RealClimate logo


Technical Note: Sorry for the recent unanticipated down-time, we had to perform some necessary updates. Please let us know if you have any problems.

Runaway tipping points of no return

Filed under: — gavin @ 5 July 2006 - (Slovenčina)

I wonder if any else has noticed that we appear to have crossed a threshold in the usage of the phrase ‘tipping point’ in discussions of climate? We went from a time when it was never used, to a point (of no return?) where it is used in almost 100% of articles on the subject. Someone should come up with a name for this phenomenon….

Regardless of the recent linguistic trends, the concept has been around for a long time. The idea is that in many non-linear systems (of which the climate is certainly one), a small push away from one state only has small effects at first but at some ‘tipping point’ the system can flip and go rapidly into another state. This is fundamentally tied to the existence of positive feedbacks and is sometimes related to the concept of multiple ‘attractors’ (i.e. at any time two different ‘states’ could be possible and near a transition the system can flip very quickly from one to another). Another ‘tipping point’ in non-linear systems occurs when as some parameter varies, the current attractor changes character or disappears. However it is currently being used interchangeably a number of potentially confusing ways and so I thought I’d try and make it a little clearer.

Positive feedback

A positive feedback occurs when a change in one component of the climate occurs, leading to other changes that eventually “feeds back” on the original change to amplify it. The classic ones in climate are the ice-albedo feedback (melting ice reduces the reflectivity of the surface, leading to more solar absorption, more warming and hence more melting) and the water vapour feedback (as air temperatures rise, water vapour amounts increase, and due to the greenhouse effect of the vapour, this leads to more warming), but there are lots of other examples. Of course, there are plenty of negative feedbacks as well (the increase in long wave radiation as temperatures rise or the reduction in atmospheric poleward heat flux as the equator-to-pole gradient decreases) and these (in the end) are dominant (having kept Earth’s climate somewhere between boiling and freezing for about 4.5 billion years and counting). But it is the postive feedbacks that make weather chaotic and climate interesting.

People often conclude that the existence of positive feedbacks must imply ‘runaway’ effects i.e. the system spiralling out of control. However, while positive feedbacks are obviously necessary for such an effect, they do not by any means force that to happen. Even in simple systems, small positive feedbacks can lead to stable situations as long as the ‘gain’ factor is less than one (i.e. for every initial change in the quantity, the feedback change is less than the original one). A simple example leads to a geometric series for instance; i.e. if an initial change to a parameter is D, and the feedback results in an additional rD then the final change will be the sum of D+rD+r2D…etc. ). This series converges if |r|<1, and diverges (‘runs away’) otherwise. You can think of the Earth’s climate (unlike Venus’) as having an ‘r‘ less than one, i.e. no ‘runaway’ effects, but plenty of positive feedbacks.

Tipping points

So are there ‘tipping points’ in climate? One way to assess that is by looking for elements of the physical system where we think that there is a threshold behaviour. Two frequently discussed examples are the overturning circulation in the North Atlantic and the summer sea ice in the Arctic. In both of these cases, the existence of these phenomena can be disrupted in models (and there is evidence of similar behaviour in the real world) by small changes in freshwater and increasing polar amplification, respectively. At some point, both could simply cease to be viable. But we are not very confident of where these points are or how sensitive the threshold is. These are examples of ‘known unknowns’.

There is also the existence of ‘unknown unknowns’ – tipping points that we are as yet unaware of. An example of this kind of surprise happened in relation to the Antarctic ozone hole, where unexpected chemistry on surfaces of ice particles lead to much more efficient destruction of ozone in the polar vortex than had been expected, making an existing concern into a serious problem. By their nature, we are not able to assess how important any such surprises might be, but it is impossible to rule them out entirely.

By far the most common examples of tipping points though are in relation to ecosystems. The extremely complex web of interdependencies that keep ecosystems dynamic and healthy give rise to plenty of potential thresholds and it is extremely difficult to predict consequences of external changes. The myriad influences on the health of ecosystems (habitat loss, logging, urbanization, species introduction etc. as well as climate change) means that it is most likely here that the tipping point concept will be most applicable. Examples such as a rise in minimum winter temperatures that allow a new insect species to gain a foothold in a new ecosystem (pine bark beetles in Alaska), or warming that leads to movement upward in altitude of ecosystem zones that end up reducing the area of existing alpine biomes. As the planet warms, it is easy to imagine an increasing number of ‘tipping points’ being passed, each related to some different sub-system of the climate or biosphere.

Points of no return

Are ‘tipping points’ the same as the ‘points of no return’ oft used in the media? For a species that becomes extinct as a result of crossing a threshold, the answer is obviously yes. But in the physical climate system, are there genii that can’t be put back in the bottle? This is really a question of time scale. Changes to aerosol concentrations can be reversed in a few weeks after an emission change. CO2 levels however are much slower to change and are already very unlikely to revert to pre-industrial values in any scenario over the next few hundred years. In this minimal sense the climate is already past the point of no return compared to pre-industrial climate.

The ‘known’ physical tipping points described above have natural timescales that determine whether ‘returns’ are possible. The Arctic sea ice, for instance, has timescales of around 5 years to a decade, and so a collapse of summer ice cover could conceivably be reversed in a ‘cooling world’ after only a decade or so (interactions with the Arctic ocean stratification may make that take a little longer though). Model simulations of the thermohaline circulation indicate that for small perturbations, recovery can occur in a few decades. For larger perturbations (i.e. complete collapses) intermediate-complexity models suggest that in some regimes these changes can be quasi-permanent, although this behaviour has not yet been fully explored in current state-of-the-art GCMs. The clues from the paleo-record indicate that there is likely a bi-modal spectrum of overturning states in glacial climates, but there is no evidence of such multiple steady states in the Holocene. Thus there is no strong reason to think either of these ‘tipping points’ are really irreversible – though that is not to imply that the process of loss and recovery wouldn’t have significant impacts.

The big ‘point of no return’ though is usually associated with the melting of the ice sheets – in particular, Greenland and the West Antarctic Ice Sheet (WAIS). Currently the ice sheets exist in part because they already exist i.e. the reason it snows on Greenland is in some large part because there is a large ice sheet there. Should the ice sheet start to melt in a serious way (i.e. much more significantly than current indications suggest), then lowering of the elevation of the ice sheet will induce more melting simply because of the effect of the lapse rate (air being warmer closer to sea level due to pressure effects). Thus if Greenland disappeared, it is unlikely that it would grow back even under current climate, let alone in a warmer world. So loss of either of these ice sheets would indeed be an effect with ‘no return’, at least on any reasonable human timescale.

