The warming trend that has accompanied anthropogenic climate change to date has reduced Colorado River flows by 0.5 million acre-feet (at the very least) over just a 14-year time period (2000–2014). This is according to new research from the University of Arizona and Colorado State University.
To explain that better: Between the years of 2000–2014, Colorado River flows declined to around 81% of the 20th-century average. During this 14-year time period, a reduction of around 2.9 million acre-feet of water flow per year occurred. So, out of the total 2.9-million-acre-feet-a-year reduction, a limited but notable portion was the result of rising temperatures — owing to lower drainage into the river and also higher evaporative rates.
This work is notable because many projections of future Colorado River flow don’t take the direct effects of rising temperatures into account.
“This paper is the first to show the large role that warming temperatures are playing in reducing the flows of the Colorado River,” commented Jonathan Overpeck, UA Regents’ Professor of Geosciences and of Hydrology and Atmospheric Sciences and director of the UA Institute of the Environment.
The new study found that the higher temperatures in the region since 2000 (1.6° Fahrenheit higher than the average since record-keeping there began) are responsible for between one-sixth to one-half of the river flow reductions seen since 2000.
That means that, as temperatures in the region continue rising, the 40 million people who currently rely on the Colorado River for their survival may be out of luck and forced to move. These people are spread throughout 7 US states and the Mexican states of Sonora and Baja California.
“We’re the first to make the case that warming alone could cause Colorado River flow declines of 30% by midcentury and over 50% by the end of the century if greenhouse gas emissions continue unabated,” Overpeck continued.
Co-author Bradley Udall, a senior water and climate scientist/scholar at CSU’s Colorado Water Institute, commented as well: “The future of (the) Colorado River is far less rosy than other recent assessments have portrayed. A clear message to water managers is that they need to plan for significantly lower river flows.”
The press release provides more:
“The team began its investigation because Udall learned that recent Colorado flows were lower than managers expected given the amount of precipitation. The two researchers wanted to provide water managers with insight into how future projections of temperature and precipitation for the Colorado River Basin would affect the river’s flows.
“Udall and Overpeck began by looking at the drought years of 2000-2014. About 85% of the river’s flow originates as precipitation in the Upper Basin — the part of the river that drains portions of Wyoming, Utah, Colorado and New Mexico. The team found during 2000-2014, temperatures in the river’s Upper Basin were 1.6° Fahrenheit (0.9° Celsius) higher than the average for the previous 105 years.
“To see how increased temperatures might contribute to the reductions in the river’s flow that have been observed since 2000, Udall and Overpeck reviewed and synthesized 25 years of research about how climate and climate change have and will affect the region and how temperature and precipitation affect the river’s flows.
“Water loss increases as temperatures rise because plants use more water, and higher temperatures increase evaporative loss from the soil and from the water surface and lengthen the growing season.”
Something that’s important to note here is that current climate models apparently can’t simulate the 20- to 60-year megadroughts that are known to have occurred in the region at various points in the relatively recent past. Also notable is that most of these current climate models didn’t predict/reproduce the current drought — so clearly there are limitations to them in that regard.
However, some earlier research has made it clear that, as temperatures in the region continue rising, such “megadroughts” will become more and more likely — and that this rising level of risk is much higher than current climate models predict.
“A megadrought in this century will throw all our operating rules out the window,” Udall noted.
There’s no doubt about that. Such an event would cause significant problems for the US, and would very likely spur a large-scale migration like that seen during the Dust Bowl.
Many other regions of the world will be facing a similar situation as the southwestern US (and northern Mexico) over the coming century with regard to water scarcity — and the geopolitical outcomes aren’t likely to be too dissimilar. Mass migration, increasing levels of conflict, and everything that goes along with those two. … The emergence of defacto state-less zones in areas of extreme water scarcity seems likely over the coming decades.
The new research is detailed in a paper published in the American Geophysical Union journal Water Resources Research.
Images by USGS