Hotter Days, Heavier Rains
 Projected for United States

By: CIRCulator Editorial Staff

Reprinted from: The Climate CIRCulator

Clouds at sunset

Picture taken at sunset in Iowa following severe weather and tornado outbreak. Image courtesy of the National Oceanic and Atmospheric Administration. No changes were made. Some Rights Reserved.

MANY TYPES OF EXTREME EVENTS will increase in size and frequency in the future. That is the conclusion of climate experts who recently compared climate models to observed data.

The scientists evaluated global climate models’ simulations of extreme events in the United States. They also examined the models’ future projections of the frequency and magnitude of extremes, from hot days to heavy rainfall. The questions were posed during a workshop at the Coupled Model Intercomparison Project phase 5 (CMIP5).

The workshop’s findings — published in the Bulletin of the American Meteorological Society — echo earlier studies. Conclusions include:

Hotter Temperatures

More high-temperature extremes and fewer low-temperature extremes are on the horizon. In other words, the highs are getting higher and the lows are also higher.

By 2100, under the high-emission scenario, the 20-year annual maximum temperature (so-called because it would occur once in 20 years, on average) likely will increase between 5 and 7 degrees Celsius in the Pacific Northwest. The 20-year annual minimum temperature could rise by 10 degrees inland and 5 to 7 degrees on the coast.

Historical simulations of extreme temperatures are largely consistent with recorded observations. The exception occurred in the mid-1980s to mid-1990s, the researchers note. Record highs occurred slightly less often and record lows occurred slightly more often in the simulations than in the observations.

Compared to observations, simulated warm extremes were less than 2 degrees Celsius warmer, while cold extremes were a little more than 2 degrees colder in the western United States.

Heavier Precipitation

Heavy precipitation events are on the rise across the United States, and that trend is projected to continue. But the climate models vary substantially, both in how much heavy precipitation already has increased and in how much it will increase going forward.

Depending on the emissions scenario, models project a 50 percent to 90 percent increase in the annual fraction of precipitation falling in the wettest rainstorms across the country by 2100.

In the Northwest, the 20-year annual maximum precipitation is projected to happen three to four times more frequently by 2100. All models project an increase in the extreme precipitation index, a regionally averaged measure of extreme precipitation that has increased more in the real world than models simulate during the last four decades. In general, there are large differences between climate models in simulating historical heavy rainfall, but that large spread across models is smaller for future projections.

The workshop’s findings were the fourth and last in a series assessing the state of the science and its ability to detect changes in various types of extremes (see The Climate CIRCulator, May 2014).

Vose, R.S., & Coauthors. (2014) Improved Historical Temperature and Precipitation Time Series for U.S. Climate Divisions. J. Appl. Meteor. Climatol., 53, 1232–1251. Online Access

This story originally appeared in the June/July 2014 issue of The Climate CIRCulator.

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