Future Differences from “Normal” Temperature

This tool is excerpted from Chesapeake Bay Watershed Climate Impacts Summary and Outlook for Winter 2021-2022.

Tool Background

The Fall 2021 edition of the Mid-Atlantic Climate Impacts Summary and Outlook discussed projected future differences from “normal” temperature. One key finding from the Fall 2021 Summary was that the “use of the 1981–2010 ‘old’ normal shows over a half-degree greater increase in future average annual temperature compared to the newly-released 1991–2020 normal.”¹ This edition continues this discussion by looking at how use of climate normals impacts understanding of future changes in “normal” precipitation.

“Climate normals are a standardized measure of typical climate conditions, such as average annual temperature and precipitation. The National Oceanic and Atmospheric Administration’s (NOAA) National Centers for Environmental Information (NCEI) calculates these “normals” for the continental United States based on observed conditions from weather stations over a 30-year period.² Climate normals were first calculated for the 1901–1930 time period and are updated each decade.³ The latest climate normals were calculated over the period of 1991–2020 and were released to the public in May 2021.

By standardizing the time period and datasets used to produce climate normals, climate normals provide a consistent and comparable frame of reference to evaluate how different current weather conditions or future weather and climate forecasts may be from the past.⁴ However, because these normals are updated every ten years, as climate conditions change, the definition of what it means to be “normal” is also changing in time.”⁵

Figure 6 shows how the climate normals from 1901 to today compare with the average precipitation from the 20th century (1901–2000). While temperatures have steadily increased compared to the 20th century average over time, we do not see a similar trend with precipitation nationwide. However, since about 1961, much of the United States, including the Mid-Atlantic region, has been getting wetter and wetter.^6,7 Therefore, as our definitions of “normal” change to wetter or drier conditions, the projected changes in precipitation due to climate change may seem more “normal” while in actuality being quite different from the amount of precipitation experienced even in the relatively recent past (e.g. in the last few decades).

Annual U.S. precipitation compared to the 20th-century average for each U.S. Climate Normals period from 1901–1930 (upper left) to 1991–2020 (lower right).

Source: NOAA NCEI; https://www.noaa.gov/news/new-us-climate-normals-are-here-what-do-they-tell-us-about-climate-change)

The interactive data tool shown in Figure 2 explores how the regular updates of climate normals affect our interpretation of projected future climate changes. Figure 7 shows how future projected total annual precipitation from multi-decadal periods compare to “normal”. This figure compares future projections to the last two climate normals (1981–2010 and 1991–2020) and visualizes the percent change from the “normal” time period to the selected future time period. The absolute difference in precipitation amounts is available for individual locations in the tooltip graphics.

Key Findings

• Future projections of average annual precipitation show greater departures from “normal” across the Mid-Atlantic when using the 1981–2010 normal as a baseline than when comparing to the 1991–2020 normal.
• Under the 1981–2010 normal, the Mid-Atlantic on average is expected to experience an increase of nearly 3 inches of precipitation under a low emissions future scenario and nearly 5 inches of precipitation under a high emissions scenario by 2100.
• By comparison, using the 1991–2020 normal, the Mid-Atlantic would expect to experience an increase of just over 2 inches of precipitation under a low emissions future scenario and almost 4.5 inches of precipitation under a high emissions scenario by 2100.
• By 2100, use of the 1981–2010 normal shows over a half-inch increase in future total annual precipitation than with the 1991–2020 normal under both future emissions scenarios.

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