the abstract reads (my outline):

Whether the characteristics of tropical cyclones have changed or will change in a warming climate — and if so, how — has been the subject of considerable investigation, often with conflicting results. Large amplitude fluctuations in the frequency and intensity of tropical cyclones greatly complicate both the detection of long-term trends and their attribution to rising levels of atmospheric greenhouse gases. Trend detection is further impeded by substantial limitations in the availability and quality of global historical records of tropical cyclones. Therefore, it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes. However, future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100. Existing modelling studies also consistently project decreases in the globally averaged frequency of tropical cyclones, by 6–34%. Balanced against this, higher resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20% in the precipitation rate within 100 km of the storm centre. For all cyclone parameters, projected changes for individual basins show large variations between different modelling studies.

In GCMs, these phenomena appear as vortex-like features in the discrete representation of the flow represented by neighboring grid boxes. It’s quite remarkable that these phenomena are present at all in these models (sometimes they are not, though), even though they may have too weak or exaggerated features. In the real world, the definition of a tropical storm is a synoptic scale low-pressure system with maximum sustained surface wind speed greater than 17 m/s, and in hurricanes greater than 33 m/s.

If we look at wind speed measurements at a given location, we see that there are relatively few days with zero wind, more often there are moderate wind speeds, and it is typically rare when the wind speed exceed the threshold defining a tropical storm or a hurricane. In statistical terms, the wind speed may be described by a distribution function – e.g. a Weibull distribution (e.g. here and here). The situation is illustrated below showing wind speed statistics, where the curve is the probability distribution function (pdf) for the wind speed and where the x-axis represents the wind speed and the y-axis the likelihood (frequency) of occurrence. The threshold marking tropical storms is shown as the first vertical line (the others mark typical TC categories), and the area under the curve to the left of this treshold (denoted “a” in the diagram) is proportional to number of observations (e.g. days) with no tropical storms. The area above (“A”) is proportional to the frequency of tropical cyclone occurrence.

Fig. 2. Wind speed statistics and tropical cyclones.

Let’s consider the implication of fewer tropical cyclones but an increase in their intensity. In terms of wind speed statistics, this suggests a shift in the pdf (grey gurve in Fig. 3), with an increase in the area under the curve with wind speeds lower than 17 m/s (“a”). This also implies a decrease in the area under the curve for which wind speeds exceed 17 m/s (“A”), as the area under the total curve of a pdf must be constant (unity by definition). But if the tropical cyclones are getting more intense (increased mean TC maximum wind speed), there must be a second threshold, e.g. 33 m/s for which the area under the curve for the new pdf is greater than for the old curve.

Fig 3. The grey curve illustrates a perceived shift in the pdf describing the wind speed statistics

It is certainly possible that the requirement in these changes in the wind speed statistics can occur, but the question is whether it is likely and whether we are able to detect this. If the shape of the wind speed is constrained to being a Weibull type, then it is easy to simulate the probability that the area under the curve is greater both for the portion of the curve with wind speeds lower than say 17 m/s and greater than 33 m/s (Monte-Carlo simulations – R-script). The fraction of Weibull shapes satisfying this, accoring to a simple Monte-Carlo simulation, is 1.9% (i.e. not very likely). Another issue is the required size of a statistical sample to be able to detect such changes, and the GCMs’ ability to provide such details (there are not that many simulations with high-resolution GCMs, the number of TCs is sensitive to a number of factors, such as ENSO, AMO, MJO, and the annual cycle – I must admit that I don’t know if the GCMs capture these dependencies well).

An analysis of empirical data provides strong indications that the number of TCs, N, varies with the surface area of warm surface (warmer than 26C). However, N also depends on the number of triggering events (organized convection e.g. African easterly waves in the North Atlantic), the wind shear, and the convective available potential energy (CAPE). The question about future trends in number and intensity of TCs depend on these aspects, in addition to the magnitude of the sea surface temperature.

It is well-known that tropical cyclones are influenced by a number of factors, such as El Nino Southern Oscillation (ENSO), the Madden Julian Oscillation (MJO), wind shear, organized convection, and sea surface temperatures. The GCMs, however, provide different accounts as to which direction the ENSO takes, struggle with reproducing the MJO, and may have biases with respect to the sea surface temperatures and wind shear. There are also some problems, as they produce a spurious “double” inter-tropical convergence zone (ITCZ), as well as biases in sea surface temperatures and wind shear.

Furthermore, small-scale processes may still not be sufficiently resolved by the GCMs used for projecting the future climates. Having said that, high-resolution global atmosphere models provide realistic-looking pictures of tropical cyclones, and the question is not whether the models in principle are capable to capture these events, but rather whether the current set-up of GCM experiments is sufficient for providing reliable information about how these will evolve in the future. The main report may shed more light on this, and we should keep in mind that the models must be evaluated against the past and reproduce known dependencies, in addition to reproducing the wind speed distributions.

There is still a debate about the past trends (see previous discussions here, here, here, and here). Has the tropical cyclone activity or the number of cyclones increased? Note, the trends may not not necessarily linear, and if one tries to fit a straight line in time, it may not provide the best picture of the situation. As long as we have no reliable records on tropical cyclones for the past, I’d argue that we don’t know how well our models are able to capture long-term changes in tropical cyclones. However, this is only one small issue (see Fig. 1), and the SREX-SPM covers many other topics on which I have little expertise.

68 Responses to “The IPCC report on extreme climate and weather events”

  1. 1
    Hunt Janin says:

    What does the new IPCC report have to say about sea level rise?

  2. 2
    Paul D says:

    Can you rotate the text image 90 degrees clockwise??
    It’s a 2 minute job in Photoshop or your favourite image editing software.

