One of the most socioeconomically important impacts of climate change is drought. In the southwestern U.S., water scarcity is has been a concern for decades. We've seen wildfires, dried-up riverbeds, and crop failures in part as a result of droughts and less snowpack, and many cities in the southwest are already restricting water consumption based on these issues.
Perhaps the most visible and historic story of water use and climate change is that of the Colorado River. Stretching from Colorado and Wyoming to (historically) the Gulf of California, it has long been the water source of the west. But since the late 1990s, its southernmost riverbed — including the outlet to the ocean — has been dry as a result of damming, increased water consumption, and climate change.
We know that the Colorado no longer reaches the ocean (with the exception of an experiment in 2014 that let it flow freely), but what's harder to pin down is how exactly climate change is affecting its discharge rate (how much water it's moving). A new study in Science has found that per degree Celsius of warming, the river's mean annual discharge has decreased about 9% — undoubtedly contributing to water scarcity in the southwest. In their models, they found that the loss of winter snow, which reflects heat back up to the atmosphere when it is present on the ground, led to warmer temperatures and more evaporation.
The authors found that even if precipitation increases in the areas where the Colorado River collects its water, it likely won't be enough to offset the increased evaporation as a result of the warming. Predicting more severe water shortages, this study offers a dire warning of the consequences of climate change, especially for regions like the southwest that are already stressed.