Australia has suffered through two back-to-back sweltering summers, with a record-setting heat wave sweeping across the country at the end of 2013 and into 2014. Now, five separate studies published today conclude that the blazing summer was linked to human-caused climate change.
The papers are part of a larger report, Explaining Extreme Events of 2013 From a Climate Perspective, published in the Bulletin of the American Meteorological Society (BAMS). It includes 22 separate studies focusing on 16 different extreme weather events that occurred last year. And while researchers concluded that human activity had increased the likelihood and severity of Australia’s heat wave, they reported that it was hard to see any direct link between climate change and other extreme events last year—including the last 2 years of California drought and Colorado’s extreme rains.
The report highlights the value—and limitations—of “attribution research,” said Thomas Karl, the director of the National Climatic Data Center (NCDC) in Asheville, North Carolina, at a press conference today. Still a young science, attribution research seeks to strengthen understanding of the factors that contribute to extreme events. That, Karl said, is tricky because extreme events are “very complex,” and multiple factors—including precipitation, temperature, soil moisture, snowpack availability, and human land and water usage—all come into play. “It’s a new task scientists have taken on in the last few years,” he said. “It’s still in the early stages. We have learned that our ability is significantly different in some variables compared to others”—for example, the influence of climate on temperature changes is easier to trace than on precipitation.
One strength of the report is that several of the scientific teams focused on the same event, said Thomas Peterson, a co-editor on the report and also of NCDC, which is part of the National Oceanic and Atmospheric Administration. Five groups, for example, examined the Australian heat wave and found a strong climate link. “Just one paper increases the value, but five together … it’s a very powerful signal,” he said.
Three different groups that studied the California drought, however, did not come up with a clear picture of how long-term climate change might have driven that event. One study found that climate did directly affect the “geopotential heights” in the region—corresponding to high and low pressure in the atmosphere—but did not find a direct link between those pressures and actual changes in temperature and precipitation leading to the drought. Another looked at long-term sea surface temperature increases, but also found no direct link to California drought risk. And a third found that climate-induced sea-surface temperature anomalies over the northeast Pacific were driving storms (and moisture) away from California, but the warming also caused increased humidity—two competing factors that may produce no net effect.
“[Attribution science] is hard, cutting-edge research,” said Stephanie Herring, of NCDC in Boulder, Colorado, and the report’s lead editor. “It remains incredibly challenging.”