National Academies Press: OpenBook
« Previous: Summary
Suggested Citation:"Introduction." National Research Council. 2011. Global Change and Extreme Hydrology: Testing Conventional Wisdom. Washington, DC: The National Academies Press. doi: 10.17226/13211.
×

Introduction

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.

BOX 1
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

Suggested Citation:"Introduction." National Research Council. 2011. Global Change and Extreme Hydrology: Testing Conventional Wisdom. Washington, DC: The National Academies Press. doi: 10.17226/13211.
×

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?

Suggested Citation:"Introduction." National Research Council. 2011. Global Change and Extreme Hydrology: Testing Conventional Wisdom. Washington, DC: The National Academies Press. doi: 10.17226/13211.
×

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.

Suggested Citation:"Introduction." National Research Council. 2011. Global Change and Extreme Hydrology: Testing Conventional Wisdom. Washington, DC: The National Academies Press. doi: 10.17226/13211.
×
Page 3
Suggested Citation:"Introduction." National Research Council. 2011. Global Change and Extreme Hydrology: Testing Conventional Wisdom. Washington, DC: The National Academies Press. doi: 10.17226/13211.
×
Page 4
Suggested Citation:"Introduction." National Research Council. 2011. Global Change and Extreme Hydrology: Testing Conventional Wisdom. Washington, DC: The National Academies Press. doi: 10.17226/13211.
×
Page 5
Next: Findings »
Global Change and Extreme Hydrology: Testing Conventional Wisdom Get This Book
×
Buy Paperback | $27.00 Buy Ebook | $21.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

Climate theory dictates that core elements of the climate system, including precipitation, evapotranspiration, and reservoirs of atmospheric and soil moisture, should change as the climate warms, both in their means and extremes. A major challenge that faces the climate and hydrologic science communities is understanding the nature of these ongoing changes in climate and hydrology and the apparent anomalies that exist in reconciling their extreme manifestations.

The National Research Council (NRC) Committee on Hydrologic Science (COHS) held a workshop on January 5-6, 2010, that examined how climate warming translates into hydrologic extremes like floods and droughts. The workshop brought together three groups of experts. The first two groups consisted of atmospheric scientists and hydrologists focused on the scientific underpinnings and empirical evidence linking climate variability to hydrologic extremes. The third group consisted of water managers and decision-makers charged with the design and operation of water systems that in the future must be made resilient in light of a changing climate and an environment of hydrologic extremes.

Global Change and Extreme Hydrology summarizes the proceedings of this workshop. This report presents an overview of the current state of the science in terms of climate change and extreme hydrologic events. It examines the "conventional wisdom" that climate change will "accelerate" the hydrologic cycle, fuel more evaporation, and generate more precipitation, based on an increased capacity of a warmer atmosphere to hold more water vapor. The report also includes descriptions of the changes in frequency and severity of extremes, the ability (or inability) to model these changes, and the problem of communicating the best science to water resources practitioners in useful forums.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!