10 years?

Jim Hansen was widely quoted earlier this year stating that there were likely only 10 years left in which serious actions could be taken to prevent ‘dangerous anthropogenic interference’ on climate occurring in the future. He described this as a ‘tipping point’, but it should be clear that he was not using the term in exactly the same way as I defined above. He very specifically was not indicating that some irreversibly large change in climate would happen in 10 years. Instead he was pointing to the trajectory of increasing CO2 emissions that continue to add to atmospheric concentrations. Actual and projected emission levels are already at the high end of Hansen’s ‘alternative scenario’ which was suggested as an achievable outcome (based on significant control efforts) that kept forcings (including Co2, CH4 and black carbon) below a level that Hansen considered would be ‘dangerous’ (specifically a level that would avoid the melting of any significant fraction of the WAIS or Greenland ice sheet). It is the inertia of societal infrastructure, the carbon cycle and the climate that implies that at any point there is a significant warming that is already ‘in the pipeline’ (and thus very difficult to avoid). We have estimated this at about 0.5 C. Hansen’s statement can therefore be read as a comment on a ‘point of no return’ of the human-climate system, rather than the climate system in a purely physical sense.

The ’10 year’ horizon is the point by which serious efforts will need to have started to move the trajectory of concentrations away from business-as-usual towards the alternative scenario if the ultimate warming is to stay below ‘dangerous levels’. Is it realistic timescale? That is very difficult to judge. Wrapped up in the ’10 year’ horizon are considerations of continued emission growth, climate sensitivity, assumptions about future volcanic eruptions and solar activity etc. What is clear is that uncontrolled emissions will very soon put us in range of temperatures that have been unseen since the Eemian/Stage 5e period (about 120,000 years ago) when temperatures may have been a degree or so warmer than now but where sea level was 4 to 6m higher (see this recent discussion the possible sensitivities of the ice sheets to warming and the large uncertainties involved). In 10 years time CO2 levels will likely be greater than 400 ppm and the additional forcing combined with the inertia of the system will be make it increasingly unlikely that we will avoid a further 1 deg C or more warming. While the ’10 years’ shouldn’t be read as an exact timetable, it is surely in the right ballpark. 30 more years of business-as-usual will make it impossible to keep temperatures from rising beyond Eemian levels (see here for some discussion of stabilisation scenarios), and decisions (on infrastructure, power stations, R&D, etc.) that are being made now will determine the emissions for decades to come.

One point or many?

Much of the discussion about tipping points, like the discussion about ‘dangerous interference’ with climate often implicitly assumes that there is just ‘a’ point at which things tip and become ‘dangerous’. This can lead to two seemingly opposite, and erroneous, conclusions – that nothing will happen until we reach the ‘point’ and conversely, that once we’ve reached it, there will be nothing that can be done about it. i.e. it promotes both a cavalier and fatalistic outlook. However, it seems more appropriate to view the system as having multiple tipping points and thresholds that range in importance and scale from the smallest ecosystem to the size of the planet. As the system is forced into new configurations more and more of those points are likely to be passed, but some of those points are more globally serious than others. An appreciation of that subtlety may be useful when reading some of the worst coverage on the topic.


207 Responses to “Runaway tipping points of no return”

  1. 101

    “It may also be that they just do not care what happens after they die.”

    A theist here, and a Roman Catholic, probably in a very small minority in this dicussion.

    I’ve been interested in the problem for years, and especially after having interviewed two scientists for a small Idaho paper on the subject of stratospheric ozone depletion in the arctic – one an atmospheric physicist and the other an atomospheric chemist.

    I sense that Rome is coming around to an awareness and concern in these matters, particularly as they affect the poor in the so-called undeveloped countries.

    I do know of individual Catholics who are concerned, and I’m thinking now of a field biologist who works in the sub-Arctic and who has done some notable work in insect infestation of sub-boreal forests.

  2. 102
    Grant says:

    It seems that changes in the diurnal temperature range (DTR) are not so easy to attribute to a cause. Nor is DTR changing everywhere by the same amount, or even with the same *sign*.

    Some googlescholar search turned up very interesting things about DTR. I can’t seem to find the work by Tyndall (or anyone else) giving the reasons “basic physics” indicates greenhouse warming will decrease DTR. But one of the papers discussing the observed trends throughout the 20th century [Karl et al. 1984, J. Climate and Appl. Meteorology, 23, 1489] simply states that

    The physical mechanism responsible for the observed decrease in the diurnal range is not known.

    At least part of the change in DTR seems to be due to atmospheric absorption of solar energy, and by latent heat from increased evaporation [Cao et al. 1992, J. Climate, 5, 920]:

    atmospheric absorption by CO2 and water vapor increases, reducing the solar heating at the surface, and surface evaporation increases faster with temperature than the transfer of sensible heat (due to the Clausius-Clapeyron relation), both of which tend to reduce the diurnal cycle. However, in the three-dimensional model, the diurnal cycle increases substantially where the snow line recedes, where the land surface becomes drier, or where there are substantial decreases in cloud cover. The diurnal cycle of surface temperature decreases where sea ice is replaced by open water because of the increase in thermal inertia of the surface.

    Another important factor seems to be changes in cloud cover. This is from [Karl et al. 1993, Bull. Amer. Met. Soc., 74, 1007]

    The decrease in daily temperature range is partially related to increases in cloud cover. Furthermore, a large number of atmospheric and surface boundary conditions are shown to differentially affect the maximum and minimum temperature. Linkages of the observed changes in the diurnal temperature range to large-scale climate forcings, such as anthropogenic increases in sulfate aerosols, greenhouse gases, or biomass burning (smoke), remain tentative.

    And this from [Dai et al. 1999 J. Climate, 12, 2451]

    “Clouds, which largely determine the geographic patterns of DTR, greatly reduce DTR by sharply decreasing surface solar radiation while soil moisture decreases DTR by increasing daytime surface evaporative cooling. Clouds with low bases are most efficient in reducing the daytime maximum temperature and DTR mainly because they are very effective in reflecting the sunlight, while middle and high clouds have only moderate damping effects on DTR. The DTR reduction by clouds is largest in warm and dry seasons such as autumn over northern midlatitudes when latentheat release is limited by the soil moisture content. The net effects of clouds on the nighttime minimum temperature is small except in the winter high latitudes where the greenhouse warming effect of clouds exceeds their solar cooling effect.
    The historical records of DTR of the twentieth century covary inversely with cloud cover and precipitation on interannual to multidecadal timescales over the United States, Australia, midlatitude Canada, and former U.S.S.R., and up to 80% of the DTR variance can be explained by the cloud and precipitation records. Given the strong damping effect of clouds on the daytime maximum temperature and DTR, the well-established worldwide asymmetric trends of the daytime and nighttime temperatures and the DTR decreases during the last 4â??5 decades are consistent with the reported increasing trends in cloud cover and precipitation over many land areas and support the notion that the hydrologic cycle has intensified.”

