Concerns have been raised in a number of venues about the implications of climate change with respect to hydrologic extremes, including floods and droughts (IPCC, 2007a,b; CCSP, 2008; Milly et al., 2008). The conventional wisdom is that greenhouse warming will result in an increased moisture load within the atmosphere, reflecting well-established physical principles embodied by the Clausius-Clapeyron relation (Box 1). Greater atmospheric moisture in turn supports “acceleration” of the hydrologic cycle, with postulated increases in the mean state and extremes of key hydrologic fluxes such as precipitation, evapotranspiration, tropospheric water vapor content, and runoff (Trenberth, 2011). These changes are often associated with a potential increase in the intensity, frequency, and/or duration of major storms (e.g., hurricanes) that result in a wide spectrum of adverse consequences, such as wind damage, erosion and sedimentation, landslides, and mudslides. These accelerations, however, do not interact uniformly with the general circulation of the atmosphere, topography, and proximity of land systems to the oceans. Thus, the allied postulation discussed here is that frequency and severity of floods and droughts will increase.
The Clausius-Clapeyron Relation
The Clausius-Clapeyron relation is a basic physical law that characterizes the transition between two given phases of matter, in this context the transition between water vapor and liquid water. It is a mathematical equation that, when applied, tells us that the water holding capacity of Earth’s atmosphere increases by about 7 percent per degree Celsius increase in temperature (or 4 percent per degree Fahrenheit). In other words, air holds more water at higher temperatures. Thus as the planet warms, more moisture is available for storm events, for example.
The expected changes in precipitation inferred from theoretical knowledge are reasonably well simulated with global climate models (NRC, 2010b) and are confirmed by observations of more intense precipitation and more severe drought worldwide compared to the past 40 to 50
years (Trenberth, 1999; Groisman et al., 2005; Kharin et al., 2007; NRC, 2010) and by increases in precipitation levels in the United States over the 20th century (Groisman et al., 2004). Yet a clear picture of how precipitation translates into the hydrologic extremes is frustrated by observations and studies made by the U.S. hydrologic science community. Recent analyses of U.S. Geological Survey (USGS) long-term streamflow records show few statistically significant trends in floods from annual maximum streamflows as a result of intense precipitation within the United States (USGS, 2005). Evidence for changes in droughts in the United States, determined by the balance between precipitation and runoff, is mixed. Trends of increasing precipitation across much of the eastern and central United States appear to have reduced drought severity and length, while a general warming in parts of the West appears to have increased atmospheric evaporative demand more rapidly than precipitation, resulting in longer and more frequent and severe droughts (Groisman et al., 2004; Andreadis and Lettenmaier, 2006).
Floods and droughts are also complicated by the presence of other factors and are not simply climate-driven phenomena. Anthropogenic land-cover change such as deforestation and reforestation, urban expansion, and the pervasive impact of water engineering—impoundment, irrigation, and water diversions, as well as other social factors—confound these signals of change (Vörösmarty et al., 2005; Trenberth, 2011). Yet floods and droughts remain a primary concern for water managers. In the context of these factors, there is a pressing need for decision-makers to better understand the complexity of these interactions and to recognize the limits and opportunities of the current knowledge base upon which their decisions will rest. The implications for water management, agriculture, and other sectors of the U.S. economy, especially in light of widely publicized predictions of increased frequency and severity of hydrologic extremes as the climate warms, have yet to be fully articulated.
The workshop, Global Change and Extreme Hydrologic Events: Testing Conventional Wisdom, was convened by the NRC Committee on Hydrologic Science in January 2010 to probe the conventional wisdom surrounding the acceleration of the hydrologic cycle and its implications. The workshop, sponsored by the U.S. Nuclear Regulatory Commission, the National Aeronautics and Space Administration, and the National Oceanic and Atmospheric Administration, provided a forum for the science and engineering applications communities to identify differing perspectives and to seek common ground on the issue of climate-change-induced floods and droughts. In addition, the workshop provided an opportunity to recognize and potentially begin to transcend the array of contrasting definitions, scientific agendas, methodologies, and observations that separate the climate science, hydrologic, and engineering applications communities as they address the hydrologic extremes question. The statement of task, organized as a series of questions was as follows:
1. Is the global hydrologic cycle accelerating and what does this acceleration look like? Is precipitation becoming more intense? Is drought frequency and severity becoming more prominent?
2. Are hydrologic fluxes associated with floods and droughts changing at the regional scale?
3. Floods and droughts from a climatologic and hydrologic perspective—how do we reconcile the two?
4. How does the science compare to the public debate?
Climate scientists observing trends in atmospheric dynamics and operating global circulation models were invited to speak along with hydrologists who study the local- to regional-scale movements and distributions of water, focusing on surface and subsurface processes across the landmass (see Appendix D for a summary of the presentations). As a result, workshop participants were presented with global, national, and regional perspectives. U.S. water managers, who routinely seek to translate science into water management solutions, and representatives from several U.S. federal agencies also attended the workshop. Thus, this report strongly reflects a U.S. perspective; it should be recognized that the United States is unique in the overall increases in precipitation that have occurred and in the water infrastructure in place (IPCC, 2001 and others). Source material for this workshop report was drawn from both formal presentations and breakout sessions, which directly engaged speakers, committee members, and other workshop participants in discussion, as well as from the committee’s deliberations.
This document is a synthesis of the workshop and the committee’s findings pertaining to the statement of task. The first section, Characterizing the Conventional Wisdom, provides an overview of the state of the science and probes whether the evidence supports ongoing changes in the frequency and severity of various hydrologic extremes (Tasks 1 and 2). The section on Translating the Science of Hydrologic Extremes to the Policy and Management Sectors examines gaps between the science and management sectors. Both sections draw heavily upon information gathered and discussed at the workshop. Finally, in the third section, A Way Forward, the committee identifies possible steps forward using the knowledge and perspectives gained from the workshop, which includes a challenge for the hydrologic community to promote the translation of research findings into planning and applications. The second and third sections address Tasks 3 and 4.