    [Response: Done. -rasmus]

  3. 3

    The spectrum of tools available to help is familiar: improved forecasts and warnings for severe weather, rigorously enforced zoning and building codes, and restoration of ecosystems that serve as buffers between people and river floods or coastal-storm surges, for instance.

  4. 4
    CM says:

    Hunt (#1), see pp. 6, 12, and 26 of the SPM linked to by Rasmus. You’re in luck (and coastal populations are not): some of the more robust statements in the report concern extreme coastal high water events.

    Otherwise, I found Rasmus’s discussion here rather more interesting than the SREX SPM, and mind you, my eyes glaze over quickly when people start using words like “Weibull distribution”. Trust the IPCC to make even “extreme climate events” sound boring.

    I’m not asking for an 8pm Discovery Channel disaster-fest. But take this (to return to sea level): “The very likely contribution of mean sea level rise to increased extreme coastal high water levels, coupled with the likely increase in tropical cyclone maximum wind speed” — now you think they’ve finally got into gear, right? — “is a specific issue for tropical small island states.”

    It’s a specific issue for them? This is the kind of heroic understatement last seen as a literary trope in the sagas of my Norse ancestors. (“There is low confidence as to whether Gunnar is inside, but there is robust evidence that his halberd is … aaagh.”)

    There’s been a lot of talk in these threads about drought projections. SREX’s cautious wordings might be grist for another round of that.

  5. 5
    Pete Dunkelberg says:

    Hunt @ 1, just search on sea level in the linked pdf.

  6. 6
    Pete Dunkelberg says:

    What is the latest date for a paper to be submitted, accepted or published in order to be considered in this report?

  7. 7
    Chris Colose says:

    Rasmus, thanks for the post,

    I do not think there should be much surprise as to why TC frequencies decline in a warmer world, something which seems to me to be something that can be argued on theoretical (at least qualitatively) grounds, and not necessarily dependent on full blown GCMs (as was also implied in Judith Curry’s comment on the SPM).

    The decline in frequency depends on the ratio between the difference of middle tropospheric moist entropy and its saturation value (this difference scales with Clausius-Clapeyron at fixed RH), to the difference between moist entropy of saturated air (with T=SST and surface pressure) and the boundary layer moist entropy. The degree to which the atmosphere is unsaturated increases in a warmer world (at fixed RH), which results in decreases of convective mass flux.

    Of course, the “frequency of hurricanes” is probably not a very useful statistic, either from a human or climate perspective. The TC’s that matter for us are often the “black swans” (to use a phrase of Kerry Emanuel) of hurricanes, those with unusual tracks (esp. with exposed infrastructure), and of course weighted by the intensity of the hurricane. From a climate perspective, I’m not aware of any theoretical argument for why there should be the number of hurricanes per year that we actually observe (remarkably close to 90 every year) but small hurricanes cannot be compared in any meaningful way to big category 5’s (in terms of impacts on energy fluxes, etc). Thus, the thing that matters for us is intensity, but also whether the changed background state provides conditions favorable for “black swans.”

    It is actually not even obvious that N is highly sensitive to various triggering events in the macroscale (e.g., globally and annually averaged) view, as some people like Emanuel have argued only recently (he gave a talk a couple weeks ago at the University of Albany on this), if climate not only impacts hurricanes but hurricanes pose a strong constraint on the climate system (such as the differential climate sensitivity between the Poles and tropics). If one could put a big wall near Africa and prevent AEW’s from forming into TC’s (or to be more realistic, modify the baroclinic-barotropic instability of the African Easterly Jet, perhaps by “greening the Sahara” and altering the meridional baroclinitiy and moisture gradient) then one could initially get a decrease in TC frequency which would then imply a change in the background state that could feedback onto conditions conducive for TC conditions (albeit with a different spatial structure).

  8. 8
    Lloyd Smith says:

    An expanding atmosphere diminishes the chances of TC formation but SST increases coupled with an expanded atmosphere may extend the time when TC’s are able to form and be sustained. In addition, SST increases, an expanded atmosphere and reductions in Arctic ice create favorable chances for ETC formation and extended life. Frozen precipitation and on shore impacts of ETC’s create a new set of challenges for which existing policies and infrastructure are not adequately prepared.

  9. 9
    Halldór Björnsson says:

    I like the word cloud. But it is not difficult to remove common words from the list. Here is my version after removing common english words and ensuring that all were lower case:

  10. 10
    Hank Roberts says:

    > Halldór Björnsson … version

    That’s informative. Recommended.

  11. 11
    Kerry Emanuel says:

    First, I think Chris Colose has done a great job summarizing the theoretical arguments in favor of a global decrease in tropical cyclone frequency and the fact that, at the end of the day, the global frequency of events may have little (or perhaps nothing) to do with the nature and frequency of potential initiating disturbances.

    The policy-maker’s summary of the IPCC report on extreme events strikes me as conservative to the point of being biased. Consider, for example, this statement on page 11:

    “Average tropical cyclone maximum wind speed is likely to increase, although increases may not occur in all ocean basins. It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged”

    Note that the qualification “although increases may not occur in all ocean basins” is not applied (as its converse should be) to the subsequent statement about the global decrease in frequency. Indeed, most if not all downscalings of GCMs of which I am aware predict that while global frequency decreases, it increases in some ocean basins. Why the qualification in the first instance and not the second?

    Another curious statement is “Heavy rainfalls associated with tropical cyclones are likely to increase with continued warming”. But increased heavy rainfall is arguably the most robust result of both direct simulations and downscalings of tropical cyclones, and it is a feature that everyone working on the problem, to the best of my knowledge, agrees on. So why just “likely”?

