Are Scientists Mapping and Predicting Changes in Precipitation Patterns?

Ask a Scientist - August 2013

Don Beams of Silver City, NM, asks "Is there scientific work underway to map and predict changes in precipitation patterns across the United States? This seems to me to be at least as important a phenomenon to track as the catastrophic weather events so often emphasized in the media." and is answered by Dr. Brenda Ekwurzel, a climate scientist in the Union of Concerned Scientists’ Climate and Energy Program.  

Climate scientists today are actively tracking changes in precipitation patterns. It is a fascinating area of research with critical implications for agriculture and public health. As I’ll explain in a minute, this research is also shedding important light on catastrophic weather events as well.

Of course, scientists have long known that our emissions of increasing levels of carbon dioxide and other heat-trapping gases into the atmosphere warm the planet like a blanket, raising global temperatures. They have known, too, that some of that extra heat evaporates water from the ocean and soil into the atmosphere. Thus, as average global temperatures rise, the warmer atmosphere tends to also hold more moisture—about 4 percent more per degree Fahrenheit temperature increase. Observations have also shown that, worldwide, water vapor over oceans has increased by about 4 percent since 1970. That means that, in general, when storms occur, there is more water vapor available in the atmosphere to fall as rain, snow, or hail especially when compared to the same region and season decades earlier. 

As you note, though, what we really want to know is not just overall precipitation rates, but regional patterns. Two key findings are emerging clearly. The first, as confirmed by the latest National Climate Assessment (pdf) is that, in general, wet regions are likely to get wetter and dry regions overall are likely to get drier. One researcher studying this phenomenon is Richard Seager, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory. Seager’s research shows that, given current warming trends, the American Southwest is soon likely to experience a condition of "permanent drought" on par with the Dust Bowl of the 1930s. 

As Seager explains, the Southwest is dry because, like many other parts of the so-called subtropics to the north and south of the equatorial tropics, the atmospheric flow already tends to move out far more moisture from the region than the amounts that storms bring into it. Now, with increasing concentrations of heat-trapping emissions, the planet's atmosphere will retain even more moisture as it warms. Evaporation from lakes and rivers is likely to increase, soil is expected to become more arid, and plants will likely shed more moisture directly to the atmosphere.

While the Southwest is likely to become more arid, however, the latest research on precipitation patterns is also showing that global warming is not just changing the overall amount of rain or drought, but increasing storms’ intensity. In fact, as the Earth warms, the heaviest one percent of rain or snow falling from storms has risen in nearly every region of the United States—almost three times the rate of increase in total precipitation between 1958 and 2011. This phenomenon seems to hold especially true in some regions. For instance, the Northeast region of the United States has seen a 74 percent increase in the amount of rain or snow falling in the heaviest storms during this same period

Wenhong Li, an atmospheric scientist at Duke University, has carefully studied precipitation patterns in the southeastern United States. Her research offers some of the emerging evidence on how global warming is influencing regional weather patterns.

Li and her colleagues closely analyzed summer precipitation patterns using highly reliable standardized data compiled from local rain gauges dating back to 1948. The findings were dramatic: Li and her colleagues show that an abnormally wet or dry summer in the Southeast has been more than twice as likely over the past 30 summers as it was during the 30 summers before that. From 1948 to 1977, there were just two unusually wet and two unusually dry summers – technically, “rainfall anomalies that exceeded one standard deviation from the norm.” From 1978 to 2007, however, there were six unusually wet and five unusually dry summers.

Using statistical detective work, Li and her collaborators have charted a clear link between gradually warming temperatures and increasing swings from very wet to very dry summers in a region’s precipitation pattern. The research illustrates the power of drawing strong statistical correlations to increase our understanding of the relationship between the warming planet and variability in weather patterns. In this case, Li’s work shows how global warming can increase both the chance of drought and the possibility of flash floods in the southeastern United States in the years ahead, both of which can have dire consequences for agriculture and for people living in flood-prone regions.

Brenda Ekwurzel is a senior climate scientist with the Climate & Energy Program. Prior to joining UCS, Dr. Ekwurzel was on the faculty of the University of Arizona Department of Hydrology and Water Resources with a joint appointment in the Geosciences Department. She holds a Ph.D. in isotope geochemistry from the Department of Earth Sciences at Columbia University's Lamont-Doherty Earth Observatory and conducted post-doctoral research at Lawrence Livermore National Laboratory, in California.

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