Future Differences from "Normal" Temperature
An interactive data tool exploring how the changing definition of a climate normal affects our interpretation of future climate changes.
This tool is excerpted from Chesapeake Bay Watershed Climate Impacts Summary and Outlook for Winter 2021-2022.
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.
Selecting Time Periods and Future-Emissions Scenarios
Use the slider to the right of the maps to adjust the 30-year period used to calculate changes in future precipitation relative to each climate normal. Users can also select the future emissions scenario (Low or High Emissions).
Viewing Variability Within a Location
Hover or tap over a point of interest. A window will pop up that displays changes in precipitation by time period and climate normal. You can also use the State and Country filters to the right of the map to zoom into a location of interest.
Localized Constructed Analogs (LOCA) is a downscaled climate data product available at 1/16-degree (6-km) resolution over the continental United States. LOCA data sets8 include the 32 climate models available in the Coupled Model Intercomparison Project 5 (CMIP5) archive, for two future greenhouse gas concentration trajectories: a low-emissions future, Representative Concentration Pathway (RCP) 4.5;9 and a high-emissions future, RCP 8.5.10 For this study, we used LOCA data over the Chesapeake Bay watershed from 1981–2100 (or 2099 for some models). Access LOCA datasets and learn more about the methodology.
Data were processed by the Northeast Regional Climate Center to calculate the average annual precipitation.
Romita Grocholski, Krista, Michelle E. Miro, Lena Easton-Calabria, Jessica Spaccio, Samantha Borisoff, and Arthur T. DeGaetano, Mid-Atlantic Regional Climate Impacts Summary and Outlook: Fall 2021. Santa Monica, CA: RAND Corporation, 2021. https://www.midatlanticrisa.org/climate-summaries/2021/12.html Return to text ⤴
https://www.ncei.noaa.gov/products/land-based-station/us-climate-normals Return to text ⤴
https://www.noaa.gov/news/new-us-climate-normals-are-here-what-do-they-tell-us-about-climate-change Return to text ⤴
https://www.ncei.noaa.gov/products/land-based-station/us-climate-normals Return to text ⤴
Romita Grocholski, Krista, Michelle E. Miro, Lena Easton-Calabria, Jessica Spaccio, Samantha Borisoff, and Arthur T. DeGaetano, Mid-Atlantic Regional Climate Impacts Summary and Outlook: Fall 2021. Santa Monica, CA: RAND Corporation, 2021. https://www.midatlanticrisa.org/climate-summaries/2021/12.html Return to text ⤴
https://www.noaa.gov/news/new-us-climate-normals-are-here-what-do-they-tell-us-about-climate-change Return to text ⤴
https://www.washingtonpost.com/weather/2021/05/04/noaa-new-climate-normals/ Return to text ⤴
More information on RCP 4.5 can be found in: https://doi.org/10.1007/s10584-011-0151-4 Return to text ⤴
More information on RCP 8.5 can be found in: https://doi.org/10.1007/s10584-011-0149-y Return to text ⤴