    Yet another factor is changes in land use/land cover (LULC) [Gallo et al. 1996, J. Climate, 9, 2941]

    The results also suggest that changes in the predominant LULC (land use/land cover) conditions, within as great as a 10000 m radius of an observation station, could significantly influence the climatological DTR.”

    While not relevant to the cause of changes in DTR, the most interesting thing I found was this [Forster and Solomon 2003, PNAS, 100, 11225]

    Using surface measurements of maximum and minimum temperatures from the Global Daily Climatological Network data set, we find evidence of a weekly cycle in diurnal temperature range (DTR) for many stations in the United States, Mexico, Japan, and China. The â??â??weekend effect,â??â?? which we define as the average DTR for Saturday through Monday minus the average DTR for Wednesday through Friday, can be as large as 0.5 K, similar to the magnitude of observed long-term trends in DTR. This weekend effect has a distinct large-scale pattern that has changed only slightly over time, but its sign is not the same in all locations. The station procedures and the statistical robustness of both the individual station data and the patterns of DTR differences are thoroughly examined. We conclude that the weekend effect is a real short time scale and large spatial scale geophysical phenomenon, which is necessarily human in origin. We thus provide strong evidence of an anthropogenic link to DTR, an important climate indicator. Several possible anthropogenic mechanisms are discussed; we speculate that aerosol-cloud interactions are the most likely cause of this weekend effect, but we do not rule out others.

    The one thing that seems to be clear is: the simple explanation given in the Scientific American blog is not correct (or at the very least, far from the whole story).

    To the moderators: I think this might be a great topic for a post. Or, it could be part of a larger topic on “fingerprints” of AGW, and the root physical mechanisms behind them.

    Now … about that change in *annual* temperature range…

  3. 103
    Wacki says:

    Paul, if the Vatican is starting to come around on the whole global warming debate please post evidence either here or your website. I don’t care how trivial. I’m extremely interested in this subject.

    Also, it’s good to see you here. We need more *openly* theistical posters here to show the proudly “godless liberals” that religion isn’t as bad as they make it out to be. It seems like it’s becoming harder and harder to say that lately due to the anti-evolution movement and the “war on science”.

  4. 104
    Hank Roberts says:

    From http://www.tyndall.ac.uk/general/history/john_tyndall.shtml
    ================================================================

    In the 1860′s, Tyndall began to suggest that slight changes in the atmospheric composition could bring about climatic variations. He was exploring radiation passing through the atmosphere and noted that,

    “The waves of heat speed from our earth through our atmosphere towards space. These waves dash in their passage against the atoms of oxygen and nitrogen, and against molecules of aqueous vapour. Thinly scattered as these latter are, we might naturally think of them meanly as barriers to the waves of heat.”

    Tyndall’s main interest was with water vapour and its impact on radiation, but he also dealt with the radiative forcing of other greenhouse gases including carbon dioxide. Most importantly he identified that there was a greenhouse effect, whether natural or anthropogenic. For water vapour he noted that:

    “â�¦this aqueous vapour is a blanket more necessary to the vegetable life of England than clothing is to man. Remove for a single summer night the aqueous vapour from the air that overspreads this country, and you would assuredly destroy every plant capable of being destroyed by a freezing temperature. The warmth of our fields and gardens would pour itself unrequited into space, and the sun would rise upon an island held fast in the iron grip of frost â�¦ its presence would check the earth’s loss; its absence without sensibly altering the transparency of the air, would open wide a door for the escape of the earth’s heat into infinitude.”
    ========================================================================

  5. 105
    Hank Roberts says:

    More:
    http://start.org/meetings/wp06/wp06-sessions/wp06_A25A.html

    Weekend Effect in Diurnal Temperature Range in China: Opposite Signals Between Winter and Summer
    “… Since the late 1970s, the weekend effect has been enhanced in both winter and summer, concurrent with rapid development and enhanced human activity in China. The direct and indirect effects of human-related aerosols on radiation, cloud, precipitation, and so on, might play an important role in generating the opposite signal in the weekend effect for different seasons. During a dry winter, the reduction of aerosol concentrations in weekend days may overwhelmingly impact on the DTR through a direct effect, i.e. by increasing total solar irradiance near the surface and raising the daytime temperature and maximum temperature, and lowering relative humidity. By contrast, in summer the indirect effect of aerosols, i.e., reduction in precipitation efficiency caused by more numerous and smaller cloud droplets, would largely be responsible for the increased numbers of rainy days, the reduction of the total solar irradiance, and the lowering of the maximum temperature and DTR.”

  6. 106
    Hank Roberts says:

    This seems relevant to sudden change because a major recession or sudden decrease in air pollution would remove a lot of particulates and we’d feel the full effect of climate change masked by aerosols.

    One of the earlier publications, from 2003.:

    http://www.nerc.ac.uk/publications/documents/pe-wint03/Perfectday.pdf.

    “… We knew about a weekly cycle in pollution and wanted to learn how this altered clouds. Rather than look directly for changes in cloud cover we examined the better quality records of the daily temperature range. As clouds keep the daytime cooler and the night-time warmer, the daily temperature range is smaller under cloudy skies. We found temperatures were up to 0.5°C different at the weekends over much of North America, China and Japan. This difference is as large as that seen in the last 100 years of global warming.

    “But as we expected to see a greater daily temperature range at the weekend due to less cloud-forming pollution, we were surprised. Over Japan and the US Midwest the effect was the opposite, weekends had a smaller daily temperature range, implying more cloud. And over Europe we found no effect at all!

    “…. we have theories. While pollution in some areas provides nuclei for water to condense on to form clouds, in other places there may be soot particles, which could absorb sunlight and cause the cloud to burn off (evaporate) during the day, leaving less cloud to warm the night. Another possibility might be that pollutants could cause changes in wind circulation patterns on a weekly basis. Or there may be a gradual change across the country because of the downwind transport of pollutants. And we canâ��t rule out the possibility there is some other human-related mechanism at work other than pollution changing clouds.”

