A major new study in Science by a dozen water experts, concluded humans are the primary cause of changes in Western river flow, winter air temperature and snow pack in the past 50 years — and things will only get worse if we don’t act soon. The abstract of the study, “Human-Induced Changes in the Hydrology of the Western United States” (subs. req’d), led by the Scripps Institution of Oceanography, states:

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 climate change detection and attribution study, using a high-resolution hydrologic model forced by global climate models, focusing on the changes that have already affected this primarily arid region with a large and growing population. The results show that up to 60% of the climate-related trends of 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.

The study’s conclusion is stark:

Our results are not good news for those living in the western United States. The scenario for how western hydrology will continue to change has already been published using one of the models used here [PCM (2)] as well as in other recent studies of western U.S. hydrology. It foretells water shortages, lack of storage capability to meet seasonally changing river flow, transfers of water from agriculture to urban uses, and other critical impacts. Because PCM performs so well in replicating the complex signals of the last half of the 20th century, we have every reason to believe its projections and to act on them in the immediate future.

The time to act is now!

This post was created for ClimateProgress.org, a project of the Center for American Progress Action Fund.