Climate Change and the Colorado River: New Study Warns of "Megadroughts"

With some 30 million people relying on it, the Colorado River is a crucial American artery that, researchers warn, could shrink by nearly half its current size by 2050.

However, estimates regarding the river are varied, with the low end falling at just 6 percent.

For this reason, researchers at the University of Washington, the Cooperative Institute for Research in Environmental Studies and NOAA banded together to investigate and explain the reason why these discrepancies exists and summarize what is known about the future of the body of water.

"The different estimates have led to a lot of frustration," lead author Julie Vano, who recently earned her doctorate in civil and environmental engineering, said. "This paper puts all the studies in a single framework and identifies how they are connected."

Published in the Bulletin of the American Meteorological Society, the report offers key information for law makers, the researchers argue.

In terms of what is clear regarding the future of the Colorado River, the scientists point to the effects of an increasingly warm climate.

"We know, for example, that warmer temperatures will lead to more evaporation and less flow," co-author Bradley Udall, who contributed to the study as director of the CIRES Western Water Assessment, said in a press release.

Other variables, however, are more difficult to know, though the scientists cite the likely overall effect being one of depletion.

"Although projections of future precipitation aren't as clear, it's likely that we're going to see a reduction in overall flow in the Colorado," Udall said.

In coming to this conclusion, the researchers evaluated recent scientific papers estimating future flows of the river and, in doing so, identified several reasons for different flow estimates.

This included climate models and future emissions scenarios researcher used, with models that ran with higher future greenhouse gas emissions typically producing warmer and drier climates, and thus smaller flows.

The team also examined the models' spatial resolution, which is key to capturing topography and its effects on snow distribution in the river's mountainous headwaters. Models with coarser resolution tended to overestimate the sensitivity of runoff to climate change.

Representation of land surface hydrology, which determines how precipitation and temperature changes will affect the land's ability to absorb, evaporate and transport water also differed.

And finally, methods used to downscale from the roughly 124-mile (200-kilometer) resolution used by global climate models to the 6- to 12-mile (10- to 20-kilometer) resolution used by regional hydrology models resulted in different estimates

In addition, the new paper works to highlight several important realities facing Western water managers reliant on the river's flow.

The early 20th century, for example, is the basis for water allocation in the basin, but was a period of unusually high flow, meaning the Colorado River water is already overallocated.

Furthermore, tree ring records suggest that the river has experienced severe droughts in the past, leading the researchers to believe it will do so again - even without any human-caused climate change. However, compounding these natural droughts with steadily decreasing flows due to climate change could result in a "megadrought," signifying decades of extremely low streamflow.