Enlarge /. The banks of Lake Mead faded from previously higher water levels.
In 2014, the Colorado River returned to the ocean for the first time in 16 years. Most of the years the river doesn't make it this far because it has been dammed and diverted along the way, providing fresh water to approximately 40 million people, and supporting agriculture and economic activity in the arid southwestern United States.
With climate change disrupting historical rainfall and temperature patterns, the Colorado River is not doing well and is becoming less and less likely to return to the sea. A paper published in Science this week reports that river flow per 1 ° C warming has dropped an alarming 9.3 percent – and falling snow levels are the main reason for this dramatic decline.
For a resource that is as critically and carefully managed as the Colorado River, precision is key. Just knowing that it is decreasing in response to climate change is not enough. It is more important to know how much this decline is likely to be.
However, it is not an easy task to find out how much the flow of a river is likely to decrease. Climate change changes all possible variables, from the actual air temperature to the amount of precipitation and the amount of snow or rain. Because these factors all converge, the researchers have made different estimates of how rivers around the world will change in the face of warming. These discrepancies, researchers from the US Geological Survey, Fr. Chris Milly and Krista Dunne, lead to enormous uncertainty in our understanding of how water scarcity affects "human livelihood, economic activity, and ecosystem health".
To better understand how the warming affects the Colorado River, Milly and Dunne looked back at first. They used historical data from 1912 to create a computer simulation of the river that divided its long length into hundreds of sub-areas with unique features such as different topography and rainfall.
The simulation allowed them to find out how different climate factors influenced the river flow. In particular, they encountered a factor that played an important role: reduced snow cover, which leads to more evaporation. Less snow means more dark ground is exposed and absorbs heat instead of being covered with white material that reflects light. The warmer soil means higher evaporation rates and therefore less water in the river.
Snow cover is a "protective shield", write Milly and Dunne. And this shield is slowly getting lost.
A dry future
What does this mean for the future of the river and the people who depend on it? To find out, Milly and Dunne have studied a number of climate models that predict how global temperatures will change in the future. Scenarios were used that depend on how well we can reduce emissions.
In a "business as usual" scenario, the model predicted that the level of the river would decrease by 19 to 31 percent by 2065. In a more optimistic scenario, the model predicted a decline of 14 to 26 percent. Even if more water falls than rain instead of snow, it is unlikely that it will be enough to counteract this decline, the researchers conclude.
Scientists have struggled to find an explanation for why the Colorado River has declined so much, says Keith Musselman, a hydrologist who was not involved in this research. The progress of this paper to determine the reasons for this decline is significant, he says. Simulations like this always have a degree of uncertainty, but the study is an "excellent example" of how they can be used with real data to build a better understanding of our water systems and how sensitive they are to changes, says Musselman ,
Despite the crucial role of mountain snow in supporting agriculture and life in the arid west of the United States, Musselman adds, we almost don't understand it enough. Taking greater account of snow and its relationship to our rivers, groundwater and reservoirs is "critical to understanding society's vulnerability to climate change".
Science, 2020. DOI: 10.1126 / science.aay9187 (About DOIs).