Friday, 29 May 2009

Quote of the day

On the Belgian forum, Ferdinand Engelbeen in an attempt to say science overestimates global warming, writes :
Ferdinand Engelbeen aerosolen
Based on the absorbtionspectra of H2O and CO2, doubling of the latter (280 => 560 ppm) will result in a 0,9 °C increase. The water vapour feedback included, the rise becomes only 1,3 °C. That is all.

The rest of the 3°C of 2xCO2 predicted by models is rather dubious :
the cooling effect of aerosols is estimated (too) high, the positive temperature effect of clouds (in reality probably negative) ...
So if you overestimate a cooling factor in your calculation, your final result ends up too warm ???


  1. Jules, if you look at the main climate forcings, aerosols and greenhouse gases are each others oppposite.

    To match the temperature trend of the previous century (a necessary but not sufficient condition for climate models), especially the 1945-1975 cooler period, aerosols are thought to have had a major role in offsetting the GHG warming.

    Indeed, there is an offset between the effect of aerosols and GHGs: if aerosols have a huge (cooling) effect, GHGs must have a huge (warming) effect and reverse. In both cases one can have a good fit with the temperature trend of the past century:

    See further RealClimate at:
    and my comments at #6 and following


    Ferdinand Engelbeen

  2. Ferdinand, you would have had a point if the equilibrium climate sensitivity was solely determined by the 20th century record. However, since the climate has been out of equilibrium for most of the 20th century, it doesn't serve this purpose very well. Time periods from the past serve this purpose better, and lead to the conclusion that the climate sensitivity is between 2 and 4.5 degrees C for a doubling of CO2. See eg and the asociated GRL paper.

  3. Jules,

    I agree that one need to look at more periods, but that should be periods which are more or less comparable with the current one, as climate sensitivity during the glacial/interglacial flips is quite different from current times (ice albedo...). From these flips we know that CO2 has little effect on temperature (a drop of 40 ppmv CO2 at the end of the Eemian has no measurable effect).

    Further there is no reason at all to expect (as all climate models include) that all forcings have the same effect for the same change in strength. Solar has a direct effect in the stratosphere, clouds and deep in the ocean surface. IR has no certain effect on clouds, little effect in the stratosphere and is absorbed in a fraction of a mm of the ocean's surface.

    This makes it very improbable that the "natural" climate sensitivity for natural changes and for 2xCO2 has the same value...

  4. Ferdinand,
    The estimates of climate sensitivity based on interglacial changes take into account the changes due to the vast amount of land ice, as eg explained in this RC post
    "...there is a lot more going on at LGM time than just the change in CO2. Some of these things are feebacks like water vapor, clouds and sea-ice, which could be reasonably presumed to be relevant to the future as well as the past. Other forcings, including the growth and decay of massive Northern Hemisphere continental ice sheets, changes in atmospheric dust, and changes in the ocean circulation, are not likely to have the same kind of effect in a future warming scenario as they did at glacial times. In estimating climate sensitivity such effects must be controlled for, and subtracted out to yield the portion of climate change attributable to CO2. Broadly speaking, we know that it is unlikely that current climate models are systematically overestimating sensitivity to CO2 by very much, since most of the major models can get into the ballpark of the correct tropical and Southern Hemisphere cooling when CO2 is dropped to 180 parts per milllion. No model gets very much cooling south of the Equator without the effect of CO2. Hence, any change in model physics that reduced climate sensitivity would make it much harder to account for the observed LGM cooling."

    I remember an explanation on RC (but can't find the reference) that 1 W/m2 of forcing should have roughly the same effect on the global equilibrium temperature repsonse, irrespective of the exact nature of the forcing. If 'fast' feedbacks are included in the forcing figure, this makes certain sense, though admittedly I don't know the details about this.


  5. Bart,

    If you look at the discussion on Realclimate about climate sensitivity which you refer to, you will find my comments and Raypierre's comments about the same items. That still is not resolved and the equal response to equal forcing changes still is debatable.

    Think e.g. about solar vs. GHG influence on sea (surface) temperature: solar warms the whole layer down to several hundred meters as visible light penetrates that deep. IR is absorbed in the upper fraction of a mm. That means that the upper fraction of a mm disperses its energy downward, emits more heat waves back to space and/or evaporates more water (all three are measured). I don't think that 1 W/m2 change by solar or CO2 ultimately has the same effect on global (ocean) temperatures in the case of ocean heating...

    The same point for the solar influences in the stratosphere: higher activity = more UV = more ozone and higher temperature difference tropics-poles, shifting the jet stream position polewards, including cloud/rain patterns and temperatures...

    So why should one expect the same overall influence with such a difference in effects?

  6. The globally averaged temperature after the climate system has equilibrated is rather insensitive to the initial differences in (the location of) the reponse.



    That a similar response may level out over the globe may be right, but it seems rather questionable to me that a similar change in forcing at very different altitudes (stratosphere vs. troposphere), ocean penetration and reaction on clouds/rain patterns would give the same response...

  8. Uit de discussie op RealClimate maak ik op dat er een klein verschil kan zijn in uiteindelijke respons, maar dat dat verschil niet erg groot is:
    "Different forcings can have different impacts (which can be measured by the efficacity - Hansen et al, 2005), and to some extent that is model dependent. But the differences are not by orders of magnitude, more like a few 10’s of percent at max."
    "GCM simulations with large changes (equivalent to doubled CO2) show a remarkable degree of equivalence (see Govindaswami and Caldeira GRL Vol. 27, 2001)."

  9. Jules,

    The result of the efficacy report by James Hansen is based on his own climate model, including all the constraints within the model.
    That means very little flexibility allowed for any difference in effect for different types of forcing.

    Thus that such a model calculates similar (+/- 10%) sensitivities for different forcings is no wonder, but doesn't prove anything.
    Even the HadCM3 model allows a doubling of solar effect with a little more freedom. But the largest effect is from the estimates of the forcings by aerosols, which are quite uncertain, even the overall global sign is uncertain...