If there is anyone out there in Global Change Land still following this group, you might be interested in the op-ed piece that Dr. Richard Lindzen wrote for the Wall Street Journal on 30 November.
http://online.wsj.com/article/SB10001424052748703939404574567423917025400.html In it, one of his points is a repeat of his claim that there is a negative feedback due to clouds. He is apparently referring to his Adaptive Iris hypothesis. His comments (heavily edited for brevity) are: “It is generally accepted that a doubling of CO2 will only produce a change of about two degrees Fahrenheit if all else is held constant........current climate models predict much higher sensitivities. They do so because in these models, the main greenhouse substances (water vapor and clouds) act to amplify anything that CO2 does. This is referred to as positive feedback. But as the IPCC notes, clouds continue to be a source of major uncertainty in current models...............It turns out that increased thin cirrus cloud coverage in the tropics readily resolves the paradox but only if the clouds constitute a negative feedback. In present terms this means that they would diminish rather than enhance the impact of CO2.....” He also gives an example of a negative feedback, discussing a period some 2.5 billion years ago when the Sun’s output is thought to have been 20-30% less than now. He claims that the negative feedback of clouds might explain the apparent lack of freezing of the oceans at the time. I’ve often wondered about Lindzen’s negative cloud feedback. My main issue with his hypothesis is the question of Ice Ages. We know with near certainty that over the past 3 million years or so, the Earth’s climate has been dominated by Ice Age conditions. We are currently experiencing an Interglacial, that is, a warm period during which the massive ice sheets have melted and the Earth’s average temperature is a few degrees warmer than the temperature during the Ice Ages. So, I take the opportunity to ask a rather obvious question: If the feedback from high clouds is strong enough to have prevented freezing during the period of low solar output Lindzen mentions, how is it that the Earth warmed enough for the ice sheets to have melted. Wouldn’t the negative feedback have prevented the warming which led to the melting of the ice? Looking at the other end of the cycle of Ice Ages, if that cloud feedback was strong enough to offset the low solar output, what was the cause of the freezing which terminated the last Interglacial, the Eemian, roughly 120,000 years ago (and all the other Interglacials over the past 3 million years as well)?? I think Dr. Lindzen may have destroyed his own hypothesis in his op-ed piece....8-) E. S. -- 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