  7. 107

    “Paul, if the Vatican is starting to come around on the whole global warming debate please post evidence either here or your website. I don’t care how trivial. I’m extremely interested in this subject.”

    First off, I’m just a layman and nothing I say is in the least authoritative for the Church. Authoritative statements for the Church as a whole come out of Rome from the Pope or from an important office.

    If you ever hear of a monsignor or a bishop making pronouncements about climate change, that is only his opinion.

    I would not use the phrase ‘coming around’, only perhaps that the Church is becoming aware of the problem of climate change. It has pastoral and humanitarian responsibilities. Insofar as climate change affects these, it must take notice of it.

    See for example, this below, especially the fourth paragraph on and references to the Pope’s position on the environment.

    http://www.stnews.org/News-2577.htm

    As for intelligent design, the link below seems to represent adequately a Catholic position.

    http://www.catholicnews.com/data/stories/cns/0600273.htm

    I agree that ID is not scientific and should not be taught as science. That doesn’t necessarily make it wrong.

  8. 108
    Hank Roberts says:

    >89, 90, 91, 102 etc.
    Grant, is it clear that you’re looking at variation in nighttime temperature records due to two very different causes? You need to sort them out if you’re questioning how that variation is explained and whether it proves anything about the greenhouse effect predictions.

    1) The fundamental ‘greenhouse effect’ basic physics that Tyndall describes as quoted above from a century ago — some gases, including water vapor and carbon dioxide, are transparent in the human visual range, but are not transparent to a range of infrared. In the atmosphere these gases act like a blanket trapping heat, and the effect is prominent at nighttime because that’s when the sky is dark and more heat can radiate away into space. In earlier threads there was discussion of how to show this at the high school science fair level.

    2) The effects of clouds and aerosols on radiative heat transfer, which are many and varied and still being studied with many questions open.

    Hold the variations in aerosols and clouds constant, and you’ll see the greenhouse gas effect.

  9. 109

    Re #87 — Blair is right, the calculator doesn’t work properly as seen in Firefox. I’m not quite sure how to fix this. It works in IE, and I haven’t tried Netscape or Opera.

    -BPL

  10. 110
    Hank Roberts says:

    Firefox OSX workaround:
    Pick planet; after input numbers blink, page-back (left-arrow)
    Numbers reappear. Change them if desired.
    Click Calculate; after result numbers blink, page-back
    Results reappear.

    No promise from me that these reappearing numbers are correct or resemble those the page is supposed to produce, haven’t taken the time to check anything.

  11. 111
    EDWARD LANWERMEYER says:

    re: 37
    Nils, the German Wikipedia “Folgen der Globalen Erwaermung” entry is quite ambitious and very well done.
    And yes, although not a climatologist practitioner, I can read it with considerable profit . The accompanying graphics included are also very timely and helpful.

    thank you on behalf of German speaking ( and reading) everywhere, for the considerable labor of completing this, including all the sublinks by topic name that follow the main entry . I have friends I will be sending this Wikipedia.de discussion to.

    ed lanwermeyer

  12. 112
    S Molnar says:

    Re #110: Same problem in Opera as in Firefox, and same workaround. Also, same disclaimer, but earth works out correctly. Of course, the other workaround is just to use IE – I hate it as much as the next guy, but Opera users always need a backup since so many websites don’t work properly in Opera (not the fault of the Norwegians, I hasten to add).

  13. 113
    Blair Dowden says:

    Barton, I tried to follow your calculation of planetary temperatures on your web site. I am afraid I lost you at equation <3>. I have no idea where the value Tau (“gray optical thickness”) comes from, and I do not understand the four cooling processes, or even what some of them mean. For example, how is absorption of radiation in the atmosphere or evaporation of seawater a cooling process?

    Perhaps some further explanation would help.

  14. 114
    George A. Gonzalez says:

    It is when considering tipping points that geoengineering responses to climate change appear central to preventing at least some of the worst outcomes of climate change. It is important to stress that the CO2 400 ppm atmospheric threshold is dangerous to cross because it will cause the mean global temperature to rise 2 degrees Celsius. So it is the 2 degree Celsius increase that must be avoided. This can be done so through geoengineering (e.g., aerosols in the stratosphere or mirrors in space). Even at 380 ppm the current biosphere stability in the long-term may be unviable. It is currently maintained by sulphate emissions from coal burning power plants. These emissions are in the atmosphere only a few years. It would appear vital to plan to replace the cooling affects of these sulphate emissions.

  15. 115
    Hank Roberts says:

    That seems the clearest statement yet of the real problem == is there anything that can replace current levels of air pollution, if high sulfur coal and oil are phased out for respiratory health reasons, that would make up for the loss of the aerosols’ negative forcing on global temperature?

    I’d think arguing over volatiles a smokescreeh to divert attention from CO2 problems, notably now ocean acidification?

  16. 116
    Vincenze says:

    More around social tipping points than a scientific ones (though the first tipping point is science)… the Climate Institute Australia released their report “The Top Ten Tipping Points on Climate Change” the other day and I’ve written about it in this article…

    http://vincenze.com/?p=148

    May be relevant to you folk.

    Cheers,

    Vincenze.

  17. 117

    ##Barton, I tried to follow your calculation of planetary temperatures on your web site. I am afraid I lost you at equation <3>. I have no idea where the value Tau (“gray optical thickness”) comes from, and I do not understand the four cooling processes, or even what some of them mean. For example, how is absorption of radiation in the atmosphere or evaporation of seawater a cooling process?

    Perhaps some further explanation would help.##

    Radiation absorbed in the atmosphere doesn’t make it to the ground. Evaporation of seawater cools the surface and warms the atmosphere.

    Optical thickness (optical depth, optical path) is a fundamental quantity in radiation physics. It has to do with the reduction in strength of a beam of radiation going through a physical medium. Tau is 1 for a situation where the strength of the beam has decreased by a factor of e (2.718…). The higher tau is, the greater the absorption, all else being equal.

    You’re probably right that I should have explained more of the background. I’ll have to see about revising the paper. Thanks for checking it out for me.

