Climate Change and Hydrological Impacts
The regional climate of the Pacific Northwest influences water temperatures, the flows of the Columbia River, and soil moisture and groundwater availability in the Columbia basin. The flows and temperature requirements for salmonids resources and threatened and endangered stocks should be evaluated in the context of historical and potential future variability and change in both water temperatures and streamflow. Prospective changes in climate are important, as climate shifts over the past 30 years have produced shifts in the distributions and abundance of many species and appear to be responsible for one species-level extinction (Thomas et al., 2004).
The regional climate influences water temperatures of the Columbia River basin. These water temperatures have been increasing over the last 45 years (1953 to 1998) in the Columbia River at a rate of about 0.38°C per decade or 1.9°C per 50 years (Figure 3-8). Some of this increase can arguably be accounted for by nonclimatic changes in the river basin such as dams and reservoirs, changes in land use, increases in water withdrawals, and other factors. However, the nearest river to the Columbia River of similar dimensions is the undammed Fraser River in Canada, which also has experienced temperature increases from 1953 to 1998 of about 0.2°C per decade or almost 1°C per 50 years (British Columbia Ministry of Water, Land and Air Protection). Average August temperatures of the Columbia River (Figure 3-8) are now about 5°C higher than the average summer temperatures of the Fraser River.
Historically, winter conditions contributing to winter snowpack, maximum streamflow in spring, and maintenance of summer and even winter flows have varied greatly over the last century. They are expected to vary and change in the future.
The influence of interyear and interdecadal variability on the hydrograph at the Dalles Dam from 1900 to 1998 have been summarized by Miles et al. (2000; see also Hamlet and Lettenmaier, 1999). A dominant source of the inter-year variability in flows has been driven by the climate variability associated with El Niño-Southern Oscillation (ENSO) and La Niña conditions. The Pacific Decadal Oscillation (PDO) also drives variability of flows (Miles et al., 2000). These two large-scale climatic drivers (ENSO and PDO) can interact to affect the lowest and the highest flows. Although these climate change drivers are important and must be noted, a detailed analysis of them was beyond the scope of this report.
Prospective future climate changes (driven by greenhouse gas emissions) have been simulated, with many simulation model results suggesting that the water supply of the Columbia River may be reduced in the next half century. Scenarios of future changes in the Columbia River hydrograph suggest that future warming will move the river toward conditions, on average, that closely resemble conditions observed during the warm phases of ENSO and PDO during the last century (Hamlet and Lettermaier, 1999; Miles et al., 2000). These simulations were generated with two general circulation models (GCMs) for the years 2025, 2045, and 2095 using expected rates of carbon dioxide emissions. One model was from the Max Plank Institute in Germany and the other was the Hadley 2 model from the Hadley Center in the United Kingdom. Both models indicate warming in all months relative to historical air temperature from 1961 to 1997. For 2045 the projected air temperature increases in individual months range from about 1° to about 4°C. The fact that the Hadley 2 model projects wetter conditions than observed historically especially in summer and fall, while the Max Planck model projects dryer conditions in the summer and fall, demonstrates the uncertainties associated with climate change model projections of changes in precipitation associated with temperature increases. As noted, the models are more consistent in projecting temperature increases.
British Columbia Ministry of Water, Land and Air Protection. Summaries in part by John Morrison, Institute of Ocean Sciences. Web materials from the Government of British Columbia at http://britishcolumbia.c-ciarn.ca/.
Hamlet, A. F., and D. P. Lettenmaier. 1999. Effects of climate change on hydrology and water resources in the Columbia basin. Journal of the American Water Resources Association. 35(6):1597-1623.
Miles, E. L., A. K. Snover, A. F. Hamlet, B. Callahan, and D. Fluharty. 2000. Pacific Northwest Regional assessment: The impacts of climate variability and climate change on the water resources of the Columbia River basin. Journal of the American Water Resources Association 36:399-420.
Thomas, C. D., A. Cameron, R. E. Green, M. Bakkenes, L. J. Beaumont, Y. C. Collingham, B. F. N. Erasmus, M. F. de Siquiera, A. Grainger, L. Hannah, L. Hughes, B. Huntley, A. S. van Jaarsveld, G. F. Midgley, L. Miles, M. A. Ortega-Huerta, A. T. Peterson, O. L. Phillips, and S. E. Williams. 2004. Extinction risk from climate change. Nature 427(8):145-148.