Extreme Weather Events of 2013: Human-Caused or Natural Variability?
A new report published by the Bulletin of the American Meteorological Society investigates the causes of
The report, "Explaining Extreme Events of 2013 from a Climate Perspective," integrates findings from 20 different research groups to assess the respective roles of human-caused climate change and natural
For example, five of the research groups studied the record heat that occurred in Australia in 2013, and all five found that human-caused
But for other 2013 events like droughts, heavy rains, and storms, the influence of human-caused climate change relative to natural factors was more mixed. For instance, the report did not find conclusive evidence for the impact of human-caused climate change on the ongoing
In another example, the report found that while human-caused warming increased the moisture content of the air over Colorado, it did not increase the likelihood of the extreme rainfall that caused widespread flooding in Boulder last September (curious about this apparent contradiction? Learn more). In fact, human-caused climate change apparently decreased the likelihood of such an event—though the report notes that further research using additional models is needed.
One reason that attribution of extreme events is difficult is that they occur infrequently, which makes trends hard to discern. For floods, observational records are also strongly influenced by human actions to reduce flood risk, which can mask underlying trends. Although regional trends in both floods and droughts have been observed, no nationwide trends are evident for these events. But there are trends in the factors that lead to them: for example, heavy rains that can cause flooding are increasing nationwide, and factors that lead to drought—increased temperature, reduced snowpack, reduced soil moisture, and others—are trending in a direction that favors more drought events in some regions. While attribution of individual events remains a challenge, droughts in the Southwest and heat waves in all U.S. regions are projected to become more severe as human-caused climate change progresses.
Understanding the relative influence of human and natural factors in extreme events can help governments and communities make informed decisions about
NOAA scientists served as three of the four lead editors on the report, which has been produced annually since 2012. Report authors included scientists from NOAA, NASA, the U.S. Geological Survey, DOE’s Oak Ridge and Lawrence Berkeley National Laboratories, U.S. academic institutions, and research entities in other countries. "This annual report contributes to a growing field of science which helps communities, businesses, and nations alike understand the impacts of natural and human-caused climate change," said Thomas Karl, director of NOAA's National Climatic Data Center and Chair of the Subcommittee on Global Change Research (USGCRP’s steering body). "[T]he environmental intelligence the report yields to decision makers is invaluable, and the demand is ever-growing."