    -BPL

  18. 118
    Brian Gordon says:

    Re: #85: alternative energy possibilities

    I often see arguments against alternative energies that boil down to this: When something as efficient, inexpensive, and convenient as oil comes along, we’ll take it seriously. However, this is a silly point of view, really:

    New energy sources will take time, money, and energy to discover, develop, and make efficient; they won’t just magically appear complete with all the same characteristics as oil (eg, suitable for use in internal combustion engines) and with the existing zillion-dollar oil infrastructure. This latter ranges from massive oil sands projects to transportation to refining to distribution. It also includes a heck of a lot of engineers and scientists researching and developing in companies and taxpayer-funded organisations, and how about the military subsidy…? We have spent literally trillions of dollars building infrastructure based on cheap oil; how much have we spent similarly supporting sustainable energy sources, or that unAmerican ideal: conservation.

    There is no guarantee that we will be able to continue our existing oil-based economy/infrastructure/lifestyle. The longer we wait, the less likely we’ll have the time, energy, and social stability to find alternatives. Every dollar currently spent on oil and coal research and development is being poured into obsolete, dangerous technology.

  19. 119
    Lynn Vincentnathan says:

    RE # 101, 103, 107, there are great statements about GW at the top of the Catholic hierarchy. The U.S. Bishops made a very good statement in 2001(link below) & are working on a GW packet for parishes, and Pope John Paul II as early as 1990 stated:

    “Today the ecological crisis has assumed such proportions as to be the responsibility of everyone…The…’greenhouse effect’ has now reached crisis proportions…â?? from “Peace with All Creation,” see http://conservation.catholic.org

    However, there is not much action at the local level. My priest won’t bring up GW because he’s afraid of the “Rush Limbaugh parishioners.” So lots of talk, no walk.

    I would appreciate it if some scientist on this site could read the U.S. Bishops’ statement, “Global Climate Change” (2001, at http://www.usccb.org/sdwp/international/globalclimate.htm
    ), and discuss their use of science. My own understanding is that they are too conservative & assume the GW problems are only in the future. (You can hear in dialogic fashion apologetics to skeptics.) My idea is that we’ve already been beset with GW problems (increases due to GW in droughts, floods, storms, heat deaths, disease spread, etc) perhaps for several decades, and that as science improves it will find these past & present harms to be more & more certain.

    Links:
    1. http://conservation.catholic.org
    2. http://www.usccb.org/sdwp/international/globalclimate.htm
    3. My own “Little Way of Environmental Healing” (brief summary) at http://www.marklynas.org/wind/person/138

  20. 120
    Dan Robinson says:

    A month or so ago in a related blog, before I was called away on other business, I asked if the earth could be pushed into a “Venus syndrome” of permanent runaway greenhouse. (I continue to fall behind in the ongoing discussion.) I accept the figures from “Lessons from Venus” that indicate “… the amount of CO2 we could add to the atmosphere by burning all available fossil fuel reserves would not move us significantly closer to the [Venus level] runaway greenhouse threshold”. Can we assume that burning most of the available surface carbon resources, in stoves, boilers and increasing wildfires also wouldn’t make a significant difference?

    But to imply simply “no runaway greenhouse” is another matter. I think there are several possible levels of this. Maybe the most pertinent is where our excessive burning of carbon triggers self-reinforcing natural forces beyond our control, with little knowlege of what will turn them around. Can anyone guess how this could evolve, say through this millenium?

    It seems we’ve been in a slowly developing runaway greenhouse since life began. We’ve evolved basically as individuals on various levels, with individual selfish interests. Individuals and local communities have in the past been controlled by natural laws of checks and balances. But the very “natural”, not truly human, and therefore blind, goal of “directing a stream of ‘negentropy’ upon ourselves” (John Dobson) has now gone global, beyond such laws.

  21. 121
    George A. Gonzalez says:

    re: #115

    I do not see how discussing the prevention of a global mean temperature rise of 2 degrees Celsius is a smokescreen for anything. If this occurs, it would likely mean the end of life on the earth as we know it. On the issue of ocean acidification, a 2 degree Celsius increase would probably push CO2 to 750 ppm in the atmosphere. What would happen to the ocean’s pH balance then?

    Respectfully yours,

  22. 122

    Re: #119

    “RE # 101, 103, 107, there are great statements about GW at the top of the Catholic hierarchy. The U.S. Bishops made a very good statement in 2001(link below) & are working on a GW packet for parishes, and Pope John Paul II as early as 1990 stated:”

    “Today the ecological crisis has assumed such proportions as to be the responsibility of everyone…The…’greenhouse effect’ has now reached crisis proportions…â?? from “Peace with All Creation,” see http://conservation.catholic.org

    I was aware of the USCCB statements but not of The Catholic Conservation Center, so thanks Lynn.

    There is a very vocal faction in Catholicism which is anti-environmentalist and takes its positions from Chrichton, Lomborg and “Professor” Limbaugh. The size of the faction is another matter, probably not very large.

    In respect to the environment, I believe that the Catholic at large is very much in the same place as the public at large. Atmospheric carbon uptake seems very far removed from the daily problems of earning a living, surviving the daily commute and the boss’s moods, and paying for the kids’ school tuition.

    I believe that there is an underlying issue, though. Many devout Catholics have the impression, not entirely unfounded, that many if not most physical scientists are deep in the bone reductionists who hold religious faith in contempt.

    The devout are not faithful, for the most part, because they read about God in a book, or have never got beyond a slavish innocent reliance on what their elders told them, but out of deep life experience.

    To be frank, they believe most scientists to be not only contemptuous of them, but blind.

    For that reason, I read with interest about a letter sent by physicist Lawrence Krauss to the Pope. It has weaknesses, but it’s a start, anyway:

    http://www.edge.org/3rd_culture/krauss06/krauss06.1_index.html

  23. 123
    pete best says:

    This discussion on tipping points seems to assume on realclimate’s stance on them is that tipping points are not likely to happen? The fact that polar ice is disappearing faster than predicted from the models along with permafrost decline and more besides seem to indicate to many climate scientists (who incidently appear profously in the Fred Pearce book – the last generation) that human induced climate change is happenning faster than can be explained by the primarily linear models.

    If the lay scientist is being fed too much ITS REALLY URGENT – WE MUST DO SOMETHING NOW rhetoric from scientific journalists does that means that the scientists are feeding to them or people are exaggerating to sell books. Climate Science seem to deny any evidence of major far from equilibrium effects citing that all that is being observed is as predicted, which other sources say is not true.

    This is troubling me somewhat

  24. 124
    pete best says:

    Re #121

    http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2006/07/06/MNGSPJQ8221.DTL

    the oceans appear to already be increasing their acidity

  25. 125
    Stephen Berg says:

    A paper just published in Journal of Climate has an interesting graphic on p. 3059. It shows the causes of climatic warming over the 20th Century and indicates that anthropogenic sources are the primary reason.

