Prepare for a much drier future — or a much wetter one. Climate change is likely to bring significantly warmer temperatures across the entire United States. But what that means for rain and snowfall will depend crucially on where in the country you live.
The far Southwest, currently in the grip of a historic drought, is likely to get significantly drier in the coming decades, according to the latest predictive climate data. The Northeast will get wetter by a similar proportion. If you're a farmer in Southern California, or own riverside property in upstate New York or New England, you should be preparing for life-changing drought and flood, respectively.
The animation above was made from data provided to BuzzFeed News by Jon Eischeid of the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colorado. It reflects the averaged output of the world's leading climate models, given starting conditions in 1900 and then left to simulate how the atmosphere and oceans behave until 2100. Each frame shows precipitation over a five-year period.
The models assumed that carbon dioxide emissions will remain on their current upward trajectory, leveling out only toward the end of the century. This is a scenario that the Intergovernmental Panel on Climate Change (IPCC) calls RCP8.5. If this is what the world allows to happen, climate models indicate that average global temperatures are likely to rise by about five degrees Celsius by 2100. (Climatologists suggest that we should try to keep the increase to less than two degrees Celsius to avoid the worst effects of climate change.)
Predicting future precipitation is harder than estimating temperature changes because simulating how moist air will circulate on a regional scale is more complicated than calculating the warming effect of greenhouse gases such as carbon dioxide.
Despite the greater uncertainties, climate models consistently predict an arid zone in the subtropics that will grow and intensify, while higher latitudes will see more rain and snow. The U.S. also gets split in two by meanders in the jet stream, a fast-flowing current of air high in the atmosphere, which is why the projections for the Southwest and Northeast are so different.
The total amount of precipitation is only one of several factors that help scientists predict the risk of future droughts and floods. Also important is how much of the water falls as snow, and the frequency and intensity of heavy storms.
In California, precipitation is seasonal, with the vast majority falling between November and March. Historically, this has created a large snowpack in the Sierra Nevada, which melts slowly in the spring, providing a predictable and manageable supply of water. But as the region warms, more will fall as rain, running off rapidly from the mountains out to the sea.
It's a similar story across much of the Southwest. For this region, the main message from the National Climate Assessment, published in 2014, is to expect a decline in the snowpack and river flows. That will decrease the reliability of water supplies for cities, agriculture, and ecosystems.
To deal with the bigger droughts, California could try to catch more water during its wet season, but that might be risky. California's reservoirs are typically not kept at full capacity even in times of plenty, because they are also used to soak up storm water and prevent damaging floods. Holding more water in winter could, paradoxically, increase flood risk even as the region dries out and becomes more susceptible to drought.
"As climate warms, it's going to be more challenging to balance flood and storage requirements," Daniel Cayan, a climatologist at the University of California, San Diego, told BuzzFeed News.
In the Northeast, meanwhile, the extra rain and snow is also expected to come mainly in the winter months, and as increasingly heavy storms. Rising sea levels, as well as threatening coastal flooding, will make it harder for this rainfall to flow out to sea. Put all of this together, and increased flooding from the region's rivers seems inevitable.
We're already seeing heavier rainstorms and increased flood risk, Radley Horton, a climatologist at Columbia University and one of the lead authors of the Northeast chapter of the climate assessment, told BuzzFeed News. Between 1958 and 2010, there was a 70% increase in the amount of precipitation falling in the 1% of heaviest storms.
The models' predictions change, however, if our carbon emissions drop. Below is an animation for another IPCC scenario, called RCP4.5, which assumes that carbon dioxide emissions will peak by mid-century, and then rapidly decline. This would likely cause the globe to warm by about three degrees Celsius, on average, by 2100. There is still a trend toward less precipitation in the Southwest, and more in the Northeast, but it's less extreme.
Whether the world's nations collectively agree to drastically reduce their carbon emissions, we are already locked into a certain amount of climate change. Even if the total amount of precipitation were to remain the same, increased temperatures mean more evaporation, drier soils, and less river flow. If summers warm by three degrees Celsius, for instance, annual flow in the Colorado River, the largest in the Southwest, could decrease by more than 13%.
Viewed from this perspective, California's epic drought is likely a harbinger of worse to come.
"We'll have to adapt — we'll get the water we get," Peter Gleick of the Pacific Institute in Oakland, California, who studies the effect of climate change on water resources, told BuzzFeed News. But adapting to a drier future climate might be a painful experience for California and other parts of the Southwest, he said. "Farmers may go out of business and ecosystems may crash."
Peter Aldhous is a Science Reporter for BuzzFeed News and is based in San Francisco. His secure PGP fingerprint is 225F B2AF 4B8E 6E3D B1EA 7F9A B96E BF7D 9CB2 9B16
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