    Finally, we have “The uncertainties in the historical tropical cyclone records, the incomplete understanding of the physical mechanisms linking tropical cyclone metrics to climate change, and the degree of tropical cyclone variability provide only low confidence for the attribution of any detectable changes in tropical cyclone activity to anthropogenic influences.” But this seems to ignore much evidence to the contrary. For example, the work of Elsner and Kossin (2008) uses homogeneous data derived from satellite (not the so called “best track” data) to reveal a distinct upward trend in the highest quantiles of tropical cyclone intensity in many places. It also ignores the rather spectacular correlation between North Atlantic tropical cyclone power and summertime tropical Atlantic sea surface temperature extending well back into the 20th century, with the latter clearly following the northern hemisphere surface temperature rather closely; yet the IPCC does not seem to have a large problem with attribution of the latter.

    As Chris points out, the frequency-intensity dichotomy is somewhat misleading, as the societal impact is more related to the frequency of intense events, which is a different matter. It is too bad that the IPCC missed an opportunity to correct this.

  12. 12
    David B. Benson says:

    Despite the reasonable accuracy of the Weibull approximation, it is not an exact characterization of the PDF of sea surface wind speeds. from
    The Probability Distribution of Sea Surface Wind Speeds. Part I: Theory and SeaWinds Observations
    in which a more precise PDF is otained based on a Fokker-Planck equation taking into account some of the surface layer physics.

    However, the more empirical
    Eugene C. Morgan, Matthew Lackner, Richard M. Vogel, Laurie G. Baise
    Probability distributions for offshore wind speeds
    Energy Conversion and Management 52 (2011) 15–26
    find that the Kappa and Wakeby distributions fit the upper tail (higher wind speeds) of a sample better than the bimodal Weibull, … and then the 2-parameter Lognormal distribution performs best for estimating extreme wind speeds, but still gives estimates with significant error. The Lognormal has a heavy tail but from the body of the paper one finds that the tail still is not heavy enough.

    I opine, for fundamental phyical reasons, that a fractional derviative Fokker-Planck equation ought to be considered. The details and references are not included here for brevity; it suffices to note that such an approach will lead to a PDF with a heavy tail rolling off (eventually) as k/w^(3/2) for some constant k and large wind speed w.

    In summary, it is possible that this IPCC report is more right than wrong regarding tropical cyclone frequency and intensity statistics; all I am fairly confident about is that the Weibull distribution is inappropriate for considering the frequency of high wind events.

    [The reCAPCHA oracle: “again guideos”]

  13. 13
    Dan Bloom says:

    So does the new IPCC report say anything about “polar cities”? See, my ideas are getting closer and closer to reality, all ye who doubted me here over the years!

  14. 14

    Meanwhile over at the BBC, Richard Black has found a draft document (possibly a version of this one, which has slightly different wording) and quotes this:

    Uncertainty in the sign of projected changes in climate extremes over the coming two to three decades is relatively large because climate change signals are expected to be relatively small compared to natural climate variability

    The wording in the draft I found is a little different:

    Projected changes in climate extremes under different emissions scenarios generally do not strongly diverge in the coming two to three decades, but these signals are relatively small compared to natural climate variability over this time frame.

    A variant on the story has spilled into a news report here in South Africa, as well as the anti-science blogosphere.

    Correct me if I’m wrong but if we are projecting forward over any period of 20-30 years, there is a reasonably high likelihood that natural variability will obscure the climate change signal. This statement however seems to me to be stronger than that. What do we know about natural variability that suggests it will make the climate change signal seem relatively small?

    We are still in a relatively deep low in the solar cycle, for example, and unless we have some good science suggesting that the next 2-3 solar cycles will be equally low, we should see a relatively steep natural increase in temperatures (but overlaid on any warming previously banked in the system). Could this be what the draft is referring to?

    Given that we have news services that even when they do not have an agenda can end up confusing the public, we’d better make sure we have a clear way of explaining all this.

  15. 15
    prokaryotes says:

    Another marker for extreme climate are microburst and uptakes. Discovered by Ted Fujita.

    Fujita is recognized as the discoverer of downbursts and microbursts and also developed the Fujita scale,[1] which differentiates tornado intensity and links tornado damage with wind speed.
    Fujita’s best-known contributions were in tornado research—he was often called “Mr. Tornado” by his associates and by the media. Much of what we now know about tornadoes was either discovered or advanced by his efforts. In addition to the Fujita scale, he was a pioneer in the development of tornado overflight and damage survey techniques, which he used to study and map

    In the book “Our Angry Earth” (out of print, though audio version is available on the internet), Isaac Asimov and Frederik Pohl, describe microburst and the connection to climate change.

  16. 16
    Pete Dunkelberg says:

    Kerry Emanuel @ 11, it is interesting that part of your comment parallels this analysis.

    Then when you mention “…the rather spectacular correlation between North Atlantic tropical cyclone power and summertime tropical Atlantic sea surface temperature….” I wonder what you and Rasmus make of this Naval study which finds in part:

    “If Caribbean SSTs are below (above) average in earlier months,
    then Beaufort Sea SIC tends to be above (below) average in October.”

    “Our results indicate that viable long-range forecasts of October SIC in the
    Beaufort Sea are possible via the use of Beaufort Sea SIC and Caribbean SSTs
    at lead times of one to five months.”


  17. 17
    Pete Dunkelberg says:

    Philip Machanick @ 14, I can’t help thinking that your comment involves ESP (error some place). The error involves events & extreme events vs the climate change signal overall. It is high time I brought up the divide and conquer strategy.

    Total ocean heat content will keep going up. This is not “natural” i.e. unforced variability. Let us stop forgetting conservation of energy. Where does the added energy of the ocean come from? But evidently this is not a discrete extreme event as the term is used. Cool La Niñas are now warmer than El Niños were prior to 1998. Evidently this is not an ‘event” as long as the difference between the two over an ENSO cycle is about the same as it was in the past.

