Evidence for intensification of the global water cycle: Review and synthesis
Introduction
There is a general consensus that global average surface air temperature increased during the 20th century, and, although there is great uncertainty about the magnitude of future increases, most assessments indicate that future warming is ‘very likely’ (Houghton et al., 2001, NAST, 2001, ACIA, 2004). There is also 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). The theoretical basis for this intensification is summarized in the Clausius–Clapyeron relation that implies that specific humidity would increase approximately exponentially with temperature. Recent modeling studies suggest that as a consequence of this relation precipitation would increase by about 3.4% per degree Kelvin (Allen and Ingram, 2002). The slope of this relation is substantially less than the 6.5% per degree Kelvin indicated by the Clausius–Clapyeron relation because evaporation would be energy limited (Allen and Ingram, 2002).
It is now well established that surface air temperatures and precipitation over land have increased during the 20th century (Folland et al., 2001). Results from recent simulations using one of about 20 coupled ocean–atmosphere–land models based on the IS92A mid-range emission scenario indicate that global mean surface air temperature, precipitation, evaporation, and runoff will increase 2.3 °C, 5.2, 5.2, and 7.3%, respectively, by 2050 (Wetherald and Manabe, 2002). As global temperature increased considerably over the 20th century, especially since the 1970s, (Jones and Moberg, 2003), it is reasonable to ask whether trends in hydrologic variables and related indicators are consistent with an intensified water cycle during that period. Consistency among indicator variables would greatly strengthen our confidence in projections of the potential vulnerability of water resources that could be caused by future temperature increases. Some aspects of an intensified water cycle, such as more frequent occurrence of extreme events, are a potential threat to at-risk populations. Tropical storms, floods, and droughts can affect human welfare directly through catastrophic damage or indirectly through adverse effects on crop productivity. Such threats are likely to occur disproportionately in developing countries with the fewest resources for mitigation and adaptation (Arnell et al., 2001, Manabe et al., 2004).
Several lines of evidence can be used to assess the hypothesis of a warming-induced intensification of the water cycle. The trend analyses reviewed in this paper were conducted at various spatial and temporal scales that are not directly comparable among studies or variables. In spite of these limitations, synthesizing the results from a number of studies can provide an insight into the response of the water cycle to past and future climatic changes. This article briefly reviews the evidence from time-series analysis of several hydroclimatic variables to assess whether there have been systematic changes in the 20th century. Some of this information was summarized in the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR) (Folland et al., 2001), but many new reports are now available that provide a more comprehensive picture. The IPCC TAR includes analysis of changes in precipitation, stream runoff, frequency and intensity of extreme events, atmospheric water vapor, cloudiness, pan evaporation, and soil moisture. New insights are now available relating to world continental runoff, tropospheric water vapor, El Nino Southern Oscillation (ENSO), glacier mass balance, evaporation, and evapotranspiration (ET). In addition, a growing body of research on hydrologic and phenological indicators shows that the length of the growing season has increased substantially suggesting that, at least in humid regions, ET has been affected.
Section snippets
Trends in hydrologic variables
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). Regional variations are highly significant. For example, zonally averaged precipitation increased by 7–12%, between 30°N and 85°N, compared with a 2% increases for 0°S–55°S, and has decreased substantially in some regions (Folland et al., 2001). Groisman et al. (2004) reported increases in precipitation over the conterminous USA during the 20th century,
Trends in evaporation and evapotranspiration
Evaporation and evapotranspiration are central to the water cycle and long-term measurements of their annual rates would be excellent indicators of the intensity of the water cycle. Unfortunately, both are difficult to measure: to date, only a few dozen ET monitoring sites operate in the world, and the period of record is quite short. Other indirect approaches to estimate evaporation and ET, or variables directly related to ET, can provide insight into the likely trends in these variables.
Summary
Substantial uncertainty regarding trends in hydrolclimatic variables remains because of differences in responses among variables and among regions as well as major spatial and temporal limitations in data. There are large gaps in spatial coverage for some hydrologic indicators; therefore, trends in these areas are unknown. The risk in assuming that trends in these areas are comparable to those measured in other related areas is that the observed trends may simply represent regional
Acknowledgements
This work was supported by USGS funds for climate research. Richard Rebich, US Geological Survey, Barry Keim, Louisiana State University, and four anonymous reviewers provided helpful comments on earlier drafts of this article. Michael Roderick (The Australian National University). Todd Walter (Cornell University), Jean-Luc Probst and David Labat (CNRS, Toulouse, France), Pavel Groisman (NOAA), and Ruth Curry (Woods Hole Oceanographic Institute) provided helpful information. Aiguo Dai, National
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