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The Greenhouse Effect Can Be Thwarted. That's What's New.
Reference
Lindzen, R.S., Chou, M.-D. and Hou, A.Y. 2001. Does the earth have an adaptive infrared iris? Bulletin of the American Meteorological Society 82: 417-432.

What was done
Using upper-level cloudiness data obtained from the Japanese Geostationary Meteorological Satellite and sea surface temperature (SST) data obtained from the National Centers for Environmental Prediction, both of which data sets pertain to the eastern part of the western Pacific (30S-30N, 130E-170W) over the period 1 January 1998 - 31 August 1999, the authors conducted a series of analyses designed to identify the possible existence of a previously-unrecognized negative climate-feedback phenomenon.

What was learned
The data revealed a very strong inverse relationship between upper-level cloud area and the mean SST of cloudy regions, such that the area of cirrus cloud coverage normalized by a measure of the area of cumulus coverage decreases about 22% per degree C increase in the SST of the cloudy region. "Essentially," the authors state, "the cloudy-moist region appears to act as an infrared adaptive iris that opens up and closes down the regions free of upper-level clouds, which more effectively permit infrared cooling, in such a manner as to resist changes in tropical surface temperature." The sensitivity of this negative iris feedback was calculated by the authors to be substantial. In fact, they say it would "more than cancel all the positive feedbacks in the more sensitive current climate models" that are used to predict the consequences of projected increases in atmospheric CO2 concentration.

What it means
Just because the greenhouse effect is real, it does not follow that an increase in its intensity will necessarily lead to a significant increase in mean global air temperature, as climate alarmists are wont to claim. This study of but one negative feedback phenomenon, for example, suggests that it may well have the power (if it holds up under further scrutiny) to reduce the 1.5 to 4C warming typically predicted to result from a doubling of the air's CO2 content to but 0.64 to 1.6C, according to the authors. And this feedback is just one of a whole host of negative feedbacks that operate to maintain earth's climate in an equable state (see, for example, Feedback Factors in our Subject Index). Furthermore, some of these negative feedback phenomena are induced by the direct effects of the increase in the air's CO2 content itself, needing no increase in air temperature to propel them into action. Hence, it is not inconceivable that an increase in the atmosphere's CO2 concentration may result in no warming at all. Or even a cooling!

Until we really understand what's happening in all areas of earth's complex climate system, we are but grasping at straws when we cling tenaciously to global warming predictions based on even the most advanced climate models in use today. And that, in fact, is why the authors of this article are so concerned. They note that the basis of the new negative feedback they have identified - the observed relationship between upper-level cloud coverage and cloud-weighted SST - is not manifest in state-of-the-art GCMs. Certainly, as this one example clearly shows, much more research will be required before we can determine that the ongoing rise in the air's CO2 content even constitutes a problem, much less specify its magnitude and prescribe ameliorative measures for dealing with it.