    “Divide and conquer”: the focus on discrete events makes it possible to slice and dice climate into small (in both time and space) bits that are hard to individually attribute to much of anything.

  18. 18
    Hank Roberts says:

    Good pointer there from Prokaryotes.

    More here
    —excerpt follows—

    “I have been talking about the greenhouse effect for 20 years at least,” says Asimov in the video. “And there are other people who have talked about it before I did. I didn’t invent it.” As we’ve stressed here recently, global warming, and the idea that humans can change the climate, is not new.

    As one blogger notes, Asimov’s words are as relevant today as they were in 1989. “It’s almost like nothing has happened in all this time.” Except that Isaac Asimov has come and gone, and the climate change he spoke of is continuing.

    Asimov’s full speech can be seen here

    This post was written for “My Big Fat Planet,” a blog hosted by Amber Jenkins on NASA’s Global Climate Change site. “

  19. 19
    Pekka Kostamo says:

    Some comments on the hurricanes… from an engineer.

    Reading hurricane forecasts from a number of sources (for no personal reason), I note that a detail often involved is the outflow of dust from Sahara. A high dust event is believed by the forecasters to prevent formation of a hurricane, even in conditions when other factors are favourable (i.e criteria for sea surface temperature and subsurface temperature profile, atmospheric profiles of humidity and wind shear are met and an easterly wave exists).

    Obviously, the dust outbreaks depend on a number of factors, such as the rains, evaporation and prevailing winds in the great desert and the Sahel south of it. I understand that modeling efforts on future climate developments in that region have given rather unconsistent results. (This in addition to potential warming impacts on the generation mechanicsms of the easterly waves themselves in the heart of Africa.)

    As a sideline to theoretical understanding, does this line of thought open an opportunity to engineer the weather (nor climate) in terms of eliminating or reducing the number of hurricanes? Killing some killer storms before they have an opportunity to get going by means of seeding susceptible easterly wave convection features off the coasts of Africa (or in mid-Atlantic) might well be very cost effective. (I am aware of the project STORMFURY – which was about steering an already mature storm away from valuable targets nearby. A different concept altogether with a different set of opportunities and problems.)

    Many technologies could converge to reduce or eliminate one of the major hazards of weather, current or future. Using better observations, modeling, aircraft & avionics than what was available 60 years ago this would be a sideline benefit of more than marginal value.

  20. 20
    Burt Armstrong says:

    On aspect of the PETM that I can not find is what was the O2 level, prior and after the 20,000 year event? Being a biologist I have an interest in it because during the age of Dinosaurs the general O2 level was higher than it is today, being around 26%. Thus one aspect of the somewhat selective die off might have been a lowering of the O2 levels that disfavored the Dinosaurs and either favoring Mammals or not being important to Mammals for their survival. One can keep in mind that Birds a living close relative of the great Dinosaurs had a very fast metabolism. This fast metabolism is one aspect that makes them dominate the skies as flying requires a massive energy component, something only the smallest mammals and of curse birds can achieve.

    I of course do accept the IPCC report but wish that they could express more accurately how worried the climate scientists really are about what we are going to get when we go past 450 ppm CO2 due to inaction sponsored by the powers that rule us.

  21. 21
    David B. Benson says:

    The Generalized Pareto distribution (GPD) is frequently applied for the statistical analysis of extreme wind speeds. from the abstract for a poster:
    Stable estimations for extreme wind speeds
    Hans Van de Vyver – Royal Meteorogical Intstitute of Belgium
    as part of 7th Conference on Extreme Value Analysis: Probabilistic and Statistical Models and their Applications, June 27th-July 1st, 2011, Lyon, France

    This is an appropriate approach which assumes already a heavy tailed distribution an extreme wind speeds.

  22. 22

    Predicting the frequency and intensity of tropical cyclones under the present conditions let alone under future conditions are daunting tasks. There is an urgent need to find a solution to global warming. Harnessing a small part of the energy produced in the atmosphere would provide enough clean energy to meet all human needs.

    There may be a possibility of harnessing some of the energy produced in the atmosphere. My experience as a process control engineer has been that the behavior of complex processes is easier to control than to predict. The following link describes a proposal for producing a anchored vortex in order to produce energy.

    The thermodynamic of tropical cyclones is well understood. The energy source is warm sea water. While warm sea water is widely available, the initiation and control of a convective vortex requires other ingredients which could be provided.

    Harnessing the energy of the atmosphere would be a major scientific undertaking. Atmospheric scientist could contribute to solving the energy and global warming problem.

  23. 23

    Pete Dunkelberg #17: You may well be right but the draft the BBC article quotes says: “climate change signals are expected to be relatively small compared to natural climate variability”. That is what I was specifically finding hard to understand. The text I found changes the statement to refer to extremes, which makes more sense.

    Possibly this was an early draft and the wording was in error. It’s unfortunate that the BBC reported this without checking with the authors on the status of the document.

  24. 24
    prokaryotes says:

    Why can we attribute Microburst to Global warming and Ozone Depletion?
    Because more cold air around the tropopause, and the trigger of precipitation, causes more downbursts.

    Global Warming Causes Stratospheric Cooling

    Cooling of the stratosphere isn’t just the result of ozone destruction but is also caused by the release of carbon dioxide in the troposphere. Therefore, global warming in the troposphere and stratospheric cooling due to ozone loss are parallel effects. As cooling increases, development of the ozone layer can be affected because a cold stratosphere is necessary for ozone depletion.

    So releasing more carbon dioxide may not only increase global warming but may also contribute to the formation of the ozone hole. The system is pretty complicated and so we try to give just an overview of it here.