    Stott, P.A.; Mitchell, J.F.B.; Allen, M.R.; Delworth, T.L.; Gregory, J.M.; Meehl, G.A.; and Santer, B.D. (2006) ‘Observational Constraints on Past Attributable Warming and Predictions of Future Global Warming’ Journal of Climate 19(13) 3055â??3069.

  26. 126
    Hank Roberts says:

    George, my point was that it’s missing the point to be planning for replacing sulfates in the stratosphere while controlling sulfate air pollution at ground level. The real issue as you note is reducing carbon dioxide.

    The already difficult target — 2 degrees above preindustrial level —

    http://www.realclimate.org/index.php?p=246

    doesn’t include ocean acidification as an issue. I hope we’ll be seeing models and targets that include changes — and rates of change — in ocean pH.

    I’d bet — but it’s a hunch only at this point — that the rate of acidification is more urgent and will require more control and faster than we thought we’d need. I hope the hosts here can bring in some of the researchers to talk about rates and rate of change and consequences, to inform that guess.

  27. 127
    Brian Gordon says:

    Re: 123: When and where, exactly, are the tipping points?

    My impression, from reading numerous RC and other sources (like the BBC), is that:

    1. Scientists like to have solid evidence to back up their assertions.
    2. The evidence on when and where various tipping points will happen, and their subsequent effects, is not yet solid.
    3. Therefore, scientists are not willing to say that we have passed a particular point.

    This may change as we understand more about the climate system. It may also mean that we’re flying by tipping points (or are committed to doing so) and just don’t have the evidence to know so.

  28. 128
    Mark Shapiro says:

    This is somewhat OT, but RC has done a couple articles on the thermohaline circulation, fortunately concluding that we don’t have to worry about a tipping point there.

    I just saw an article in American Scientist Online by Richard Seager explaining why the THC is not so critical to Europe’s climate. The main thrust is that Europe has a maritime climate rather than a continental climate. The case seemed clear and reasonable to me. It is at:

    http://www.americanscientist.org/template/AssetDetail/assetid/51963?fulltext=true&print=yes

    Any other opinions?

  29. 129
    Joseph O'Sullivan says:

    This was an excellent explanation of the concept of tipping points. I think that in the popular press at least there is a confusion of tipping point with time lags.

    When the scientific community states that we are already locked into climate change in the near future because of the pollution we have already emitted, it is a logical conclusion to the idea of a “point of no return” or “tipping point”. The complications and subtlety that provide a more accurate explanation are lost.

    Ecosystems do provide examples of tipping points. Gavin’s paragraph on this was very informative. There has been work by scientists that indicates that are tipping points in ecosystems that have been crossed due to human activities.
    Pauley and other marine biologists have shown that drastically reduced populations in marine fishes caused by overfishing may never recover because overfishing has created irreversible changes in ecosystem structure.

    #124 (Pete Best) 121 (George Gonzalez)
    There is a new synthesis report on the impacts of anthropogenic ocean acidification on marine life.
    http://www.ucar.edu/news/releases/2006/acidification.shtml

  30. 130
    S Molnar says:

    Re #128: A related paper by Seager was discussed in a comment and reply on a previous thread:
    http://www.realclimate.org/index.php/archives/2005/05/gulf-stream-slowdown/#comment-2268

    Or, for you tinyURL fans out there:
    http://tinyurl.com/gtz9h

    RealClimate has such a wealth of information – too much for me to keep straight – that I find the search window at the top of the page indispensable.

  31. 131
    Mark Shapiro says:

    S Molnar –

    Thanks for the refs. The comments & replies are great.

  32. 132
    George A. Gonzalez says:

    re: # 124, 126 and 129

    Thanks!!!!

  33. 133
    Blair Dowden says:

    Re #117: Barton, I have wanted to do the same kind of calculation you have done, and I appreciate your effort. But I have to disagree with some of what you have done.

    A simple radiation balance model only needs incoming solar energy and albedo. It makes no difference where the energy is distributed between the land, ocean and atmosphere. The existance of an ocean does not matter, as albedo and water vapor are already taken care of. Of course, Gavin and the boys may take a different view about all this, but their models are rather more complex.

    Optical thickness has nothing to do with the greenhouse effect. It can be approximated by a logarithmic (or even square root, as you have done) relationship. The only figure I have is 3.7 watts per square meter for each doubling, but I don’t know how to seed this to start with. This will not work for Venus, because at high concentrations arbon dioxide absorption bands expand with increased atmospheric pressure (collisional broadening) and higher temperatures (doppler broadening) [according to Raypierre.]

    Your gray gas is fine, but maybe you should keep it that way instead of distinguishing between water vapor and carbon dioxide. The calculation will show that about 99% of the greenhouse effect on Earth is from water vapor. Even the craziest skeptics only claim 95%, probably because they think that is the highest figure they can get away with. It does not take into account the vertical structure of the atmosphere, which is way too hard. Maybe it is best to treat them as the same gas.

    But you want to handle water vapor feedback, which would be nice. I don’t know how to do this (ie. what is the increase in water vapor for every degree C), but maybe you could just hack in the known ratio that 65% of the greenhouse effect is water vapor. That would spoil the purity.

    I don’t know if this helps, or just confuses. I would love to see a working model I can believe in.

  34. 134
    Blair Dowden says:

    Re #121: George, a 2 degree C temperature rise would not “mean the end of life on the earth as we know it.” The Earth was about 3 degrees warmer for several million years during the Pliocene, between 2 and 6 million years ago, and life did quite well, perhaps better than today. Now, a quick transition to that climate could be rather disruptive to our already overpopulated human species, and it should be noted that sea levels were 25 meters higher during that time. This is not something we should want to do, but neither is it the end of the world.

    About ocean acidification. This has nothing to do with temperature, or even carbon dioxide levels, both of which have been much higher in the past and the ocean did fine. The problem is the rate of increase of carbon dioxide levels, because the ocean does not have time to transfer it from the surface to the deep ocean. The faster carbon dioxide levels rise, the worse the problem will be. This is not just speculation or modeling, because there was an interval of rapid carbon dioxide rise about 55 million years ago, known as the Paleocene Eocene Thermal Maximum, for which there is evidence of a large dying of calcium carbonate shelled organisms. So it is something to take seriously.

