Let me say first that I am opposed to simple answers to complex question. See for instance:
http://www.realclimate.org/index.php/archives/2008/09/simple-question-simple-answer-no/ Here is a definition and a couple of simple calculations from Wikipedia. 'In Intergovernmental Panel on Climate Change (IPCC) reports, equilibrium climate sensitivity refers to the equilibrium change in global mean near-surface air temperature that would result from a sustained doubling of the atmospheric (equivalent) CO2 concentration (ΔTx2). This value is estimated, by the IPCC Fourth Assessment Report (AR4) as likely to be in the range 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values... Sample calculation using industrial-age data Rahmstorf (2008)[3] provides an informal example of how climate sensitivity might be estimated empirically, from which the following is modified. Denote the sensitivity, i.e. the equilibrium increase in global mean temperature including the effects of feedbacks due to a sustained forcing by doubled CO2 (taken as 3.7 W/m2), as x °C. If Earth were to experience an equilibrium temperature change of ΔT (°C) due to a sustained forcing of ΔF (W/m2), then one might say that x/ (ΔT) = (3.7 W/m2)/(ΔF), i.e. that x = ΔT * (3.7 W/m2)/ΔF. The global temperature increase since the beginning of the industrial period (taken as 1750) is about 0.8 °C, and the radiative forcing due to CO2 and other long-lived greenhouse gases (mainly methane, nitrous oxide, and chlorofluorocarbons) emitted since that time is about 2.6 W/m2. Neglecting other forcings and considering the temperature increase to be an equilibrium increase would lead to a sensitivity of about 1.1 °C. However, ΔF also contains contributions due to solar activity (+0.3 W/m2), aerosols (-1 W/m2), ozone (0.3 W/m2) and other lesser influences, bringing the total forcing over the industrial period to 1.6 W/m2 according to best estimate of the IPCC AR4, albeit with substantial uncertainty. Additionally the fact that the climate system is not at equilibrium must be accounted for; this is done by subtracting the planetary heat uptake rate H from the forcing; i.e., x = ΔT * (3.7 W/m2)/(ΔF-H). Taking planetary heat uptake rate as the rate of ocean heat uptake, estimated by the IPCC AR4 as 0.2 W/m2, yields a value for x of 2.1 °C. (All numbers are approximate and quite uncertain.) Sample calculation using ice-age data "... examine the change in temperature and solar forcing between glaciation (ice age) and interglacial (no ice age) periods. The change in temperature, revealed in ice core samples, is 5 °C, while the change in solar forcing is 7.1 W/m2. The computed climate sensitivity is therefore 5/7.1 = 0.7 K(W/m2)-1. We can use this empirically derived climate sensitivity to predict the temperature rise from a forcing of 4 W/m2, arising from a doubling of the atmospheric CO2 from pre-industrial levels. The result is a predicted temperature increase of 3 °C."[6] Based on analysis of uncertainties in total forcing, in Antarctic cooling, and in the ratio of global to Antarctic cooling of the last glacial maximum relative to the present, Ganopolski and Schneider von Deimling (2008) infer a range of 1.3 to 6.8 °C for climate sensitivity determined by this approach.' http://en.wikipedia.org/wiki/Climate_sensitivity I don’t believe that we do understand decadal changes in radiative forcing. Solar forcing is uncertain. Temperature doesn’t respond linearly to greenhouse gas forcing - the planet warms and cools (much ado about natural variation) while greenhouse gases accumulate. I don’t believe we understand polar ice clouds or changes in solar UV and stratospheric ozone responses. We don’t understand the causes of decadal changes in significant ocean and atmospheric climate indices. Clouds are an area where there is an admitted shortfall in understanding. See for instance significant cloud changes on decadal timescales at - http://isccp.giss.nasa.gov/zD2BASICS/B8glbp.anomdevs.jpg Goode and Palle suggest that shortwave reflectance reduced by 4 W/m2 between the mid 1980’s and 1998 and increased by 2 W/m2 since. TOA radiation anomalies change all the time and especially abruptly. See: http://bbso.njit.edu/Research/EarthShine/literature/Palle_etal_2008_JGR.pdf I especially don’t believe that we understand enough about paleoclimatic forcing to justify such a simple calculation. The other way to approach