    The tropopause is shown as dotted line (the troposphere below and the stratosphere above). For CO2 it is obvious that there is no cooling in the troposphere, but a strong cooling effect in the stratosphere. Ozone, on the other hand, cools the upper stratosphere but warms the lower stratosphere.

    The impact of decreasing ozone concentrations is largest in the lower stratosphere, at an altitude of around 20 km, whereas increases in carbon dioxide lead to highest cooling at altitudes between 40 and 50 km (Figure 3). All these different effects mean that some parts of the stratosphere are cooling more than others.

    Microburst – They generally are formed by precipitation-cooled air rushing to the surface, but they perhaps also could be powered from the high speed winds of the jet stream deflected to the surface

    The main jet streams are located near the tropopause, the transition between the troposphere (where temperature decreases with altitude) and the stratosphere

    The formation of a downburst starts with hail or large raindrops falling through drier air. Hailstones melt and raindrops evaporate—this is an endothermic process that demands a lot of energy (in the form of latent heat) so the air is cooled. Cooler air has a higher density than the warmer air around it, so it falls

    The atmospheric currents become more distubed and unbalanced, like ripples in an ocean. This causes more air bursting.

    Outflow boundaries create low-level wind shear which can be hazardous during aircraft takeoffs and landings. If a thunderstorm runs into an outflow boundary, the low-level wind shear from the boundary can cause thunderstorms to exhibit rotation at the base of the storm, at times causing tornadic activity. Strong versions of these features known as downbursts can be generated in environments of vertical wind shear and mid-level dry air. Microbursts have a diameter of influence less than 4 kilometres (2.5 mi), while macrobursts occur over a diameter greater than 4 kilometres (2.5 mi). Wet microbursts occur in atmospheres where the low levels are saturated, while dry microbursts occur in drier atmospheres from high-based thunderstorms. When an outflow boundary moves into a more stable low level environment, such as into a region of cooler air or over regions of cooler water temperatures out at sea, it can lead to the development of an undular bore.

    The national temporal distribution of catastrophic windstorms during 1952–2006 has a flat trend, but their losses display a distinct upward trend with time, peaking during 1996–2006.

  25. 25
    Urs Neu says:

    I am not sure if the analysis of overall wind speed pdf is an adequate approach to look at the frequency-intensity question, since tropical cyclones (TCs) are individual closed systems within the atmosphere and have a different behavior than normal wind flow. There are kind of tipping points in the development of TCs, which might lead to a very different behavior of a certain part of the atmosphere (and its wind speed) if a TC is built or not. Thus I am not sure if this can be described by a regular pdf curve, especially at the high end.

    One of the points discussed at the moment is the question on what is more important, the change in overall tropical SSTs (via latent heat input) or changes in relative SSTs in a certain basin (i.e. regional SST change relative to overall tropical SST change; via vertical stability, e.g. Vecchi and Soden, doi:10.1038/nature06423). I suppose it’s both (not only from the experience that climate generally tends to make things as complicate as possible…, but for both influences being physically plausible), but frequency and intensity might not be influenced by both in the same way.

    It is striking and in some way also puzzling that the observed long-term trend (i.e. over a century) in Atlantic TC activity seems mainly to come from an increase in the number of tropical storms that never reached hurricane strength, but not in hurricane number (Landsea et al. 2009/ doi: 10.1175/2009JCLI3034.1; Vecchi and Knutson 2011/ doi: 10.1175/2010JCLI3810.1). While I agree that evidence for a long-term increase in hurricane activity might be weak, I do not agree to the argument of Landsea et al. that the increase in TCs is spurious (see However, an increase in weaker TCs does not really fit in the picture that we expect, nor does the recent Atlantic hurricane season (rather low number of hurricanes (6) and major hurricanes (3) compared to the total number of TCs (18)). I don’t know at the moment how to put that in a consistent pattern with expectations.

    Since we have no reliable information on the long-term history of the most intense hurricanes, we cannot compare the recent evolution to history. However, even if there is no centennial long-term trend and hurricane activity has not been higher in recent years than it was in the mid 20th century or at the end of the 19th century, this does not mean, that the recent observed increase (as described by Kerry Emanuel above) is not due to human activity. Since there are several factors – which can vary by natural as well as human causes – that can influence tropical storm genesis and intensity, a long-term trend might not be very helpful for attribution. These factors can change individually, and one factor may be the reason for an increase at one point while another or other factor(s) may trigger an increase later on. Thus – supporting Kerry Emanuels point – due to the lack of sufficient historical information the indication of uncertainty in historical time series might be somewhat pointless in the attribution discussion. Nevertheless, this point seems always to be weighted quite strongly in the hurricane discussion. What we need for attribution is evidence for a significant change of one or several factor(s) due to human activity, the corresponding processes, and model results. While there is evidence for the two former points, we have still problems with model resolution and differences between model results and observations, although Kerry Emanuel has explanations for the latter (Emanuel 2007/ DOI: 10.1175/2007JCLI1571.1). Although uncertainties concerning future tropical storm activities might have even increased somewhat during the last years (which might be frustrating in way and makes researchers cautious), we should not forget about what seems to be clear – as said by Kerry Emanuel.

  26. 26
    Pete Dunkelberg says:

    Philip Machanick,

    I think physics is your home ground, but since your topic is making sense of the material you quoted I’ll try to help. Both quoted statements are saying

    “Burn baby burn. Go ahead and burn all the carbon that can be extracted from the earth, it won’t make a noticeable difference for a very long time.”