  35. 135
    George A. Gonzalez says:

    re #134

    The problem with a 2 degree C increase in global temperature does not lie with the rise in temperature itself, but with the fact that such a rise in temperature will engage positive feedbacks (e.g., the massive die off of the rainforest). This will release more CO2 into the atmosphere than emitted throughout the 20th century (in a shorter space of time). As a result, CO2 atmospheric levels will be pushed to 750 ppm. Thus, a 2 degree C rise in global mean temperature will cause a dramatic and relatively sudden global mean temperature increase.

  36. 136
    Hank Roberts says:

    I’m waiting on the ocean chemists to tell us if primary production will drop off before we can burn that much coal — remember where our oxygen comes from!

    http://www.oceansonline.com/oceanicfoodwebs.htm

    Melting the Arctic sea ice could hit hard at ocean biology, as will acidification. Here’s a decent summary page:

    ‘… The halocline is far more important in polar waters in producing water column stratification. Melting of sea ice places a lens of fresh water on top of salt water, creating a very stable water column that lends itself to some of the most intense phytoplankton blooms experienced on the planet.”

    “… Once more, I want to emphasize (read–pound into your head), the importance of physical processes … and chemical processes … in determining biological processes (spring and all phytoplankton blooms, switching from a “classical” food web to a microbial food web, deepening of the chlorophyll maximum, etc.). Remember that all of the energy and matter that fuels oceanic food webs comes from the phytoplankton, who use sunlight to create living matter from inorganic elements.”
    ——

    That’s snipped from a good illustrated page, recommended reading.

    If we lose the, what, 30 percent? of the oxygen that’s currently being put into the atmosphere by ocean organisms, we may not manage to burn all that coal that fast.

    http://www.agu.org/meetings/os06/os06-sessions/os06_OS16B.html
    “Gross oxygen production (GOP) is a fundamental but poorly known characteristic of planktonic marine ecosystem especially with reference to climate change.”

  37. 137
    C. W. Magee says:

    Re: 120.
    I think the projected incease in solar luminosity means that Earth will have a runaway venus-style climate eventually. But “eventually” is something like a billion years in the future. And changes in CO2 will not significantly effect this event.

  38. 138
    Blair Dowden says:

    Re #135: George, your reasoning is circular. The projected 2 or 3 degree rise in global average temperature already includes feedbacks. I see no reason why tropical rainforests will die when paleoclimate data tells us that they covered more area in the time when temperatures where higher than today. The problems with a rapid temperature rise are serious, and we should do what we can to avoid this happening, but I think you are overstating the problem.

    If we want to convice people to reduce carbon dioxide production, we need to be credible. That means not making claims that are not based on the scientific knowledge that we have.

  39. 139
    D_WillRobinson says:

    The very term, “Climate Change is the work of Republican spin doctor, Frank Luntz, who contends, “‘Climate change’ is less frightening than ‘global warming.’ … While global warming has catastrophic connotations attached to it, climate change suggests a more controllable and less emotional challenge.”

    You can read further on Luntz’s fine evil work here.

  40. 140

    [[Re #117: Barton, I have wanted to do the same kind of calculation you have done, and I appreciate your effort. But I have to disagree with some of what you have done.

    A simple radiation balance model only needs incoming solar energy and albedo. It makes no difference where the energy is distributed between the land, ocean and atmosphere. The existance of an ocean does not matter, as albedo and water vapor are already taken care of. Of course, Gavin and the boys may take a different view about all this, but their models are rather more complex.]]

    ANYBODY’S model is going to be more complex. You’re talking about estimating the Earth’s effective or emission temperature, which, due to the presence of an atmosphere, is usually going to be considerably less than the SURFACE temperature.

    [[Optical thickness has nothing to do with the greenhouse effect.]]

    If you’ll forgive me, this is an unbelievably ignorant statement. See if you can get ahold of a copy of John Houghton’s “The Physics of Atmospheres” (the most recent edition is 2002, I think) or K.N. Liou’s “Introduction to Atmospheric Radiation.” The greater the infrared optical thickness of an atmosphere, the hotter the surface will be COMPARED TO THE EFFECTIVE TEMPERATURE.

    [[The calculation will show that about 99% of the greenhouse effect on Earth is from water vapor]]

    Not true. This has been covered in RealClimate before. CO2 accounts for about a quarter of the effect, not 1%.

    There’s no reason you should like my model in particular, but many, many gray greenhouse models exist out there for you to research if you’re interested. Michael Hart came up with a nice iterative one in 1978 (“The Evolution of the Atmosphere of the Earth,” Icarus 33:23-39. Walker et al. came up with a very different one in 1981 (Walker, J.C.G., P.B. Hays, and J.F. Kasting. “A Negative Feedback Mechanism for the Long-term Stabilization of Earth’s Surface Temperature.” J. Geophys. Res. 86(C10):9776-9782). McKay and Davis have come up with several using Mars as a test case, see, for instance, C. McKay and W. Davis, “Duration of Liquid Water Habitats on Early Mars,” Icarus, 90:214-221, 1991. Some of these, e.g. McKay’s, can be found on the web. My model has a number of flaws, some of which you pointed out, but it’s not doing anything spectacularly novel and I’m pretty sure it’s not doing anything spectacularly incorrect, either.

  41. 141
    George A. Gonzalez says:

    re: #138

    The claim that my logic is circular is baseless. One variable causing a temperature increase (e.g., anthropogenic CO2 emissions) can trigger other variables (e.g., methane releases from tundra) that will compound and accelerate temperature increases.

    Respectfully yours,

  42. 142
    Eric says:

    This discussion lacks comprehensive analysis of feedback. There’s
    cherry picking of known positive feedbacks (e.g. melting glaciers and
    tundra) and speculation about new ones (e.g. less snow in Greenland).

    But there’s little mention of negative feedback from increasing anthro
    CO2 and the subsequence water vapor feedback. Water vapor is
    speculated to correspond with “less cloudiness”, but that’s about it.
    The DTR discussion is interesting but not really relevant to climate,
    the more important effect would be higher or lower temperature in the
    daytime or nighttime.

    Here’s some negative weather feedbacks to consider: concentrated
    tropical convection is shown in models to be a negative feedback.
    Diurnal clouds are a negative feedback, in our temperate zone they
    occur after rain, when there is colder air aloft (generally when there
    is a more amplified jet stream), and other times.

    Determining which of those preconditions results from negative
    feedbacks (or are attenuated by positive) will require more modelling.
    Clouds in lower latitudes are a negative feedback.

