Extreme events such as heavy rains, severe storms, drought, and heat waves can have devastating effects on infrastructure, the economy, and vulnerable segments of the population. A growing field of climate science seeks to understand the drivers behind extreme events and how they connect to broader climate trends. Building on efforts to monitor the global climate (see Highlight 1), a recent report published in BAMS 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. NOAA scientists served as three of the four lead editors on the report, working with authors from NOAA, NASA, DOI’s U.S. Geological Survey (USGS), DOE’s Oak Ridge and Lawrence Berkeley National Laboratories, U.S. academic institutions, and research entities in 13 other countries.

Five of the 20 research groups studied the record heat in Australia, and all five found that human-caused climate change increased the likelihood and severity of such an event. But for some of the other 2013 events, the evidence 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. However, it did find that human activity increased the likelihood of unusual atmospheric pressure patterns associated with drought. In another example, the report found that while human-caused warming increased the moisture content of the air over Colorado, it did not increase—and in fact apparently decreased—the likelihood of the extreme rainfall that caused widespread flooding in Boulder.

Understanding the relative influence of human and natural factors in extreme events can help governments and communities make informed decisions about minimizing and responding to the effects of climate change. In particular, being able to anticipate increases in specific types of extreme weather within a given region can motivate actions to reduce risk.