2011-03-03 10:20:50New paper finding paleo evidence for cloud negative feedback
John Cook


One of the strongest arguments against low climate sensitivity (and negative feedback) is past climate change. So this new paper is interesting:

Cloud response to summer temperatures in Fennoscandia over the last thousand years

Cloud cover is one of the most important factors controlling the radiation balance of the Earth. The response of cloud cover to increasing global temperatures represents the largest uncertainty in model estimates of future climate because the cloud response to temperature is not well-constrained. Here we present the first regional reconstruction of summer sunshine over the past millennium, based on the stable carbon isotope ratios of pine treerings from Fennoscandia. Comparison with the regional temperature evolution reveals the Little Ice Age (LIA) to have been sunny, with cloudy conditions in the warmest periods of the Medieval at this site. A negative shortwave cloud feedback is indicated at high latitude. A millennial climate simulation suggests that regionally low temperatures during the LIA were mostly maintained by a weaker greenhouse effect due to lower humidity. Simulations of future climate that display a negative shortwave cloud feedback for high-latitudes are consistent with our proxy interpretation.

Not sure of the significance. This talks about high latitude feedback while my understanding was the typical picture of cloud feedback portrayed by skeptics involved tropical cloud cover.

BTW, I added this to our database of peer-reviewed papers. If in doubt, I put it in the neutral column - I'm not sure if this one supports the skeptic position or not yet. Comments welcome.

2011-03-03 10:49:23
Mark Richardson

iirc the dominant shortwave effect is in the tropics (obviously - there's more shortwave there!).

And the longwave component is vital. Iirc all clouds have a negative shortwave feedback effect. If the clouds are high up though, then they are a net positive because of their LW feedback.

2011-03-03 11:06:19Comment
Robert Way


From the conclusion of the paper

"If, as indicated by the Laanila summer sunshine reconstruction, cloud cover was less in colder periods, and therefore incoming shortwave radiation greater, cloud cover at this latitude in a warmer climate should be greater and therefore incoming shortwave radiation less than at present. Most future climate simulations from the IPCC AR4 suite do display this behavior. [15] Treering stable carbon isotope ratios from both northern Finland and northern Norway indicate a negative shortwave cloud feedback in this high‐latitude region in summer in the LIA with the Laanila series presented here also showing a negative feedback in the Medieval, in line with model findings for the 21st century [Trenberth and Fasullo, 2009]. "

2011-03-03 17:35:17
Ari Jokimäki


Remember also that that this is high latitude study. Negative shortwave cloud feedback suggests more clouds and that in high latitudes might suggest even more positive longwave cloud feedback. The greenhouse effect of clouds is important there (well, here, actually :) ) because there is very little sunshine during winter. There was a study recently finding that clouds in the arctic have increased especially during winters. Not having read the paper I don't know if they discuss longwave cloud feedback at all.