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Updated 12 October, 2003
Consequences of Global Warming for the Northwestern US: Water Resources
and Marine Ecosystems
USGCRP Seminar, 19 May 1998
Ms. Melissa Taylor
Dr. Dennis P. Lettenmaier
Dr. Nathan Mantua
How can we assess the impacts of climate change on the natural resources in any given region? Lessons learned from the past century of observed climate impacts on sectors like hydrology and fisheries serve as real-life measures for vulnerabilities and sensitivities to changing climate parameters.
For at least the past century of climate variability in the Pacific Northwest, El Ni -- o-Southern Oscillation (ENSO) has been a major driver at year-to-year time scales, while the Pacific Decadal Oscillation (PDO) has contributed strongly at decade-to-decade time scales. Spatially, the ENSO and PDO patterns have similar signatures in the Pacific Northwest: warm phases of both patterns are associated with mild winter land and coastal sea surface temperatures, and below average precipitation, snowpack and streamflows; cold phases of ENSO and PDO typically bring cool land and sea surface temperatures, above average snowpack, and abundant water supplies.
Of the potential effects of global warming, the implications for hydrology (the natural system by which precipitation makes its way into streams and eventually the oceans) and water resources (the "built" or managed system that makes freshwater available for human uses) are among the most important to society. In many parts of the world, including much of the U.S., the demand for consumptive (e.g., water supply) and non-consumptive (e.g., navigation, hydroelectric power generation, industrial cooling, instream flow) supplies of fresh water is barely balanced by sustainable surface and groundwater sources. In addition, water is essential for crop growth, and water management is an important factor in the reliability and sustainability of food supplies.
The hydrology of the Pacific Northwest, like much of the Western U.S., is dominated by two factors: a Mediterranean climate (most precipitation occurs in the winter months) and a dominant influence of topography, with much of the precipitation at high elevations occurring as snow in winter, which does not contribute to stream flow until the following spring or summer. The hydroclimatology of the region is also strongly affected by the Cascade Mountains. Rivers east of the Cascades are dominated by spring snowmelt, with seasonal peak flows occurring in late spring and early summer. Rivers on the west side are dominated by winter rains, augmented by spring snowmelt at higher elevations, so that many streams have both a winter and spring peak. Furthermore, most major floods on the west side occur as a result of intense rains and snowmelt in the late fall, while east side floods occur as a result of a mixture of winter rain-on-snow events and spring melt.
The water requirements of the Pacific Northwest are met primarily by surface water. The major river in the region, the Columbia, is managed by an extensive system of over 100 reservoirs, which is operated for irrigation, flood control, hydropower, navigation, recreation, and fisheries protection and enhancement. Nonetheless, the total storage in the Columbia River reservoirs is equivalent to only about a third of the river's mean annual flow. Therefore, the reservoir system acts primarily to store water from the spring high flow period to be released during the summer and fall; it does not, to any significant extent, store water from one year to the next. Reservoirs on smaller west-side streams are likewise small relative to the mean annual flow of the rivers. West-side streams are operated primarily for municipal water supply, fisheries protection and enhancement, and hydropower.
Climate-induced changes in marine ecosystems trigger a cascade of ecological impacts throughout the marine food-web. Such impacts are often most visible in their impacts on higher-order predators like sea birds, marine mammals, and commercially popular fish stocks. The effects of anthropogenic climate change (greenhouse warming) on marine ecosystems will most likely occur via multi-scale atmosphere/ocean circulation changes, and not by direct (radiatively driven) heating of the oceans.
From the perspective of marine ecosystems, Pacific interdecadal climate shifts between warm and cold phases (Pacific Decadal Oscillation) have been linked to decade-to-decade changes in Pacific salmon production from western Alaska all the way to central California. In addition, shorter-lived El Ni -- o-related changes to the marine environment have caused temperature and spatial dislocations of many open-ocean bird and fish species, as well as important changes in overall ecosystem productivity. Common to both El Ni -- o- and PDO-related marine climate fluctuations are regionally specific swings in primary and secondary productivity (via phyto- and zooplankton production, respectively) that trigger a cascade of ecological impacts throughout the marine food web.
