Long-Term Data Reveals Changing Climate Trends

In the Southern Appalachians, droughts and storms are becoming more frequent and intense. Photo by Chris Evans, courtesy of Bugwood.org.

Long-term data from a U.S. Forest Service experimental forest reveals that air temperatures have risen significantly since the late 1970s, while droughts are becoming more severe and frequent, and precipitation more intense and variable. The findings were recently published in the journal Hydrology Research.

For almost 80 years, the main climate station at Coweeta Hydrologic Laboratory (Coweeta), a Forest Service Southern Research Station (SRS) unit located in the Nantahala Mountains of western North Carolina, has continuously collected temperature and precipitation data. Over time, four more climate stations have been added and for nearly 30 years, they’ve continually recorded air temperature, humidity, wind speed and direction, soil and litter temperature, and a host of other climatic data. These long-term datasets are invaluable to scientists who want to separate actual climate trends from natural weather variability. Climate and weather differ in their timeframes: weather is today’s rain, sun or snow, while climate is an average of weather conditions over many years.

For the findings reported in Hydrology Research, SRS hydrologist  Stephanie Laseter and her colleagues analyzed data from the last 75 years and found that the average air temperature at Coweeta has been rising since the late 1970s, while droughts have become more severe and more frequent. Precipitation has also become more variable and more extreme: droughts and intense storms are both occurring more often, and are more severe. “In fact, 2009 was the wettest year on record, but 2007 was the driest year recorded since 1934,” says Laseter. “During droughts, native insects like the southern pine beetle add to stress on trees and lead to more rapid mortality.”

Drought can make tres more susceptible to disease. Photo by Robert Anderson, courtesy of Bugwood.org

“Even during years when the total amount of precipitation is especially high, researchers have noticed that some months may be drier than average. The general trend seems to be drier summers and wetter falls,” says Laseter.

The trends Laseter and her colleagues identified at Coweeta are consistent with regional and global observations and predictions. Globally, the 20 warmest years on record have all occurred since 1981 and most models agree that across the Southeast, air temperatures will continue to rise. Many parts of the United States have also experienced more droughts and more floods over the past 50 years, and as the climate warms, these extreme precipitation events are expected to increase across the globe.

How these trends will affect future precipitation in the Southeast is unclear. “The timing and spatial distribution of extreme precipitation events are among the most uncertain aspects of future climate scenarios,” says Laseter.

Read the full text of the article.

For more information, email Stephanie Laseter at slaseter@fs.fed.us

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