Climate Change and Extreme Precipitation

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Steve Vavrus, senior scientist, Center for Climatic Research, Nelson Institute for Environmental Studies, UW–Madison, sjvavrus@wisc.edu

When we think about climate change, the weather variables that most readily come to mind are temperature and precipitation. Obviously, temperature rises in a warming climate, but how will precipitation change? Weather records collected across Wisconsin since the late 1800s indicate that our climate has recently become wetter, with more extreme precipitation. In fact, the 2010s was by far the wettest decade in state history and was accompanied by many damaging and even deadly floods.

One reason for this trend is that warm air can hold more moisture than cold air. Wisconsin’s wettest three months (June, July, and August) are also our warmest months, and summer is typically the season with the most extreme daily precipitation. As the atmosphere warms, these summer storms wring out even more precipitation. Under future warming scenarios, climate models predict that most of the world will receive a higher proportion of their precipitation in heavier doses.

These maps show that Wisconsin is projected to have an increase in the frequency of heavy precipitation days.
Figure 1: Projected change in the frequency of heavy precipitation days (more than two inches of rainfall) by mid-century (2041-2060) versus the recent past (1981-2010). Projection is based on a set of statistically downscaled global climate models driven by the high-end RCP8.5 greenhouse gas emissions scenario. Illustration courtesy of Dan Vimont and David Lorenz, Center for Climatic Research, Nelson Institute for Environmental Studies, UW-Madison. View a larger version of this graphic

However, only certain places will become wetter overall, because rainfall and snowfall also depend on atmospheric circulation patterns that guide weather systems. For example, when jet stream winds steer high-pressure cells our way, we can get stuck in a prolonged dry pattern even while long-term precipitation increases.

Climate models project that Wisconsin will become wetter in the future compared with the climate of today and the recent past. Most estimates suggest that our state’s total precipitation will increase by around 10 percent during the current century, and that the greatest changes will occur in winter and spring.

Interestingly, the 2010s was even wetter than that estimate of future precipitation levels, which suggests that either climate models are underestimating future changes or that random variability has made Wisconsin wetter recently. Isolating the influence of human-caused climate change on observed precipitation trends is challenging, because natural variations in the climate system also contribute to extreme weather and can cause both a long string of rainy days and an extended drought.

As the “signal” from a warming climate increasingly outweighs the “noise” from natural variability, the future changes predicted by climate models should emerge. In Wisconsin this means a high likelihood that heavy downpours will become more frequent and intense (see figure 1). It appears, however, that the existing spatial pattern of precipitation will largely persist: Southern and western Wisconsin will likely be impacted the most, but everywhere in the state will probably experience an upward trend.

Some of the most important questions that University of Wisconsin-Madison climatologists are researching to improve predictions include:

  1. Will changing global circulation patterns make the expected increase in extreme precipitation more severe or less severe?
  2. Will extreme precipitation events be more frequent, more intense, or both?
  3. Will a wetter Wisconsin occur gradually or abruptly?

Learn More About Extreme Precipitation Research at UW-Madison