    Increased land vegetation is a negative feedback. Burning vegetation
    is a negative feedback from aerosols. Ocean circulation is a negative
    feedback (vs stratification), increased plankton in the ocean is a
    negative feedback.

  43. 143
    George A. Gonzalez says:

    re: #142

    Eric,
    There are no doubt negative feedbacks. The problem is that the evidence strongly indicates that anthropogenic CO2 emissions, and positive feedbacks are overwhelming the negative feedback.

  44. 144
    Hank Roberts says:

    George, what evidence do you rely on for this conclusion?
    I’m guessing you’re the college teacher who has a book out relevant to the politics of environmental decisions — but I don’t have the book and am not sure. If you’ve published your references, would you tell us where to look them up?

    You are, I think, saying you expect more warming than Hansen et al. or the IPCC expect — you disagree with what the climatologists are telling us here is known and predictded from the research to date?

    I’m just an interested reader here. I thought about the same as what you’re saying, based on everything I knew, before I began reading RC seriously. Since I began trying to understand what they mean by climate sensitivity, I’ve backed off on what I claim I know.

    Now, when Hansen’s worried, I’m worried — and he’s worried. So I’m not saying you’re wrong. But I’m saying you’re being far more specific and definite than the climatologists about what’s going to happen.

    As to trouble coming –there are more possibilities than you are focusing on. Some are already included in the climate sensitivity calculation as feedbacks. Others like methane burps and dieoff of the aragonite-shelled plankton are not.

  45. 145
    Hank Roberts says:

    To our hosts — would you consider reopening the ocean acidification thread?
    http://www.realclimate.org/index.php?p=169

    It didn’t get a lot of discussion before it closed; this area includes the unexpected feedbacks for which we have known good science — ocean chemistry.

  46. 146
    John McCormick says:

    RE #141

    George you are right on point to keep reminindg us of the threats temperature increase present. Temperature increase equals: expansion of the tropical zone (observed); increased SST (observed); meltback of Arctic ice and change of Arctic ecosystem (under observation); meltback of global glaciers and temperature increase at higher elevations (observed); melting tundra and permafrost with their CO2 and CH4 feedback (in intensive care unit). And, we can add diminished snowpack and early melt in the Western North America and more frequent and extensive fires in the West (positive feedbacks).

    Higher temperatures beget higher temperatures, in my lifetime, despite increased cloudiness.

    As the earth’s human population approaches 7 billion –a time likely to coincide with China achieving the #1 CO2 emiter status, my pessimism needle is heading towards the red zone.

    Positive feedbacks are like a swarm of killer bees coming at us from every direction. Swatting them becomes useless as their venom weakens and eventually kills us. Should we have started the aerial spraying before the swarm arrived? Would there be time and will the chemicals be effective — at which point someone might advocate a non-pesticide approach?

    George, I am hot and cold about your encouraging geoengineering, though you are not advocating any particular line of research or end product. I am reading David Keith’s papers on geoengineering and there is much to be considered before taking steps towards realistic researach efforts (greater concern for ocean acidity being high on my list).

    However, there is no reason to believe we can function OK as the temperature approaches the 2 degree mark. For the sake of continuing civility around the planet, bio-engineering must also be a high priority. Stronger drought, pest and heat resistent crops and silviculture may be our means to buying a little, precious time to come upon a long term solution. I believe there will be universal understanding of the climate problem when barley and hops yields drop dramatically and beer sells at $14 per six pack.

    I know this thread is devoted to tipping points but a tipping point witnessed means there is no time to regain the balance on a global scale.

    A serious discussion on including —ALSO— adaptive measures to the world community’s approach to climate change is in order.

  47. 147
    Brian Gordon says:

    Re: 141, 146: How nigh is the end?

    “Stronger drought, pest and heat resistent crops and silviculture may be our means to buying a little, precious time to come upon a long term solution.”
    Here’s one reason I don’t trust geoengineering solutions: say we develop and introduce all kinds of genetically-modified crops in an attempt to feed ourselves.
    A. Who is developing, manufacturing, marketing (globally) this GM stuff? Big corporations, and we can see how concerned they are about climate change….
    B. Who will sell you the special chemicals required to fertilize and keep weed and pest-free these wondrous new crops? I think we know….
    C. What happens to biodiversity when these crops are widespread? If the crops can be pollinated naturally, they will take over the land. If they cannot reproduce themselves, farmers (and therefore, everyone) are locked into annual seed-buying deals with big business.
    D. When will the ‘climate problem’ be over, so we can go back to the original crops? Maybe never. Or, maybe the original crops can no longer survive, because their habitat has been occupied by more resilient GM crops.

    Suffice it to say that not all of us are convinced that products produced in a lab and tested on rats are as ‘safe’ as plants we have evolved with over many millenia.

    “I believe there will be universal understanding of the climate problem when barley and hops yields drop dramatically and beer sells at $14 per six pack.”
    Beer in Canada has been up around there for some time, and…wait a minute, you may be on to something with this…. :-)

  48. 148
    Steve Sadlov says:

    I want to put forth something here. Quantum behavior. We see it in physics. We see it in “digital” electronics. The term “threshold” is an apt term. “Tipping point” has a more ambiguous meaning. Thesis – we may be at, near or just a past a threshold. Possible deduction – a state change is underway. Following question – what is that state? Has the earth been there before, if so, when? Additional question – if such a state is a likely outcome, is it “bad,” “good,” or neither?

  49. 149
    Eric says:

    George, on balance man is likely forcing positive. How much positive
    feedback could there be, especially once you do some smoothing to
    better compare to the naturally smoothed proxies? I see very little
    possibility of positive feedback having any current effect.

    Since the CO2 forcing can account for much of 0.6 C increase, how do
    we know “instant” positive feedbacks like water vapor will overwhelm
    negative feedbacks before slightly longer timescale feedbacks have a
    chance to kick in? I don’t see how a tipping point can occur when we
    may not be tipping (in feedback) at all yet.

    There is ample evidence of positive forcing natural cycles. There is
    evidence of some negative anthro forcing and other positive non-CO2
    anthro forcing, on balance positive. So not a lot of room for
    positive feedback and not a lot of time to take well-smoothed
    measurements.

  50. 150
    Eric says:

    John, fires are a negative feedback due to aerosols.

    [Response: I think it may be the other way round. They produce aerosols yes, but a lot of them are black carbon (a warming influence), and they also produce NOx, CO and CH4 (ozone precursors) as well as CO2 of course. I'll try and find a study that does the whole picture and report back.... - gavin]


Switch to our mobile site