Generally speaking, processes important to marine ecosystems take place at regional and smaller scales. Global-scale climate models now used to investigate the impacts of increased concentrations of greenhouse gases globally, are not yet as useful at these regional and smaller scales.
Based upon observed climate impacts on marine ecosystems, the following impacts are likely to occur as a response to future anthropogenic climate change: 1) species distributions will change; 2) there will be winners and losers; warm phases of the PDO correspond to high productivity in the Gulf of Alaska and low productivity in the California Current (vice-versa with the cold phase of the PDO); 3) ecosystem surprises are to be expected; and 4) if present day El Nino and PDO-like warm episodes are a model for future climate changes, warm water pelagic fish (e.g., albacore, mackerel, sardines) will become more common in nearshore and higher latitude waters of the northeastern Pacific.
The dominant effect of a warmer climate on the streams of the Pacific Northwest would be that less wintertime precipitation would fall as snow and more would fall as rain, resulting in decreased snowpack accumulation, and therefore increased winter flows and decreased spring and summer flows. This pattern would alter the flow patterns away from spring peaks and toward a rainfall-dominated peak in the winter. This change would, in general, create more stress on reservoir systems, as the natural storage of snowpacks would have to be replaced with reservoir storage to meet current water demands. Furthermore, the potential would exist for increased fall and winter flooding, especially in west-side streams, and perhaps in some smaller east side streams that are not generally susceptible to winter floods in the current climate. Consequences of these changes for water resource management include the need for more deliberate spillage in west-side rivers and the possibility for decreased water supply in summer, given reservoir storage limitations.
In order to understand these consequences more fully, the climate scenarios from three atmospheric General Circulation Models (GCMs) were used in conjunction with regional hydrologic and reservoir models to assess the impacts of the scenarios on Pacific Northwest hydrology and water resources. Notwithstanding that current GCMs cannot provide detailed regional-scale, watershed-specific information, modeling studies can, nonetheless, indicate the general nature of the response of the hydrology, and managed water resource systems, to changes in the region's climate.
The model studies show that the projected shifts in the timing of runoff (associated primarily with temperature), and volumes of runoff (associated primarily with changes in precipitation) would have important implications for energy production, fish protection, and irrigation water supply in the region. The changes in seasonal timing of runoff associated with the warmer climate scenarios tend to be advantageous for hydropower production during the winter high demand period, but may jeopardize subsequent reservoir refill and hydropower production for the following year. In one of the climate scenarios, however, considerably reduced precipitation would result in failure to meet firm-energy production requirements more often under current climate conditions. The changes in the seasonal pattern of streamflow generally would have negative implications for fish protection, especially, for instance, in terms of the reliability of the Columbia River reservoir system to meet the statutory minimum flow requirements for McNary Dam.
The ability of the reservoir system to meet irrigation demands generally would decline under the climate warming scenarios; particularly those accompanied by significant decreases in streamflow volumes. These changes would be especially important in the upper Snake River basin, in part because of the high irrigation demands there, and in part because the seasonal pattern of Snake River streamflows is more sensitive to climate warming than is the main stem of the Columbia. Recreation would be impacted as well. Recreation benefits for the Columbia River reservoirs depend on high reservoir levels during the summer, targets which would be more difficult to meet with reduced spring streamflows. On the other hand, the severity of spring floods would generally be reduced in the Columbia River system.
A growing body of research has shown a close connection between fluctuations in the northeastern Pacific marine ecosystems and large scale features of Pacific climate. Large amplitude, year-to-year climate fluctuations, often associated with El Nino/La Nina, have dramatic impacts on marine ecosystems in the northeast Pacific. Typical El Nino-related environmental changes include a warming of the coastal upper ocean, raised sea levels, increased poleward coastal currents, and a deepening of the ocean surface layer. Off the west coast of the continental US, these frequent warming events often lead to a reduction in phytoplankton and zooplankton production, which in turn sets the stage for dramatic crashes in overall fishery productivity. Large die-offs have been observed among higher-level predators like sea-birds, marine mammals, and some salmon populations during the strong climate warming events of 1983 and 1997/98.
