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Negative Climate Feedback and Short Response Time Seen in Mt. Pinatubo Eruption
Reference
Douglass, D.H. and Knox, R.S.  2005.  Climate forcing by the volcanic eruption of Mount Pinatubo.  Geophysical Research Letters 32: 10.1029/2004GL022119.

What was done
The authors "determined the volcano climate sensitivity and response time for the Mount Pinatubo eruption, using observational measurements of the temperature anomalies of the lower troposphere, measurements of the long wave outgoing radiation, and the aerosol optical density," perhaps inspired by what Hansen et al. (1992) had said of this eruption, i.e., that it had the potential to exceed "the accumulated forcing due to all anthropogenic greenhouse gases added to the atmosphere since the industrial revolution began," and should "provide an acid test for global climate models."

What was learned
Douglass and Knox's analysis revealed "a short atmospheric response time, of the order of several months, leaving no volcano effect in the pipeline, and a negative feedback to its forcing."

What it means
One of the issues raised by the results of this study, in the words of the University of Rochester physicists who conducted it, "is the origin of the required negative feedback."  In response, they report that "negative feedback processes have been proposed involving cirrus clouds (Lindzen et al., 2001)," and that "Sassen (1992) reports that cirrus clouds were produced during the Mt. Pinatubo event."  In addition, they note that the adaptive infrared iris concept of Lindzen et al. "yields a negative feedback factor of -1.1, which is well within the error estimate of the feedback found by us."  They also note that the short intrinsic response time they derived (6.8 1.5 months) "confirms suggestions of Lindzen and Giannitsis (1998, 2002) that a low sensitivity and small lifetime are more appropriate" than the "long response times and positive feedback" that are characteristic of the reigning climatic paradigm.

In this regard, it is also worth mentioning that the adaptive infrared iris phenomenon has the capacity, in Lindzen et al.'s words, to "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.  As a result, Douglass and Knox conclude that "Hansen et al.'s hope that the dramatic Pinatubo climate event would provide an 'acid test' of climate models has been fulfilled, although with an unexpected result."

References
Hansen, J., Lacis, A., Ruedy, R. and Sato, M.  1992.  Potential climate impact of Mount Pinatubo eruption.  Geophysical Research Letters 19: 215-218.

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.

Lindzen, R.S. and Giannitsis, C.  1998.  On the climatic implications of volcanic cooling.  Journal of Geophysical Research 103: 5929-5941.

Lindzen, R.S. and Giannitsis, C.  2002.  Reconciling observations of global temperature change.  Geophysical Research Letters 29: 10.1029/2001GL014074.

Sassen, K.  1992.  Evidence for liquid-phase cirrus cloud formation from volcanic aerosols: Climate indications.  Science 257: 516-519.

Reviewed 18 May 2005