INTRODUCTION:

Dr. Roger Pulwarty
Program Manager, Regional Integrated Assessments, National Oceanic and Atmospheric Administration (NOAA), Silver Spring, MD

SPEAKERS:

Dr. Connie A. Woodhouse
Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO

Dr. David Rind
National Aeronautics and Space Administration/Goddard Institute for Space Studies (NASA/GISS), New York, NY

The drought that occurred this past summer, primarily in Oklahoma and Texas, had a devastating impact on agriculture and agriculture-based jobs. Losses to the two states are estimated to eventually top $7 billion dollars. This drought started in April and was over in most areas by the end of September. In contrast, the drought of the 1950s lasted five years and covered much of the southern and central Great Plains. The Dust Bowl drought of the 1930s lasted about seven years and in its worst year, almost 70% of the United States experienced severe or extreme drought conditions. Just how unique were the droughts of the 1930s and 1950s? Given the extensive impacts of more recent, less severe droughts, it is important to know whether these droughts are part of the naturally varying climate, occurring on a periodic basis, or if these droughts are indeed exceptional events.

Instrumental records of temperature and precipitation dating back one hundred years or less, exist for much of the United States. Such records however, are too short for assessing whether, for example, the droughts of the 1930s and 1950s are unusual or outside of the realm of natural climate variability for the Central U.S. Consequently, in the absence of regional-scale climate models, and in order to obtain a more complete picture of the natural climate variability of the Central United States, it is necessary to make use of "proxy" or indirect records of past climates obtained from tree rings, lake and sand dune sediments, riverbeds, and archaeological remains, to extend the instrumental records of climate variability back thousands of years. What has emerged from this effort is a long-term record of drought and drought variability for the Central U.S., derived from numerous proxy records, and spanning the last two thousand years.

The three main findings of this analysis are as follows.

1. The paleoclimatic record of drought for the central United States suggests that 1930s-magnitude droughts are not unusual and that the 20th century is representative of the frequency of occurrence of such droughts. Specifically, tree-ring reconstructions of precipitation/drought indicate that droughts similar in duration and extent to the droughts of the 1930s and 1950s have occurred once or twice per century for the past 400 years.

2. A longer climate record, made up of paleoclimate data from the western United States, as well as tree-ring data from the central United States, and other types of proxy climate information, suggests that two droughts, of much longer duration than any droughts experienced this century, occurred in the past 800 years; one lasting from about 1580-1600, and one that occurred in the last quarter of the 13th century.

3. Although far fewer paleoclimatic records exist back to the year AD 1, the beginning point of this study, those that do exist provide some evidence for a possible drought regime shift, that is, a fundamental change in the characteristics of drought and/or drought variability. These records suggest that prior to roughly AD 1300, droughts were longer and/or more frequent. Since this time, droughts, for the most part, have been characterized by moderate severity and have generally lasted less than a decade.

Assuming this 2,000-year record of drought variability provides a reasonable estimate of the natural climate variability one might anticipate in the central United States in the future, a Dust Bowl-scale drought once or twice per century would be a likely occurrence. The data also suggest the possibility of an even longer drought occurring in the future. Although scientists are beginning to understand what causes drought-promoting conditions to persist on the scale of seasons to a year or two, far less is known about what causes droughts of longer duration. Long-term droughts might be linked, for example, to relatively slow but persistent changes in Pacific and Atlantic Ocean sea-surface temperatures. However, more research is needed to understand the causes of long-term droughts and to develop the ability to predict such droughts.

Increased droughts are to be expected in a warmer world, and so are increased floods. A warmer atmosphere can hold more moisture, and evaporate more water from the surface. Thus, when it is not raining, available soil water should be reduced. When it is raining, it could very well rain harder. Most researchers agree then that a warmer world will have greater hydrologic extremes. In addition, there is a basic imbalance that develops as climate warms, between the loss of moisture from the soil by evaporation and replenishment via precipitation. The land has a smaller heat capacity than the ocean, so it should warm faster. Evaporation from the land proceeds at the rate of its warming, while precipitation derives primarily from evaporation at the ocean surface. As the latter is increasing more slowly, in a warmer world, precipitation will not increase as rapidly as evaporation due to the fact that the oceans warm more slowly than the land surface (evaporation over the ocean is slower than over the land). Hence, more droughts are anticipated in a warmer world, but the specific location of such droughts is somewhat uncertain.