Perhaps even more important to northeastern Pacific marine ecology are the decade-to-decade environmental shifts associated with the Pacific (inter)Decadal Oscillation (PDO). The PDO has been described as an interdecadal El Nino-like pattern of climate variability. Warm phases of the PDO bring decadally-persistent El Nino-like environmental changes. Long-lived (20 to 30 year) climate fluctuations associated with the PDO have been linked to dramatic and persistent changes in the large marine ecosystems of the North Pacific Ocean. Since the late 1970's (the last switch from cold to warm PDO regimes) these changes include crashes in Alaska Murre (sea-birds) and Stellar Sea Lion populations, significant reductions in Halibut growth rates, sharp declines in Alaska King Crab and shrimp fisheries, altered salmon migration routes, and an era characterized by record salmon production in Alaska but very low salmon production in Washington, Oregon, and California.
Climate-related changes to streamflow regimes will also play a major role in determining the future of Pacific salmon. The PDO-related changes in salmon abundance previously noted are thought to result mostly from changes in the marine environment. For Alaska salmon, the typical positive PDO year brings enhanced streamflows and nearshore ocean conditions favorable to high productivity. Generally speaking, the converse appears to be true in the Pacific Northwest.
The specter of a greenhouse climate with warmer, wetter winters and warmer, drier summers in the Pacific Northwest suggests significantly reduced snowpack. Such streamflow regimes would be less favorable for salmon than those now observed with El Nino and PDO. Such scenarios paint a picture of an increased frequency of scouring, nest-damaging fall and winter floods, with reduced flows and elevated stream temperatures in the critical low flow summer periods.
Dr. Dennis Lettenmaier is a hydrologist with interests in continental and global-scale land surface hydrology, and smaller-scale sensitivity of catchment hydrologic processes to land cover change. He is presently involved in several projects seeking to improve the representation of the land surface, especially the representation of streamflow and evapotranspiration, in climate and numerical weather prediction models. He is a Professor of Civil Engineering at the University of Washington, where he has been on the faculty since 1976.
Since 1995 Dr. Lettenmaier has worked with NOAA's Joint Institute for the Study of the Atmosphere and Oceans (JISAO) at the University of Washington, where he participates in a NOAA Global Change project, “An Integrated Assessment of the Dynamics of Climate Variability, Impacts, and Policy Response Strategies for the Pacific Northwest”, as the hydrology and water resources team leader. He has participated in several assessments of the effects of climate change on hydrologic and water resources, including the 1989 EPA Report to Congress, for which he directed the study on California Water Resources, and a recent U.S. Army Corps of Engineers study of the climatic sensitivity of six water resources systems throughout the continental U.S. He is a Fellow of the American Geophysical Union and the American Meteorological Society. Dr. Lettenmaier received his Ph.D. degree from the University of Washington's Department of Civil Engineering in 1975.
Dr. Nathan Mantua is an atmospheric scientist whose interests are in understanding ocean-atmosphere climate dynamics. He is presently involved in interdisciplinary studies related to climate variability, seasonal-to-interannual climate prediction, and the human and ecological dimensions of climate change.
Since 1995 Dr. Mantua has worked with NOAA's Joint Institute for the Study of the Atmosphere and Oceans (JISAO) at the University of Washington where he has played a key role in a NOAA/ Global Change project titled “An Integrated Assessment of the Dynamics of Climate Variability, Impacts, and Policy Response Strategies for the Pacific Northwest.” This study is an interdisciplinary effort focused on understanding the role of climate information in resource management. The JISAO team is investigating both short- and long-term climate issues, the former in terms of the use of seasonal-to-interannual climate predictions, the latter in assessing the Pacific Northwest's vulnerability to potential anthropogenic climate change.
Dr. Mantua has also had a life-long involvement with the commercial and sport salmon fishing industries, and as a result, has a unique understanding of the connections between climate and fishery science. His versatility in these two fields has led to a number of collaborations with fisheries scientists at the University of Washington and at other research institutions. He was recently appointed to the Scientific Steering Committee (SSC) for the international Global Oceans Ecosystems Dynamics (GLOBEC) program, a NOAA/NSF (Global Change)-sponsored effort devoted to better understanding the role of climate variations in marine ecosystems. Dr. Mantua received his Ph.D. degree from the University of Washington's Department of Atmospheric Sciences in 1994, and shortly thereafter, was awarded a NASA Global Climate Change Fellow.