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Floods (General) -- Summary
Climate alarmists predict there will be more frequent and severe flooding around the world in response to global warming. But are they correct? Perhaps the best way to answer this question is to determine what has happened in the past in this regard; for if floods were either more or less frequent and severe during prior times of notable global warmth (or the opposite during times of notable global cold conditions), such situations could well occur again. Hence, we here review the findings of several studies that have taken this approach to the subject in analyses that apply to the world as a whole. For findings pertaining to specific continents, see the other sub-headings under the general category of Floods in our Subject Index.

Most recently, Huntington (2006) has noted 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."

Exploring 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, Huntington found 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)," 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)."

Over the totality of earth's land area, therefore, there does appear 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. However, the study of Smith et al. (2006) demonstrates that over the period 1979 to 2004, when climate alarmists claim the earth experienced a warming that was unprecedented over the past two millennia, there was no net change in global precipitation, i.e., the sum of precipitation over the planet's land and water surfaces).

In another effort to evaluate climate model projections of increased floods (and contemporaneous droughts!) associated with global warming, Svensson et al. (2005) examined 20th-century river flow data for a group of 21 stations distributed about the globe whose record lengths varied from 44 to 100 years, with an average of 68 years. This effort revealed there were slightly more stations with significant negative trends (reduced flooding) than significant positive trends (increased flooding), while nearly all stations showed increasing low-flow trends, approximately half of which were significant at the 90% level. These results, according to Svensson et al., indicate "there is no general pattern of increasing or decreasing numbers or magnitudes of floods, but there are significant increases in half of the low-flow series," which observations are totally inconsistent with model predictions of increased flooding and drought in response to global warming; and if the "unprecedented" temperature increase of the 20th century could not produce the model-predicted outcome, there is little reason to believe that further warming will either.

In a discussion of the issue with a more philosophical bent, Baker (2004) reports "it is generally assumed that various environmental changes, including hydrological ones, can be anticipated as logical consequences of ... predicted climate change," and that "these predictions are presumed to provide the rock-solid foundation for policies to cope with the change." It is his feeling, however, that "these methods of international global change science are highly skewed toward a rather particular set of philosophical presumptions," and that "a more appropriate organization to the global change research programme would not only make more effective use of palaeoflood hydrology, it would also much more optimally benefit humankind."

In an analysis of this procedural dichotomy, Baker compares the two different ways of assessing future flood risks. The approach that is favored by the IPCC uses what he calls "idealized conceptual models," which predict future flood behavior on the basis of global climate model (GCM) predictions of CO2-induced global warming and how that warming is projected to alter the planet's hydrologic cycle. The other approach seeks to determine how real-world flooding in ages past responded to different global temperature regimes; and it assesses future flood risks on the assumption that floods of the future will likely reflect the consequences of a similar hydrologic response to projected changes in the thermal regime of the planet.

With respect to the first of these approaches, Baker reports that it leads to "the claim that the magnitudes of extreme floods may be increasing because of global warming." With respect to the second, he notes that a preliminary survey of historical and palaeoflood records suggests that "the floods of recent years do not generally exceed ... those of past clusters, and much larger floods are usually indicated in the past," during periods such as the Little ice Age, when it was much colder than it is currently. Consequently, the two approaches produce diametrically opposed predictions about the future.

So what is one to believe? Does one accept predictions about the future that are contrary to relationships known to have existed in the past? Or does one go with the method that has a proven track record? Baker concludes that "for 20 years there has been an overemphasis on developing [GCMs] as the principal tool for dealing with threats to habitability of the planet," stating that "in contrast to unverified predictions of the future given by GCMs, palaeoflood data provide evidence of real-world cataclysms that people can understand sufficiently to alter their perceptions of hazards." And as we read the palaeorecord, it would appear that flooding will become less extreme than it is now if the planet continues to warm.

Of much the same philosophical persuasion is Pielke (1999), whose paper near the end of the 20th century discusses the oft-reported fallacy that "damaging flooding in recent years is unprecedented because of 'global warming'." He notes, for example, that the media has been prone to associate "almost every extreme weather event with global warming," and that flooding has been no exception. However, as he further indicates, "it is essentially impossible to attribute any particular weather event to global warming," but he notes there are several important non-climatic factors that will have the potential to influence flooding in the future, including deteriorating dams and levees, changes in land use, building in flood-prone areas, governmental policies, and other societal influences. These are things about which we really can do something - and should! - rather than wasting our time, energy and resources tilting towards windmills (and the like) as replacements for fossil fuels.

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.

Baker, V.R. 2004. Palaeofloods and global change. Journal of the Geological Society of India 64: 395-401.

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.

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

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.

Pielke, R.A., JR. 1999. Nine fallacies of floods. Climatic Change 42: 413-438.

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.

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.

Svensson, C., Kundzewicz, Z.W. and Maurer, T. 2005. Trend detection in river flow series: 2. Flood and low-flow index series. Hydrological Sciences Journal 50: 811-824.

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

Last updated 11 June 2008