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What’s ‘normal’ US weather? It’s hotter than ever, NOAA says, and often wetter

Drought conditions in early April left the San Luis Reservoir in California depleted.
Drought conditions in early April left the San Luis Reservoir in California depleted.David Paul Morris/Bloomberg

WASHINGTON — The official calculation of what constitutes “normal” US climate has been updated — and to virtually nobody’s surprise, it’s a warmer picture than ever. It’s also, for much of the nation, wetter.

On Tuesday, the National Oceanic and Atmospheric Administration released an updated set of climate averages for the contiguous United States based on the 30-year period from 1991 to 2020, including more than 9,000 daily reporting stations. It refers to these averages as “climate normals,” and updates them once every decade.

Compared to previous 30-year periods, the climate has turned unambiguously warmer.

“We’re really seeing the fingerprints of climate change in the new normals,” Michael Palecki, manager of NOAA’s effort to update the climate normals, said at an April news conference. “We’re not trying to hide that; we’re in fact reflecting that.”

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The 30-year average temperature for the 48 contiguous states climbed to a record high of 53.28 degrees in the most recent 30 years, Palecki confirmed in an e-mail.

Since 1901-1930, the first period for which climate normals were calculated, the contiguous United States has warmed 1.7 degrees. That’s roughly on par with the global rate of warming over that period, though the United States was lagging the rest of the world until the last several decades.

The United States has seen its two largest jumps in temperatures during the two most recent 30-year periods for climate normals. They rose 0.5 degrees from the period 1971-2000 to 1981-2010 and 0.46 degrees from 1981-2010 to 1991-2020. Since 1901-1930, all but two of the 30-year periods have shown an increase in temperature.

The new normals reveal that the climate is not only becoming warmer but also wetter. Preliminary data showed a national precipitation average of 31.31 inches for 1991-2020, up by 0.34 inches over the 1981-2010 value of 30.97 inches. The 20th-century average was 29.94 inches.

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“In the last three normals, we’ve been driving toward a much wetter environment in most of the US,” Palecki said.

However, precipitation trends vary by region. Between 1981-2010 and 1991-2020 it turned wetter across much of the eastern two-thirds of the nation, but drier across most of the Southwest.

The impact of climate change on precipitation is more complex than on temperature. Many parts of the nation are projected to get wetter over time, especially toward the northern states. However, rainfall and snowfall appear to be trending toward clusters of intensified precipitation, separated in some cases by longer dry periods, particularly in California. There are also signs that a multi-decade severe drought may have already set in over the US Southwest and northwest Mexico.

The landscape-drying influence of hotter temperatures will tend to increase the effects of drought even where average precipitation doesn't change.

“It’s not surprising that precipitation maps don’t show the same unmistakable fingerprint of climate change that the temperature maps do,” noted Rebecca Lindsey at climate.gov. “And yet, it’s probably not a coincidence that the last four maps in the series [shown below] . . . are nationally the four wettest-looking maps in the collection.”

NOAA’s climate averages are used in a wide array of settings. Weathercasters call on the values to tell us how a day’s temperatures compare to the norm for that calendar date. Some utilities and state regulators use climate averages when setting rates for the electricity and natural gas that heat and cool buildings. Farmers use the averages to help with long-range planning of what to plant and when.

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"What we're trying to do with climate normals is to put today's weather in a proper context so we understand whether we're above normal or below normal and also we're trying to understand today's climate so people know what to expect," Palecki said.

The great challenge in depicting “normal” climate is that climate is no longer stationary, as increases in greenhouse gases push temperatures ever upward. That means even a recent 30-year average may not capture the true likelihood of a given temperature right now, especially as a set of climate norms approaches the end of its useful life.

Small differences can have a major effect on interests such as utilities, for which tiny temperature increments can translate into big costs.

Some resource managers are looking as much toward future change as they are toward the recent past, according to University of Oklahoma’s Renee McPherson. An associate professor of geography and environmental sustainability, McPherson also serves as university director for the South Central Climate Adaptation Science Center of the US Geological Survey.

“It used to be that the normals would give a great idea of what the climate has been like for someone’s 30-year career, so new resource managers could get up to speed quickly on how and why a more senior manager made the choices they did based on past climate,” McPherson said in an e-mail.

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“Now we’re seeing enough change from one decade to the next that we need to prepare managers differently. They need to understand these are not static, so the direction of change is as important, or more important, as the values of the normals themselves.”