Future Differences from “Normal” Temperature

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

Tool Background

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.¹ Climate normals were first calculated for the 1901-1930 time period and are updated each decade.² 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.³ 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. While these changes are shifting our baseline for scientific analysis, this regular updating of what is considered to be normal is something that humans do as well. A recent study showed that people adjust what they think is normal in terms of weekly temperatures on approximately a 5-year timescale.⁴ This finding is significant as it indicates that changing temperatures over time due to climate change may be normalized in public thought and experience.

Figure 1 shows how each climate normal since 1901–1930 compares with the average temperature from the 20th century (1901-2000). Due to the effects of climate change, U.S. “normals” have been getting steadily warmer over time with a majority of the warming occurring since 1981, as shown in Figure 1.⁵ In fact, since the 1901-1930 time period, the continguous United States has experienced 1.7 degrees-Fahrenheit of warming on average.⁶ Therefore, as we base our definitions of “normal” on warmer and warmer temperatures, the changes that are projected to occur due to climate change may appear more “normal” while still representing a large departure from temperatures experienced just a few decades ago.

Annual U.S. temperature compared to the 20th-century average for each U.S. Climate Normals period from 1091-1930 (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 in Figure 2 explores how the changing definition of a climate normal affects our interpretation of future climate changes. Figure 2 shows how future projected average annual temperatures from multi-decadal periods compare to normal. This figure includes the last two climate normals (1981–2010 and 1991–2020) and illustrates the percent change from the “normal” time period to the future time period. The absolute difference in temperatures can be seen for individual locations in the tooltip graphics.

Key Findings

• Compared to the 1981-2010 normal, future projections of average annual temperature show larger increases, particularly in the northern Mid-Atlantic.
• Under the 1981–2010 normal, the Mid-Atlantic on average is expected to experience nearly five degrees of warming under a low emissions future scenario and nine degrees of warming under a high emissions scenario by 2100.
• By comparison, using the warmer 1991-2020 normal, the Mid-Atlantic would expect to experience just over four degrees of warming under a low emissions future scenario and almost 8.5 degrees of warming under a high emissions scenario by 2100.
• By 2100, use of the 1981-2010 normal shows over a half-degree greater increase in future average annual temperature than with the 1991–2020 normal under both future emissions scenarios.

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