    “Burn baby burn” (Better: burn dummies burn) is three whole words, which can be further condensed to just two slightly longer words:

    “Agency capture”

    This can be further condensed to just one word:


    Recall that denialism is a mishmash of ABC all aimed at D:

    A. It isn’t happening
    B. It’s not our fault (anything but CO2)
    C. It will be good for us anyway (all that “cheap” energy (ignoring massive costs))

    and therefore there is no good reason to change our energy sources (meaning: current cash flows)
    and if enough people believe this Big Carbon wins a big

    D. Delay.

    Evidently, when a report such as the IPCC’s depends on total consensus even one “political balance” type in the group is enough for a dark side victory.


    Back in physics land, can we recognize the climate change signal?

    1. Let’s have a baseline. How about the century 1880 – 1979 for averages, and the total land area impacted by flood + drought in any ten year period for extreme events. [individual events are a distraction from easier ways to recognize the signal]

    2. Conservation of energy. Total ocean heat content keep increasing, total ice keeps decreasing which also requires energy, seal level keeps increasing, total atmospheric energy including latent heat keeps increasing. Why?

    3. Increasing aridity and drought: predicted, observed and projected. See Dai’s papers including his 2011 followup on his well known 2010 Drought _Under Global Warming: A Review_ for starters.

    4. Flood: insufficiently researched but a noticeable problem

    5. Bunched precipitation (more of an area’s total precipitation coming in fewer heavier events): Predicted, observed, quite a few papers but observed effect exceeds predicted effect. By the way I prefer looking at bunched precipitation as a middle part of a whole water cycle response to warming, with drought and flood on the sides of the same distribution.

    Conclusion: the notion that the climate change signal doesn’t and won’t stand out from natural variation is bizarre. Even the most basic physics (conservation of energy) knocks it down.

  27. 27
    Pete Dunkelberg says:

    Louis Michaud @ 22: “There is an urgent need to find a solution to global warming.”

    The solution is no secret: Stop burning carbon. Leave it in the ground. Deploy existing alternate energy technology.

    If the additional alternate energy source you propose
    works out, of course this is a good thing.

  28. 28
    KeithWoollard says:

    I am sorry, but your second graph makes absolutely no sense to me. Why would having fewer but stronger TCs affect the wind distribution on normal (non-TC) days? Are you therefore saying that in areas that never experience TCs, the wind speed increases with average temperature? Have you any scientific basis for this assumption?

  29. 29
    John N-G says:

    The Weibull distribution examples are informative, but David P. Benson (12 and 21 above) argues that other distributions may be more appropriate and Urs Neu (25 above) argues that TC’s may be a separate category not fitting neatly into the distribution of overall wind speeds. I agree with Urs Neu for the specific reason that there are theoretical arguments (reasonably consistent with observations) for the maximum intensity of TC’s, implying that the upper end of the PDF may be bounded.

    If the value of the upper bound increases, the frequency of the most intense tropical cyclones must increase, no matter what happens to the overall shape of the PDF. If you consider any appropriately bounded analog to a Weibull distribution (or any other distribution for that matter), the odds of random changes to all the parameters governing that distribution causing a simultaneous increase in the frequency of most intense events and a decrease in frequency of events above a lower and much easier to satisfy threshold is close to 50%, not 1.9%.

  30. 30
    RichardC says:

    22 Louis – Interesting idea. I’ve got some questions. First, figure 4 shows a 4m diameter model and the vortex rising from it seems to increase the effective height of the structure by about 80%. I assume they took the photograph at the instant the absolute best performance was achieved, so the claim that a vortex in a production unit of 80 meters height could sustain a 15,000m vortex seems wildly optimistic. Could you explain?

    I also would think that the vortex above the structure would be less efficient than that within the structure. Is that correct? Assuming an 80% increase in height (estimated from figure 4 in your link), the system seems negligibly more efficient dollar to dollar than a simple chimney of far less diameter, which can be taller than the AVE structure you propose.

    Finally, natural vortexes move. Winds push them around. A vortex anchored at the bottom would seem to be far more subject to destruction by natural air currents than a hurricane or tornado. Plus, doesn’t the anchoring of the bottom completely eliminate the possibility of using sea surface temperatures as a driver? The sea surface would cool and the vortex would die. This still leaves the possibility of using solar power or fossil fuels as the driver, but it just doesn’t seem that the idea will make much difference when compared to a simple power chimney – though I’m intrigued and want to learn more. TIA for further info.

  31. 31
    CM says:

    Philip, Pete, re: SREX drafts and “relatively small” climate change signal,

    I don’t really see what the issue is here. The SREX is about extremes. If the draft text at some point read as in the BBC story, presumably the drafters thought it was obvious that “signal” referred to extreme events, not (say) long-term trends in global mean temperature, ocean heat content or glacier retreat. Then they realized that it would not be obvious, at least when the denialosphere was through with it, and changed it.

  32. 32
    CM says:

    Prokaryotes, re: downbursts,

    The link you assert between stratospheric cooling and downbursts doesn’t make sense to me. Precipitation-cooled air is not cooled in the stratosphere, and if there’s a link between stratospheric cooling and downward deflection of the jet stream, this layman would like to have it spelled out. Correct me if I’m wrong, but I don’t think any of the references you have stitched together spell out such a hypothesis, nor have I found any literature discussing it by a little bit of googling. Apparently those who study downbursts look mainly at the gradient in equivalent potential temperature between the surface and the mid-troposphere. Though I could imagine other reasons why global warming should increase downbursts, I haven’t actually seen any literature suggesting it would. So where does this suggestion come from?

  33. 33
    Lewis Guignard says:

    RE: Pete Dunkleburg.

    One supposes we should shut down all the coal fired electrical generation plants world wide to satisfy the desire, by some, to eliminate mankinds production of CO2.

    There is one main question: Can man control the climate? Not influence, but control. Then there is the question of how.

