Overview - Negative Climate Feedbacks

Global warming is essentially the response of the earth/atmosphere system to increases in its resident energy storage - from increased inputs and changes in the degree to which these energy inputs are retained. During the last one or two decades various theories have been presented that postulate the existence of negative feedbacks in the earth/atmosphere system by which increases in climate forcing lead to changes that allow for more of this energy input to escape to space, thereby creating a "damping" that prevents significant global warming. As of this writing, the most recent and noteworthy of these is the so-called "Iris" hypothesis of Richard S. Lindzen and co-authors.

Lindzen, who is the Sloan professor of Atmospheric Sciences at M.I.T. and a member of the Intergovernmental Panel on Climate Change, is an outspoken climate change skeptic who is frequently quoted by the industry and Far-Right special interests that have been feverishly fighting climate change science in public forums and in Washington D.C. Lindzen stands out though in that he is one of a tiny handful of global warming skeptics who has actually published noteworthy research in the field in the last 15 years. He has a reputation among his colleagues for being short-tempered toward those he disagrees with, and frequently resorts to the sort of condescension and ad-hominem that are more characteristic of the ideological think tanks and industry groups he often consults for than the scientific community. Nevertheless, unlike these groups his publications have for the most part past peer-review and made genuine contributions to the climate science knowledge base. His work as a skeptic has provided a necessary check and balance on climate change researchers who otherwise might have been less careful and thorough. He does acknowledge that global warming is happening (which increasingly, even skeptics are being forced to admit). But he believes the warming will be far less severe than most climate change researchers think due to negative feedbacks in the atmosphere that he believes will counteract the effects of anthropogenic greenhouse gases.

The last 10 years or so of Lindzen’s career have been devoted largely to finding and demonstrating such negative feedback mechanisms. His latest and most credible attempt to date, published in Feb. 2001 in the Bulletin of the American Meteorological Society (BAMS) with colleagues M.D. Chou and A.Y. Hou, proposes that a tropical deep convection mechanism exists (which he acknowledges that he does not yet understand) that negatively correlates rising sea surface temperatures (SST's) with tropical high cloud cover. In his model, as SST's increase, high cloud cover decreases, resulting in an increase in outgoing long wavelength radiation (OLR) which radiatively cools the Earth. This effect, if it exists, would give the Earth an adaptive infrared "iris" similar to the human eye's that would act as a negative feedback which Lindzen and his colleagues believe will prevent global warming. This theory has come to be known as Lindzen's Iris.

Lindzen and his colleagues based their theory on observations of cloud data from the Japanese GMS-5 satellite and the results of computer models of cloud radiative fluxes. In the first of these papers, Fu et. al. show that Lindzen and his colleagues overestimated high cloud radiative emissions and water vapor feedbacks, and also that their assumptions about low cloud vs. high cloud albedo and radiative effects were incorrect. In the next two papers Lin et. al. and Chambers et. al. revisit the Iris Effect using data from the Tropical Rainfall Measuring Mission (TRMM) satellite, and replacing the modeled tropical cloud radiative fluxes used by Lindzen et. al. with actual radiative cloud fluxes measured directly by the Clouds and the Earth's Radiant Energy System (CERES) satellite. They show that Lindzen et. al. underestimated both high cloud albedos and OLR fluxes. These in turn lead to a positive feedback rather than a negative one.

The final link is to an article from the NASA Earth Observatory discussing the Lindzen's Iris and the arguments that have been presented both for and against it. Lindzen and his colleagues have challenged these results and controversy surrounds the Iris Effect discussion. But it is worth noting that they are based on satellite measurements that are more up to date and relevant than his were and the results of these critics more or less independently replicate each other whereas his results have have not been replicated. Furthermore, the sort of deep convection mechanism proposed for the Iris Effect by Lindzen and his colleagues would have a short time constant for response to climatic forcings compared to the scale over which reliable measurements of global average temperature increases have been observed. We have to wonder why, if the Iris mechanism is as strong as they claim, does global average temperature continue to rise at unprecedented rates?

Over the last 10-15 year, Lindzen has maintained an unusually high degree of confidence in the existence of negative feedback mechanisms that invalidate any and all concerns about global warming. At times, this confidence has bordered on the extreme. He has been highly critical of anyone who doubts the existence of such mechanisms or believes that global warming will be a serious problem, and his frequent ad-hominem attacks against such people are highly inappropriate for a scholar of his stature. However, though the importance of Lindzen's contributions to this area of research are indisputable, his confidence in his views must be evaluated in light of the fact that after more than 10 years of searching, neither he nor anyone else has yet produced any evidence for the existence of large scale negative feedbacks like the Iris Effect that has been replicated by anyone else in the field - a fact that continues to be as true of his Iris Effect as with his previous negative feedback claims. Until this happens, the burden of proof is solidly on Lindzen and his colleagues.


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