Causes of global change

Human-Caused or Natural
February 19, 2015 – 10:37 am

Flooded streets in Boulder, CO, in 2013A new report published by the Bulletin of the American Meteorological Society investigates the causes of extreme weather and climate events that occurred around the world in 2013.

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 climate variability in 16 extreme events. The attribution of individual extreme events to human or natural influence is a challenging and evolving field of science, but this approach—considering results from different research groups—enables increased confidence when multiple groups come to similar conclusions independently.

For example, five of the research groups studied the record heat that occurred in Australia in 2013, and all five found that human-caused climate change increased the likelihood and severity of such an event. Heat waves in Europe and Asia were also found to be more likely as a result of human influences on climate.

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 drought in California. It did, however, find that human activity increased the likelihood of unusual atmospheric pressure patterns associated with the drought. Hence, although the attribution of the drought remains uncertain, a connection to human-caused climate change cannot be ruled out yet.

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.

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