To address the question of where droughts are likely to occur, one first needs to have a reasonable sense of what the future magnitude of warming will be, and what the latitudinal distribution of warming will be. For example, the greater the warming at high latitudes relative to low latitudes, the more likely there will be increased drought over the U.S. in summer. In contrast, substantial tropical warming could give us El Nino-like precipitation, with intensified flooding along the southern tier of the U.S. All of these conditions are likely to intensify as the global temperature rises.

Output from General Circulation Models (GCMs) has been used in an attempt to be more specific. Utilizing soil moisture output from the GCMs, the IPCC (Intergovernmental Panel on Climate Change) Working Group I (The Science of Climate Change) has focused on the moderate probability that summertime droughts will afflict middle latitudes (i.e., developed countries). IPCC Working Group II (Assessment of the Possible Impacts of Climate Change) using the temperature and precipitation output from GCMs has focused on the vegetative stress that GCMs project will occur at low and subtropical latitudes (hence, for developing countries). Clearly, available water (in the form of soil moisture) and the health of vegetation (productivity) are two related but somewhat different aspects of drought, for which an integrated assessment of their combined effects will likely give a more thorough picture of future drought possibilities.

What, therefore, can be concluded about the likelihood of future drought in any particular area? Each of the IPCC projections (GCM and Assessment model output) has a scientific basis for suggesting that developed and developing countries might be strongly affected. Least likely to suffer drought are high latitudes, where flooding would seem to be a more likely problem. Modeling improvements in both GCMs and Impact Assessment models are required before scientists can estimate more precisely where the effects of floods and droughts are likely to be most severe. In any case, because hydrological extremes should increase in a warmer world, being prepared for such events would seem to be a prudent strategy.

Dr. Connie A. Woodhouse is a Research Associate at the Institute of Arctic and Alpine Research (INSTAAR) at the University of Colorado, and also works at the National Oceanic and Atmospheric Administration's (NOAA) National Geophysical Data Center (NGDC) in Boulder, Colorado. Her research focus is on the reconstruction of past climates of the central and western United States using tree rings, and on the investigation of the relationships between regional climates and patterns of atmospheric circulation.

Dr. Woodhouse's work has been published in a number of peer-reviewed journals including the Bulletin of the American Meteorological Society, Climate Research, International Journal of Climatology, and theJournal of Climate. Her work on the paleoclimatological record of drought has recently been featured on CNN and CBS and in many national newspapers. Dr. Woodhouse is a member of the American Geophysical Union, the American Meteorological Society, the Association of American Geographers, and the Tree-Ring Society. She is a member of the advisory committee for the Western Water Initiative, sponsored by the Cooperative Institute for Research in Environmental Sciences (CIRES) and NOAA; and is a consultant for NOAA/NGDC World Data Center A for Paleoclimatology.

Dr. Woodhouse received her Ph.D. degree in Geosciences from the University of Arizona in 1996.

Acknowledgements: The work presented in this seminar is derived primarily from a recently published review article in the Bulletin of the American Meteorological Society (v. 79, pp. 2693-2714), with Jonathan T. Overpeck.

Dr. David Rind has been affiliated with the NASA Goddard Space Flight Center's Institute for Space Studies (GISS) in New York since 1976, where he is currently a Senior Climate Research Scientist. Since 1976 he has also been affiliated with the Department of Earth and Environmental Sciences of Columbia University, where he is now an Adjunct Full Professor.

Dr. Rind has served on various scientific panels, including the National Research Council committees on Earth System History, Solar Influences, and Global Change; the American Meteorological Society's Committee on Climate Change; and the NOAA Paleoclimate Advisory Panel. He has been a member of various science teams, including the SAGE II and SAGE III water-vapor measuring satellite instruments team; and the Earth Observing System's Interdisciplinary Teams investigating: climate variability, the mutual impact of climate, atmospheric chemistry and aerosols (CACTUS), and the forecasting of seasonal climatic impacts on agriculture (CAFE). He has been the recipient of such honors as the NASA Special and Superior Achievement Award, and has been an American Geophysical Union Charney Lecturer (Spring, 1995).

Dr. Rind is the author of 150 peer-reviewed scientific papers, specializing in a range of fields including climate and stratospheric dynamics, general circulation modeling, trace gas and solar climate forcing, paleoclimate studies, satellite remote sensing, land surface interactions, and sea ice and ocean modeling.

Dr. Rind received his Ph.D. from Columbia University in 1976.