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Precipitation (Trends - Global) -- Summary
Huntington (2006) notes there is "a theoretical expectation that climate warming will result in increases in evaporation and precipitation, leading to the hypothesis that one of the major consequences will be an intensification (or acceleration) of the water cycle (DelGenio et al., 1991; Loaciga et al., 1996; Trenberth, 1999; Held and Soden, 2000; Arnell et al., 2001)," and in reviewing the scientific literature on precipitation, he concludes that on a globally averaged basis, "precipitation over land increased by about 2% over the period 1900-1998 (Dai et al., 1997; Hulme et al., 1998)."

New et al. (2001) also reviewed several global precipitation data sets, analyzing the information they contain to obtain a picture of precipitation patterns over the 20th century. In their case, they determined that precipitation over the land area of the globe was mostly below the century-long mean over the first decade and a half of the record, but that it increased from 1901 to the mid-1950s, whereupon it remained above the century-long mean until the 1970s, after which it declined by about the same amount to 1992 (taking it well below the century-long mean), whereupon it recovered and edged upward towards the century mean. Hence, for the entire century, there was indeed a slight increase in global land area precipitation; but since 1915 there was essentially no net change.

For the oceanic portion of the world between 30N and 30S, however, the record of which begins in 1920, there was a discernable change in precipitation over the course of the record; however, it was an overall decrease of about 0.3% per decade. Hence, for the world as a whole, which is 70% covered by water, there may well have been a slight decrease in precipitation since about 1917 or 18.

Concentrating on the last half of the 20th century, Neng et al. (2002) analyzed data from 1948 to 2000 in a quest to determine the effect of warm ENSO years on annual precipitation over the land area of the globe. In doing so, they found that some regions experienced more rainfall in warm ENSO years, while others experienced less. However, in the words of the researchers, "in warm event years, the land area where the annual rainfall was reduced is far greater [our italics] than that where the annual rainfall was increased, and the reduction is more significant [our italics] than the increase." Consequently, whereas state-of-the-art climate models nearly always predict more precipitation in a warming world, the data of Neng et al.'s study depict just the opposite effect over the land area of the globe. Hence, with respect to one of the most basic of all climate-model predictions, there appears to be a total lack of vindication in the real world, where it really counts.

Most recently - and noting that "the Global Precipitation Climatology Project (GPCP) has produced merged satellite and in situ global precipitation estimates, with a record length now over 26 years beginning 1979 (Huffman et al., 1997; Adler et al., 2003)" - Smith et al. (2006) used empirical orthogonal function (EOF) analysis to study annual GPCP-derived precipitation variations over the period of record. In doing so, they found that the first three EOFs accounted for 52% of the observed variance in the precipitation data. Mode 1 was associated with mature ENSO conditions and correlated strongly with the Southern Oscillation Index, while Mode 2 was associated with the strong warm ENSO episodes of 1982/83 and 1997/98. Mode 3, on the other hand, was uncorrelated with ENSO but was associated with tropical trend-like changes that were correlated with interdecadal warming of tropical sea surface temperatures.

Globally, Smith et al. report that "the mode 3 variations average to near zero, so this mode does not represent any net change in the amount of precipitation over the analysis period." Consequently, over the period 1979-2004, when climate alarmists claim the world warmed at a rate and to a degree that was unprecedented over the past two millennia, Smith et al. found that most of the precipitation variations in their global data set were "associated with ENSO and have no trend." As for the variations that were not associated with ENSO and that did exhibit trends, they say that the trends were associated "with increased tropical precipitation over the Pacific and Indian Oceans associated with local warming of the sea." However, they note that this increased precipitation was "balanced by decreased precipitation in other regions," so that "the global average change [was] near zero."

Over the earth as a whole, therefore, it would appear from Smith et al.'s study, as well as from the other studies described above, that one of the major theoretical expectations of the climate modeling community remains unfulfilled, even under the supposedly highly favorable thermal conditions of the last quarter-century, which observation suggests that their other major theoretical expectation, i.e., catastrophic CO2-induced global warming, will likely remain unfulfilled too.

Adler, R.F., Susskind, J., Huffman, G.J., Bolvin, D., Nelkin, E., Chang, A., Ferraro, R., Gruber, A., Xie, P.-P., Janowiak, J., Rudolf, B., Schneider, U., Curtis, S. and Arkin, P. 2003. The version-2 global precipitation climatology project (GPCP) monthly precipitation analysis (1979-present). Journal of Hydrometeorology 4: 1147-1167.

Arnell, N.W., Liu, C., Compagnucci, R., da Cunha, L., Hanaki, K., Howe, C., Mailu, G., Shiklomanov, I. and Stakhiv, E. 2001. Hydrology and water resources. In: McCarthy, J.J., Canziani, O.F., Leary, N.A., Dokken, D.J. and White, K.S. (Eds.), Climate Change 2001: Impacts, Adaptation and Vulnerability, The Third Assessment Report of Working Group II of the Intergovernmental Panel on Climate Change, Cambridge, University Press, Cambridge, UK, pp. 133-191.

Dai, A., Fung, I.Y. and DelGenio, A.D. 1997. Surface observed global land precipitation variations during 1900-1998. Journal of Climate 10: 2943-2962.

DelGenio, A.D., Lacis, A.A. and Ruedy, R.A. 1991. Simulations of the effect of a warmer climate on atmospheric humidity. Nature 351: 382-385.

Held, I.M. and Soden, B.J. 2000. Water vapor feedback and global warming. Annual Review of Energy and Environment 25: 441-475.

Huffman, G.J., Adler, R.F., Chang, A., Ferraro, R., Gruber, A., McNab, A., Rudolf, B. and Schneider, U. 1997. The Global Precipitation Climatology Project (GPCP) combined data set. Bulletin of the American Meteorological Society 78: 5-20.

Hulme, M., Osborn, T.J. and Johns, T.C. 1998. Precipitation sensitivity to global warming: comparisons of observations with HadCM2 simulations. Geophysical Research Letters 25: 3379-3382.

Huntington, T.G. 2006. Evidence for intensification of the global water cycle: Review and synthesis. Journal of Hydrology 319: 83-95.

Loaciga, H.A., Valdes, J.B., Vogel, R., Garvey, J. and Schwarz, H. 1996. Global warming and the hydrologic cycle. Journal of Hydrology 174: 83-127.

Neng, S., Luwen, C. and Dongdong, X. 2002. A preliminary study on the global land annual precipitation associated with ENSO during 1948-2000. Advances in Atmospheric Sciences 19: 993-1003.

New, M., Todd, M., Hulme, M. and Jones, P. 2001. Precipitation measurements and trends in the twentieth century. International Journal of Climatology 21: 1899-1922.

Smith, T.M., Yin, X. and Gruber, A. 2006. Variations in annual global precipitation (1979-2004), based on the Global Precipitation Climatology Project 2.5 analysis. Geophysical Research Letters 33: 10.1029/2005GL025393.

Trenberth, K.E. 1999. Conceptual framework for changes of extremes of the hydrological cycle with climate change. Climatic Change 42: 327-339.

Last updated 9 May 2007