Background
The author 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)." In addition, he reiterates the long-held climate-model-derived notion that "an intensification of the water cycle may lead to changes in water-resource availability," i.e., "floods and droughts," as well as "an increase in the frequency and intensity of tropical storms."

What was done
Huntington explores these several theoretical expectations via a review of the current state of science regarding historical trends in hydrologic variables, including precipitation, runoff, soil moisture and a number of other parameters.

What was learned
"On a globally averaged basis," according to Huntington, "precipitation over land increased by about 2% over the period 1900-1998 (Dai et al., 1997; Hulme et al., 1998)." He also notes that "an analysis of trends in world continental runoff from major rivers from 1910-1975 found an increase in runoff of about 3% (Probst and Tardy, 1987)," and that a recent re-analysis of these trends for the period 1920-1995 "confirmed an increase in world continental runoff during the 20th century (Labat et al., 2004)."

All of these findings suggest that global warming may indeed have intensified the global hydrologic cycle over the course of the 20th century. However, Huntington also reports that "the empirical evidence to date does not [our italics] consistently support an increase in the frequency or intensity of tropical storms and floods." As for droughts, he says the "evidence indicates that summer soil moisture content has increased [our italics] during the last several decades at almost all sites [our italics] having long-term records in the Global Soil Moisture Data Bank (Robock et al., 2000)."

What it means
Over the totality of earth's land area, there appears to have been a slight intensification of the hydrologic cycle throughout the 20th century, which may or may not have been caused by the concomitant warming of the globe; but it also appears there was no intensification of deleterious weather phenomena such as tropical storms, floods and droughts. In addition, the study of Smith et al. (2006) demonstrates that over the period 1979 to 2004, when climate alarmists claim the planet experienced a warming that was unprecedented over the past two millennia, there was no net change in global precipitation (over both land and water). Consequently, several of the most basic "theoretical expectations" of the climate modeling enterprise appear to have no real-world support in 20th-century hydrologic data.

References
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.

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.

Labat, D., Godderis, Y., Probst, J.L. and Guyot, J.L. 2004. Evidence for global runoff increase related to climate warming. Advances in Water Resources 27: 631-642.

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.

Probst, J.L. and Tardy, Y. 1987. Long range streamflow and world continental runoff fluctuations since the beginning of this century. Journal of Hydrology 94: 289-311.

Robock, A., Konstantin, Y.V., Srinrivasan, J.K., Entin, J.K., Hollinger, N.A., Speranskaya, N.A., Liu, S. and Nampkai, A. 2000. The global soil moisture data bank. Bulletin of the American Meteorological Society 81: 1281-1299.

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.