climate sensitivity is with models. This I believe is even more unclear. This is a good discussion by Knutti – which is much more readable than the PNAS McWilliams (2007) paper. ‘Since anyone can contribute, it is simply unclear how to interpret the set of models in the first place. But because most modeling groups only contribute their “best” model, all of them carefully calibrated to the same observations, and no group is deliberately trying to push their model to extreme behaviour, it is possible that the multi-model ensemble underestimates the uncertainty in the climate system. Although this is hard to confirm or reject, there may even be an element of ‘social anchoring’ and a tendency towards consensus: for quantities that cannot be measured (for example climate sensitivity), it is easier to be in the middle of the crowd than far outside.' Knutti, R., The end of model democracy?, Editorial for Climatic Change, published online, DOI 10.1007/s10584-010-9800-2. http://www.iac.ethz.ch/people/knuttir/papers ‘Structural instability’ of climate models and ‘sensitive dependence’ of climate and models are ‘humbling twin properties’ that suggest that there are theoretical limits to climate predictability. (McWilliams, 2007) Forgive me for being a skeptic but in the spirit of Kuhn and Popper I believe that we should continually doubt. I think that there is quite an inventory of things we don’t understand and that the net effect is one of the climate uncertainty rather than certainty. One uncertainty is whether the current warming hiatus will persist for another decade or two. On Mar 2, 12:28 am, "Roald B. Larsen" <[email protected]> wrote: > On Feb 25, 10:06 pm, Robert I Ellison <[email protected]> > wrote: > > > This article on climate computing was published recently. Only fair > > that I link to it here as well. > > >http://www.quadrant.org.au/blogs/doomed-planet/2010/02/ellison > > > Climate sensitivity is always a fiction. There are non-linear > > processes sorted out in computer programs. So the idea of climate > > sensitivity as a number is related to a doubling of CO2. It is not > > the same number at 0.5 or 1.5 times CO2 because of the non-linear > > responses. > > > More generally climate sensitivity is thought of in qualitative terms > > - higher or lower sensitivity to greenhouse gas changes. In terms of > > abrupt climate change - I think both are answers to the wrong > > question. > > http://www.realclimate.org/index.php/archives/2007/08/the-co2-problem... > [...] > > Step 5: Climate sensitivity is around 3ºC for a doubling of CO2 > > The climate sensitivity classically defined is the response of global > mean temperature to a forcing once all the ‘fast feedbacks’ have > occurred (atmospheric temperatures, clouds, water vapour, winds, snow, > sea ice etc.), but before any of the ’slow’ feedbacks have kicked in > (ice sheets, vegetation, carbon cycle etc.). Given that it doesn’t > matter much which forcing is changing, sensitivity can be assessed > from any particular period in the past where the changes in forcing > are known and the corresponding equilibrium temperature change can be > estimated. As we have discussed previously (www.realclimate.org/ > index.php/archives/2004/12/index/#ClimateSensitivity ), the last > glacial period is a good example of a large forcing (~7 W/m2 from ice > sheets, greenhouse gases, dust and vegetation) giving a large > temperature response (~5 ºC) and implying a sensitivity of about 3ºC > (with substantial error bars). More formally, you can combine this > estimate with others taken from the 20th century, the response to > volcanoes, the last millennium, remote sensing etc. to get pretty good > constraints on what the number should be. This was done by Annan and > Hargreaves (2006), (www.realclimate.org/index.php/archives/2006/03/ > climate-sensitivity-plus-a-change/ ) and they come up with, you > guessed it, 3ºC. -- You received this message because you are subscribed to the Google Groups Global Change ("globalchange") newsgroup. Global Change is a public, moderated venue for discussion of science, technology, economics and policy dimensions of global environmental change. Posts will be admitted to the list if and only if any moderator finds the submission to be constructive and/or interesting, on topic, and not gratuitously rude. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/globalchange