    Climate has changed and will change. The last 2000 years is not even a base line. Let us take the last 100,000 years. In the last 400,000 years, a better baseline for a 4.5 billion year old planet, ice ages are the norm. Is that a better world than one of CO2 and warm weather. I’ll take thawing in Canada and Russia anytime over that.

  34. 34
    prokaryotes says:

    Hello CM, the idea that GW creates more MB is layed out in the book “Our Angry Earth” from Isaac Asimov and Frederik Pohl, which i mentioned in post #15. Asimov had an master degree in chemistry, though his analysis about it was very sound. It is my hypothesis that this could be due to stratospheric cooling and precipitation uptake.

    Please post any relevant link to an article/study which suggest otherwise or in particular about downburst/downdraft or microburst genesis. Because i couldn’t find a lot of information about microburst in general. And very little about climate change interconnection.

    There is this older article about downdrafts/microburst in general:

    The size and dynamics of a microburst may be dependent upon the scale and type of precipitation in a maturing cell. The genesis of a microburst from a downdraft may proceed as follows (see Fig. 4 and Fig. 7):

    A concentrated rain shaft or virga shaft, on a scale of about 1 km (0.5 n mi), forms a concentrated downdraft with a very sharp edge,
    across which the horizontal gradient in buoyancy force generates strong torques resulting in a sheath of toroidal vorticity surrounding the downdraft.
    The impact with the surface, and interaction with the surface friction layer, cause this sheath of vorticity to gather up into a ring vortex that spins up as it is forced to expand diameter by the outflow about the base of the downdraft.

    To be more precise, maybe due to Stratospheric cooling, the Jet Stream is altered, which could cause more downdrafts, thus creating a favorable environment for microburst genesis. Maybe the link to stratospheric cooling is weak at this time, but then there is the precipitation uptake from global warming.

    Polar stratospheric clouds or PSCs, also known as nacreous clouds (play /ˈneɪkriːəs/, from nacre, or mother of pearl, due to its iridescence), are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 meters (50,000–80,000 ft). They are implicated in the formation of ozone holes;[1] their effects on ozone depletion arise because they support chemical reactions that produce active chlorine which catalyzes ozone destruction, and also because they remove gaseous nitric acid, perturbing nitrogen and chlorine cycles in a way which increases ozone destruction.

    Nacreous Clouds
    They need the very frigid regions of the lower stratosphere some 15 – 25 km (9 -16 mile) high and well above tropospheric clouds. They are so bright after sunset and before dawn because at those heights they are still sunlit.

    They are seen mostly during winter at high latitudes like Scandinavia, Iceland, Alaska and Northern Canada. Sometimes, however, they occur as far south as England. They can be less rare downwind of mountain ranges. Elsewhere their appearance is often associated with severe tropospheric winds and storms.

    Also relevant and in agreement with more severe storms observed during the winter months.

    How a freak diversion of the jet stream is paralysing the globe with freezing conditions

    One of the main factors is a change in the position of the jet stream – the fast-moving current of air that moves from west to east, high in the atmosphere.

    Changes in the jet stream’s path can cause massive changes in weather conditions across the globe

  35. 35
    Ray Ladbury says:

    Correction: PDF stands for probability DENSITY function. The choice of a Weibull is probably sound on grounds of extreme-value therory. For the shape values being considered here, the skew will be positive.

  36. 36
    Tom Scharf says:

    After all the hyperventilating in 2006 after Katrina and a very active Atlantic hurricane season and the opportunistic climate disaster linkage hindsight, the actual numbers dropped off a cliff:

    Global cyclone energy is at or near all time lows since modern measurements began. This even with best estimates were for very activate seasons year after year.

    It has been 2000+ days and counting since a Cat3 hurricane has made US landfall. A record.

    This analysis is very flawed when it barely even mentions actual recent measurements versus recent model predictions. This is just another example of burying poor model performance, then proclaiming new numbers as if poor prior performance was irrelevant to the task.

    What am I looking for? Mea Culpa? No. Just an honest assessment that the models are performing poorly, and are unreliable, and that is why we are discussing all these updates and new theories, right?

    From a skeptical point of view, the “oversight” of not clearly showing trends of the last 30 years versus CO2/temperature seems fraught with a desired political narrative.

    Real data matters. Real prior performance matters. The hurricane link is tenuous to non-existent. Prove me otherwise.

  37. 37
    Ray Ladbury says:

    Lewis Guignard,
    I am sure that the saber-toothed tigers and mastodons are heartened to think you find the climate they thrived in to be relevant. As to how we manage to feed 10 billion people and support a complicated global infrastructure with a climate like that 100000 years ago, I would love to hear your thoughts. Are you planning to domesticate wooly mammoths?

  38. 38
    prokaryotes says:

    Category 4 Kenneth the strongest East Pacific late-season hurricane on record

    2011, 19 atlantic tropical storms, make this year the 3rd busiest on record!

    Re-analysis has shown that a tropical disturbance that formed between Bermuda and Nova Scotia on September 2 briefly attained tropical storm status

    The addition of the unnamed tropical storm to the record books brings this year’s tally of named storms to nineteen, tying 2011 with 2010, 1995, and 1887 as the 3rd busiest year for tropical storms. Only 2005 and 1933 had more named storms since record keeping began in 1851. An average season has just eleven named storms.

  39. 39
    SecularAnimist says:

    Tom Scharf wrote: “It has been 2000+ days and counting since a Cat3 hurricane has made US landfall. A record.”

    Also totally irrelevant since whether or not hurricanes “make US landfall” has nothing to do with anything.

    Having said that, I’d point out that it has been barely 90 days since Hurricane Irene, a mere Category 1 hurricane, made US landfall, causing at least 45 deaths and over seven billion dollars in damage, cutting electric power for over seven million homes and businesses for days, triggering an evacuation order for 370,000 residents of New York City, and causing unprecedented flooding throughout the Northeast.

