Drawing from the latest decade of weather data, the new normals are a reflection of climate change
By Bob Henson and Jason Samenow May 4, 2021
The official calculation of what constitutes “normal” U.S. climate has been updated — and to virtually nobody’s surprise, it’s a warmer picture than ever before.
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 with 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.”
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 email.
Since 1901-1930, the first period for which climate normals were calculated, the contiguous United States has warmed 1.7 degrees, or about 1 degree Celsius. That’s roughly on par with the global rate of warming over that period, although the United States was lagging the rest of the world until the last several decades.
The country 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.
“You can see that there’s a huge difference in temperature over time as we go from cooler climates in the early part of the 20th century to ubiquitously warmer climates here in the last two sets of normals,” Palecki said.
While the normal U.S. temperature is on the rise, some regions have warmed more than others and, over short time periods, some smaller areas haven’t warmed at all.
Temperatures actually dropped slightly for 1991-2020 compared with 1981-2010 across a part of the north-central United States extending into south-central Canada. Such departures can happen even amid longer-term warming if the decade being added (the 2010s) happens to be slightly cooler in a particular location than the decade being dropped (the 1980s).
The increase in temperature observed in Alaska reflected in the latest normals means that Fairbanks is “no longer a sub-Arctic climate in the widely used Köppen classification” for climate zones according to Rick Thoman, a climate specialist at the University of Alaska-Fairbanks. Instead, it resides within a “warm summer continental” zone, he tweeted.
Warmer and wetter
The new normals reveal that the U.S. 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.
“In the last three normals, we’ve been driving toward a much wetter environment in most of the U.S.,” 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 United States 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 megadrought may have already set in over the southwest United States 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.”
The changing meaning of ‘normal’
NOAA’s U.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 goes into heating and cooling buildings. Farmers use the averages to help with long-range planning of what to plant and when.
“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 U.S. 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, where 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 U.S. 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 email.
“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.”
NOAA has been experimenting with supplemental climate normals that may better reflect what to expect in the 2020s. Some researchers have argued that climatological periods shorter than 30 years could be more accurate for monthly temperature averages used in the near future. Thus, the new NOAA climate normals include a supplemental set of 15-year data for the period 2006-2020, a first for any climate-norms update.
Other nations around the world are also updating their climate norms to reflect the 1991-2020 period, as mandated by the World Meteorological Organization (WMO). Until recently, the WMO recommended such updates each decade but required them only every 30 years. Thus, the last full worldwide update of national and local climate norms was for 1961-1990, a metaphorical lifetime ago in a world whose climate is rapidly changing.
* NOAA notes that while the new normals approximate climate averages, they involve complex statistical methods and processing to calculate.
Bob Henson is a meteorologist and journalist based in Boulder, Colo. His books include “The Thinking Person’s Guide to Climate Change” and “Weather on the Air: A History of Broadcast Meteorology.”