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New Studies Erode Confidence in Climate Model Predictions
Volume 5, Number 14: 3 April 2002

Two data-based studies published in the 1 February 2002 issue of Science - Chen et al. (2002) and Wielicki et al. (2002) - reveal what Hartmann (2002) calls a pair of "tropical surprises."  The first of the seminal discoveries was the common finding of both Chen et al. and Wielicki et al. that the amount of thermal radiation emitted to space at the top of the tropical atmosphere increased by about 4 Wm-2 between the 1980s and the 1990s; the second was that the amount of reflected sunlight decreased by 1 to 2 Wm-2 over the same period, with the net result that more total radiant energy exited the tropics in the latter decade.

These changes are highly significant.  The measured thermal radiative energy loss at the top of the tropical atmosphere, for example, is of the same magnitude as the thermal radiative energy gain that is generally predicted for an instantaneous doubling of the air's CO2 content.  Yet as Hartman correctly notes, "only very small changes in average tropical surface temperature were observed during this time."  So what went wrong?  Or as we should probably more correctly phrase the question, what went right?

One thing, of course, was the competing change in solar radiation reception that was driven by changes in cloud cover, which allowed more solar radiation to reach the surface of the earth's tropical region and warm it.  These changes were produced by what Chen et al. determined to be "a decadal-time-scale strengthening of the tropical Hadley and Walker circulations."  Another helping-hand was likely provided by the past quarter-century's slowdown in the meridional overturning circulation of the upper 100 to 400 meters of the tropical Pacific Ocean, which was recently reported by McPhaden and Zhang (2002).  This circulation slowdown also promotes tropical sea surface warming, by reducing the rate-of-supply of relatively colder water to the region of equatorial upwelling.

So what do all of these observations have to do with evaluating the ability of climate models to correctly predict the future?  For one thing - and one very important thing - they provide several new phenomena for the models to replicate as a test of their ability to properly represent the real-world.  In the words of McPhaden and Zhang, for example, the time-varying meridional overturning circulation of the upper Pacific Ocean provides "an important dynamical constraint for model studies that attempt to simulate recent observed decadal changes in the Pacific."  If the climate models can't reconstruct this simple wind-driven circulation, in other words, why should we believe anything else they tell us.

In an eye-opening application of this principle, Wielicki et al. tested the ability of four state-of-the-art climate models and one weather assimilation model to reproduce the observed decadal changes in top-of-the-atmosphere thermal and solar radiative energy fluxes that occurred over the past two decades.  And how did the models do?

The results were truly pathetic.  No significant decadal variability was exhibited by any of the models; and they all failed to reproduce even the cyclical seasonal change in tropical albedo.  The administrators of the test thus kindly concluded that "the missing variability in the models highlights the critical need to improve cloud modeling in the tropics so that prediction of tropical climate on interannual and decadal time scales can be improved."

Hartmann was a little more candid in his scoring of the test, saying it indicated "the models are deficient."  Amplifying this assessment, he noted that "if the energy budget can vary substantially in the absence of obvious forcing," as it well did over the past two decades, "then the climate of earth has modes of variability that are not yet fully understood and cannot yet be accurately represented in climate models."

In conclusion, doesn't it seem strange that if (1) the energy budget of the planet can vary substantially in the absence of any obvious forcing, and if (2) earth's climate has modes of variability that are (a) not yet fully understood and (b) cannot yet be accurately represented in climate models - as these studies demonstrate is truly the case - the global political power brokers would not at least consider the possibility that the models upon which the Kyoto Protocol is based might not be painting an accurate picture of the future?  It sure seems so to us.  But, hey, that's politics, not science.  And that about tells you all you need to know about the issue.

Dr. Sherwood B. Idso
Dr. Keith E. Idso
Vice President

Chen, J., Carlson, B.E. and Del Genio, A.D.  2002.  Evidence for strengthening of the tropical general circulation in the 1990s.  Science 295: 838-841.

Hartmann, D.L.  2002.  Tropical surprises.  Science 295: 811-812.

McPhaden, M.J. and Zhang, D.  2002.  Slowdown of the meridional overturning circulation in the upper Pacific Ocean.  Nature 415: 603-608.

Wielicki, B.A., Wong, T., Allan, R.P., Slingo, A., Kiehl, J.T., Soden, B.J., Gordon, C.T., Miller, A.J., Yang, S.-K., Randall, D.A., Robertson, F., Susskind, J. and Jacobowitz, H.  2002.  Evidence for large decadal variability in the tropical mean radiative energy budget.  Science 295: 841-844.