    And only days later, a mere tropical storm, Lee, caused another billion dollars in damage, and again causing historic, destructive and disruptive flooding in Pennsylvania, New York and elsewhere.

    And of course this is in the context of the USA experiencing fourteen billion-dollar-plus weather disasters in 2011, far exceeding the previous record of nine set in 2009, as meteorologist Jeff Masters details at WunderGround.

  40. 40
    Dan H. says:

    While 2011 may have been busy with regards to the total number of storms, the intensity of the storms as dropped off in recent years.

    [Response: My tolerance level for your continuing unbacked assertions, reference to highly biased information sources, and pretty much outright denial of the evidence is now zero. Got that?–Jim]

  41. 41
    CM says:

    Prokaryotes #35,

    Ah, I see. You made it up. That is, you asserted your untested hypothesis as well-established fact. Don’t do that. It’s confusing, irritating, and distracts from serious conversations. Like the one we could be following on this thread, where some experts in the field have shown up to discuss the IPCC’s pronouncements on tropical cyclones.

  42. 42
    prokaryotes says:

    CM, i suggest you then focus on what you interpret as the real conversations. Because i find your replies rather confuse. So please follow your own advice, cheers.

  43. 43
    KeithWoollard says:

    Rar @ 37. Mastodons survived through many glacials and interglacials just fine. We killed them off

  44. 44
    Dan H. says:

    Would you prefer that I link to the peer-reviewed paper from which I lifted the plot? Check out the accumulated cyclone energy over the past four decades.

  45. 45
    Lewis Guignard says:

    RE: Ray Ladbury

    Your point appears to be that the only climate you think relevant is recent, the last few hundred years – during mankinds ability to measure it. Yet the earth cares not whether we can measure it or not. My point remains:

    Can man CONTROL the climate? If so, how?

    The ideas of eliminating or controlling CO2 production are actually ideas whose end game is one of making man live as he did 200 years ago. Yet this action does nothing to assure a stable climate, especially one which is reflective of the 1970’s. Can we be assured of that by our actions or are we just crying wolf and saying man did it and we must then sacrifice a virgin as a paen to the gods?

    But if there is success in stopping the production of CO2, where then your food to feed the 10 B or so humans. Do you think battery powered farm tractors and trucks will suffice to farm the land and oceans and move the produce to market? Thatis not even a joke. The batteries would weigh so much as to make the machinery unusable. As the EPA and those of similar ilk move to make the cost of CO2 production higher, the cost of electricity, even more importantly, the supply will become limited and the price of everything higher which, by simply laws of economics, will reduce usage. So everyone’s life will be one of less. Finally, the excess which allows so many scientists a free ride to research the most esoteric of ideas will go away and they too will have to get real jobs feeding themselves and their families.

    For me: burn the coal. Farm the far north with liquid hydrocarbon fuel fired machinery and keep enough excess for the scientists to keep researching.

  46. 46
    ccpo says:

    Lewis, your time here may be short, indeed, for your comments are far less interesting and engaging even than Dan’s, being almost completely rhetorical. I suspect Jim will have little tolerance.

    To answer your question, we obviously can change and control climate. We are in the process of doing so, quite obviously. Your question really is, can we stabilize it? the answer to that would also be yes – assuming we all agreed to. but, technically, dead easy. We know that over the last 800,000 years CO2 has not gone above 300 – and not usually over 280-ish. Those times were roughly parallel to today, so would seem a good benchmark. Stabilize at 260 – 280 and life should be quite rosy barring an asteroid or some such.

    Let’s set aside resource constraints for the moment. If we change nothing else but re-grow forest ecosystems, change food production to regenerative practices we can begin pulling down carbon. We can do less of both if we also change behavior and society to be less energy intensive. Or draw down faster.

    Given we can calculate the carbon cycle pretty well, we,as a global society, certainly have the ability to manage our carbon emissions or manage the mitigation by managing the sequestration or release of carbon via carbon farming. And there are a fairly long list of other measure to take, as well.

    The only thing keeping us from saving ourselves – assuming methane emissions aren’t already self-propagating – is choosing not to play nice.

    I have posted these things many times, so will not post again now. You can find them by searching on my pseudonym easily enough, I’d say. Let me know if you can’t find them.

  47. 47
    prokaryotes says:

    Re OT Post #45

    I suggest you google “electric tractor” then or “electric tractor store”.

  48. 48
    Ray Ladbury says:

    Lewis Guignard: The ideas of eliminating or controlling CO2 production are actually ideas whose end game is one of making man live as he did 200 years ago.”

    Utter bullshit! You seem to think the only sources of energy are fossil fuels and batteries. You had better be wrong, as the end of fossil fuels is unavoidable and not too far at hand regardless of their implications for climate change. What is more, if we had taken the hint in the ’70s and started serious research into a sustainable energy future, we wouldn’t be in this mess now. Even if we’d started in the ’80s, when it became clear that climate change was real, we’d probably be OK. Instead, you denialists have made your prophecy of scarcity and ruin self-fulfilling.

    Oh, and by the way, farmland is NOT an infinite resource. The Canadian shield will never replace the US Great Plains as a producer of wheat, since it lacks topsoil.

    And frankly, your assertion about the worth of scientific research is as absurd as it is insulting. What is more, you don’t even appreciate the absurdity of your using a marvel of science and technology as a vehicle for broadcasting your ignorance.

  49. 49
    SecularAnimist says:

    Dan H. wrote: “Check out the accumulated cyclone energy over the past four decades.”

    Nothing in the paper you linked to even addresses the effect of anthropogenic global warming on hurricanes.

  50. 50
    Dan H. says:

    You are correct.