Home > Uncategorized > Feedbacks mean that the warming resulting from a doubling of CO2 may be much higher than 3 C, say more paleo-climate studies

Feedbacks mean that the warming resulting from a doubling of CO2 may be much higher than 3 C, say more paleo-climate studies

I have discussed climate “sensitivity” a number of times earlier; here is a good place to start for those interested: http://mises.org/Community/blogs/tokyotom/archive/2008/10/14/what-do-we-know-about-climate-models-and-climate-quot-sensitivity-quot-a-recent-bibliography.aspx

I just ran across more bad news about the size of the risks we face. Given the risks, it seems rather perverse to me that the “do nothing (except externalizing risks and buckling our own seat belts for the longer haul)” approach is the one that is considered “conservative”.

Joe Romm, Climate Progress (emphasis added):

The disinformers claim that projections of dangerous future warming from greenhouse gas emissions are based on computer models.  In fact, ClimateProgress readers know that the paleoclimate data is considerably more worrisome than the models (see Hansen: ‘Long-term’ climate sensitivity of 6°C for doubled CO2).  That’s mainly because the vast majority of the models largely ignore key amplifying carbon-cycle feedbacks, such as the methane emissions from melting tundra (see Are Scientists Underestimating Climate Change).

Science has just published an important review and analysis of “real world” paleoclimate data in “Lessons from Earth’s Past” (subs. req’d) by National Center for Atmospheric Research (NCAR) scientist Jeffrey Kiehl.  The NCAR release is here: “Earth’s hot past could be prologue to future climate.”  The study begins by noting:

Climate models are invaluable tools for understanding Earth’s climate system. But examination of the real world also provides insights into the role of greenhouse gases (carbon dioxide) in determining Earth’s climate. Not only can much be learned by looking at the observational evidence from Earth’s past, but such know ledge can provide context for future climate change.

The atmospheric CO2 concentration currently is 390 parts per million by volume (ppmv), and continuing on a business-as-usual path of energy use based on fossil fuels will raise it to ∼900 to 1100 ppmv by the end of this century (see the first figure) (1). When was the last time the atmosphere contained ∼1000 ppmv of CO2? Recent reconstructions (24) of atmospheric CO2 concentrations through history indicate that it has been ∼30 to 100 million years since this concentration existed in the atmosphere (the range in time is due to uncertainty in proxy values of CO2). The data also reveal that the reduction of CO2 from this high level to the lower levels of the recent past took tens of millions of years. Through the burning of fossil fuels, the atmosphere will return to this concentration in a matter of a century. Thus, the rate of increase in atmospheric CO2 is unprecedented in Earth’s history.

I will repost the references at the end, since this is a review article (see also U.S. media largely ignores latest warning from climate scientists: “Recent observations confirm … the worst-case IPCC scenario trajectories (or even worse) are being realised” — 1000 ppm)


Kiel concludes: (emphasis in original)

Earth’s CO2concentration is rapidly rising to a level not seen in ∼30 to 100 million years, and Earth’s climate was extremely warm at these levels of CO2. If the world reaches such concentrations of atmospheric CO2, positive feedback processes can amplify global warming beyond current modeling estimates. The human species and global ecosystems will be placed in a climate state never before experienced in their evolutionary history and at an unprecedented rate. Note that these conclusions arise from observations from Earth’s past and not specifically from climate models.




 Romm has referred to other recent studies that indicate these extended feedbacks may occur  relatively quickly:


The conclusion from this analysis—resting on data for CO2 levels, paleotemperatures, and radiative transfer knowledge—is that Earth’s sensitivity to CO2radiative forcing may be much greater than that obtained from climate models (1214).

Indeed, in the release, Kiehl notes his study “found that carbon dioxide may have at least twice the effect on global temperatures than currently projected by computer models of global climate.”

Why is the ‘real world’ warming so much greater than the models?  The vast majority of the models focus on the equilibrium climate sensitivity — typically estimated at about 3°C for double CO2 (equivalent to about ¾°C per W/m2) — only includes fast feedbacks, such as water vapor and sea ice.  As Hansen has explained in deriving his 6°C ‘long-term’ sensitivity: (emphasis in original)

Elsewhere (Hansen et al. 2007a) we have described evidence that slower feedbacks, such as poleward expansion of forests, darkening and shrinking of ice sheets, and release of methane from melting tundra, are likely to be significant on decade-century time scales. This realization increases the urgency of estimating the level of climate change that would have dangerous consequences for humanity and other creatures on the planet, and the urgency of defining a realistic path that could avoid these dangerous consequence.

For background on the tundra (and methane), see Science: Vast East Siberian Arctic Shelf methane stores destabilizing and venting:  NSF issues world a wake-up call: “Release of even a fraction of the methane stored in the shelf could trigger abrupt climate warming.” …

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