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Climate Model Problems: II. Hydrologic Cycle
Volume 11, Number 1: 2 January 2008

In this second of our continuing series of editorials on climate model problems, we focus on earth's hydrologic cycle, reviewing the work of Allan and Soden (2007), who quantify trends in precipitation within ascending and descending branches of the planet's tropical circulation and compare their results with simulations of the present day and projections of future changes provided by up to 16 state-of-the-art climate models. The precipitation data for this analysis come from the Global Precipitation Climatology Project (GPCP) of Adler et al. (2003) and the Climate Prediction Center Merged Analysis of Precipitation (CMAP) data of Xie and Arkin (1998) for the period 1979-2006, while for the period 1987-2006 they come from the monthly mean intercalibrated Version 6 Special Sensor Microwave Imager (SSM/I) precipitation data described by Wentz et al. (2007).

So what did the researchers learn?

Allan and Soden report that "an emerging signal of rising precipitation trends in the ascending regions and decreasing trends in the descending regimes are detected in the observational datasets," but that "these trends are substantially larger in magnitude than present-day simulations and projections into the 21st century," especially in the case of the descending regions. More specifically, they state that, for the tropics, "the GPCP trend is about 2-3 times larger than the model ensemble mean trend, consistent with previous findings (Wentz et al., 2007) and also supported by the analysis of Yu and Weller (2007)," who additionally contend that "observed increases of evaporation over the ocean are substantially greater than those simulated by climate models." What is more, Allan and Soden note that "observed precipitation changes over land also appear larger than model simulations over the 20th century (Zhang et al., 2007)."

What is one to make of this conflict between models and measurements?

Noting that the difference between the two "has important implications for future predictions of climate change," Allan and Soden say "the discrepancy cannot be explained by changes in the reanalysis fields used to subsample the observations but instead must relate to errors in the satellite data or in the model parameterizations [our italics]." This same dilemma was also faced by Wentz et al. (2007), as described in our Editorial of 15 August 2007; and they too stated that the resolution of the issue "has enormous impact," but likewise concluded that the questions raised by the discrepancy "are far from being settled."

To us, the issue seems a bit less difficult. Given a choice between model simulations and observational reality, we will cast our lot with the latter every chance we get. Granted, this choice implies a huge problem with the former. But why should that be a surprise to anyone? The earth, with its oceans and atmosphere, and its myriad life forms, is a most complex place; and to believe that we have condensed all of its many climate-related phenomena -- many of which are shrouded in mystery, and some of which may even remain undetected -- to a set of equations that rigorously define our climatic future in response to an increase in but a single factor, i.e., anthropogenic CO2 emissions, seems to us to be irrationality incarnate ... and to base policy on it, the height of folly.

Sherwood, Keith and Craig Idso

References
Adler, R.F. et al. 2003. The version-2 Global Precipitation Climatology Project (GPCP) monthly precipitation analysis (1979-present). Journal of Hydrometeorology 4: 1147-1167.

Allan, R.P. and Soden, B.J. 2007. Large discrepancy between observed and simulated precipitation trends in the ascending and descending branches of the tropical circulation. Geophysical Research Letters 34: 10.1029/2007GL031460.

Wentz, R.J., Ricciardulli, L., Hilburn, K. and Mears, C. 2007. How much more rain will global warming bring? Science 317: 233-235.

Xie, P. and Arkin, P.A. 1998. Global monthly precipitation estimates from satellite-observed outgoing longwave radiation. Journal of Climate 11: 137-164.

Yu, L. and Weller, R.A. 2007. Objectively analyzed air-sea heat fluxes for the global ice-free oceans (1981-2005). Bulletin of the American Meteorological Society 88: 527-539.

Zhang, X. et al. 2007. Detection of human influence on twentieth-century precipitation trends. Nature 448: 461-465.