The overall goal of the National Assessment is to analyze and evaluate what is known about the potential consequences of climate variability and change for the Nation in thecontext of other pressures on the public, the environment, and the Nation's resources. Itis also addressing the question about why we should care about, and how we might effectively prepare for, climate variability and change.
Observations have shown that the hydrological cycle of the western United States changed significantly over the last half of the 20th century. We present a regional, multivariable climatechange detection and attribution study, using a high-resolution hydrologic model forced by globalclimate models, focusing on the changes that have already affected this primarily arid regionwith a large and growing population. The results show that up to 60% of the climate-related trendsof river flow, winter air temperature, and snow pack between 1950 and 1999 are human-induced.These results are robust to perturbation of study variates and methods. They portend, in conjunction with previous work, a coming crisis in water supply for the western United States.
Major rivers worldwide have experienced dramatic changes in flow, reducing their natural ability to adjust to and absorb disturbances. Given expected changes in global climate and water needs, this may create serious problems,including loss of native biodiversity and risks to ecosystems and humans from increased flooding or water shortages.Here, we project river discharge under different climate and water withdrawal scenarios and combine this with data on the impact of dams on large river basins to create global maps illustrating potential changes in dischargeand water stress for dam-impacted and free-flowing basins. The projections indicate that every populatedbasin in the world will experience changes in river discharge and many will experience water stress. The magnitude of these impacts is used to identify basins likely and almost certain to require proactive or reactive managementintervention. Our analysis indicates that the area in need of management action to mitigate the impacts ofclimate change is much greater for basins impacted by dams than for basins with free-flowing rivers. Nearly one billion people live in areas likely to require action and approximately 365 million people live in basins almost certainto require action. Proactive management efforts will minimize risks to ecosystems and people and may be lesscostly than reactive efforts taken only once problems have arisen.
Freshwater resources, because of a host of human assaults, but especially because of dams, are the most degraded of the Earth's major ecosystems. Now the future of every dam on Earth is threatened-- not by environmental protests or economic constraints-- but by the Greenhouse Effect and the world's changing climate. Historical and geological evidence over past millennia indicate that even small changes in climate can cause major changes in the size of floods. Insurers increasingly are convinced that global warming is to blame for the greater frequency and severity of violent storms, floods and droughts since the late 1980s.
Hydrologists cannot predict exactly how much water will flow into a planned reservoir. To make a "best guess," they project past streamflow data into the future. Overestimates of average flows mean that many dams fail to yield as much power and water as predicted, the Buendia-Entrepenas reservoir in Spain is an example.
Sedimentation, despite over 60 years of research, still may be the most serious technical problem faced by the dam industry. In the US, large reservoirs lose storage capacity at an average rate of 0.2% per year, in China the rate is closer to 2.3%. Despite all the uncertainties surrounding reservoir sedimentation, authorities very rarely stop planned projects due to a lack of adequate sediment data.
McCully, Patrick, International Rivers Network
American Rivers produced abstract