Temperature changes

Key takeaways

• The Northeast U.S. (NEUS) has warmed by 1.4˚C (2.5˚F) and has experienced a steady increase in total annual precipitation since 1895. Climate projections presented in this report indicate a continuation of these observed trends in the future.
• Coastal regions of the NEUS show pronounced increases in temperature.
• Historical temperature change in the interior NEUS has been weaker in comparison, creating a spatially heterogeneous pattern of warming.
• Average and extreme temperatures are projected to increase but the exact magnitude of the increases is dependent on the future trajectory of greenhouse gas emissions.

The observations show a clear warming trend between 1895 and 2022 (Fig. 1.2a) in the NEUS, reflecting an overall warming of 1.4˚C (2.5˚F) since 1895. Over the same period, the global average temperature has increased by about 1.1˚C (2˚F). Higher warming in the NEUS compared to the global average is expected since land warms faster than the oceans. This warming trend is projected to continue in the future, as elaborated later. Note that the observed warming in the NEUS is not spatially uniform across the region. Specifically, the coastal regions of the NEUS, from Maine to New Jersey/Delaware, have experienced enhanced warming compared to the interior parts west of the Appalachians. This pattern of warming is present in all seasons but is more pronounced in summer months. In fact, the coastal NEUS and the adjacent Northwest Atlantic continental shelf (NWAS) regions have concurrently emerged as climate change hotspots as highlighted in studies by Pershing et al. (2015, 2021) and Karmalkar and Horton (2021).

The spatial pattern of observed warming depicted in Fig. 1.2b underscores the exceptional nature of coastal warming in the NEUS, especially when compared to the other parts of the eastern U.S. Investigation into mechanisms responsible for the observed exceptional long-term coastal warming in the NEUS hints at the interplay between changes in ocean and atmospheric circulation in the North Atlantic region (Karmalkar and Horton, 2021).

Figure 1.2: (a) The annual mean surface air temperature in the Northeast U.S. between 1895 and 2022. The black line shows the twentieth-century average, and the blue line shows the trend over the entire period. (Source: NOAA Climate at a Glance) (b) Temperature change over the historical period across the contiguous US. (Source: NCA5; Marvel et al., 2023)

Figure 1.3: The annual mean (a) maximum and (b) minimum temperature in the Northeast U.S. between 1895 and 2022. The black line shows the twentieth-century average, and the blue and red lines in a and b, respectively, show trend lines over the entire period. (Source: NOAA Climate at a Glance)

Consistent with the observed trend in mean temperature, both minimum and maximum temperatures in the region have risen over the historical period. Figures 1.3a and 1.3b illustrate the yearly variations and trends in annual (365-day averaged) maximum and minimum temperatures in the NEUS. Over the period from 1895 to 2022, both maximum and minimum temperatures exhibit a steady upward trend. These changes reflect region-wide increases in both daytime high temperatures and night-time low temperatures.

The future increases in regional temperatures will depend on the future emissions trajectories (Fig. 1.4). The annual mean temperature by the end of this century is projected to reach about 55˚F in a medium emissions scenario, RCP4.5, and about 60˚F in a high emissions scenario, RCP8.5, increasing by about 5-10˚F above the average over the recent three decades (1990-2019). The future warming in the NEUS is not sensitive to the emissions scenarios until the middle of this century, with all scenarios following the same general trajectories until the 2040s and diverging thereafter.

Figure 1.4: Projected changes in the annual (ANN) average temperature. (a) the end-of-century (2071-2100 mean) projections relative to the present day (1991-2020) mean. The projections show mean values across the nine climate models for the medium emissions scenario, RCP4.5.

(b) The historical and projected temperature in ˚F as simulated by climate models. Blue and red lines show median projections under medium and high scenarios, RCP4.5 and RCP8.5, respectively. The shading indicates the spread in projections across nine climate models and all gridboxes within the region. The time series projections shown in b were calculated across all gridboxes (and not for spatial averages) to capture the spatial variability and indicate the minimum and maximum projections possible within the region.

Table 1.1: Northeast and state-level projections (ordered north to south) for annual mean surface air temperature for two periods in the future under the medium emissions scenario, RCP4.5. The 5th to 95th percentile spread in projections is based on nine climate models and spatial variability within each region.

An increase in heat extremes is one of the obvious consequences of rising temperatures. One indication of the expected increase in heatwaves is the steady rise in annual mean maximum temperature (daytime highs) in the NEUS over the last century. We calculate the change in this metric by examining the number of days per season or per year that cross the maximum daily temperature thresholds of 90˚F, 95˚F, and 100˚F. For example, Figure 1.5 shows the spatial pattern of change in days > 95˚F by the end of this century (2071-2100 mean) relative to the present day (1991-2020 mean). In the medium emissions scenarios, the entire region will experience an increase in the number of hot days.

Figure 1.5: Projected changes in the number of days with maximum temperature, Tmax, above 95˚F. (a) the end-of-century (2071-2100 mean) projections relative to the present day (1991-2020) mean. To indicate the highest values plausible for this variable, the projections show maximum values at every gridbox across the nine climate models for the medium emissions scenario, RCP4.5. (b) The historical and spatially projected values for the number of days with Tmax above 95˚F as simulated by climate models. Blue and red lines show median projections under medium and high scenarios, RCP4.5 and RCP8.5, respectively. The shading indicates the spread in projections across nine climate models and all gridboxes within the region. The time series projections shown in b were calculated across all gridboxes (and not for spatial averages) to capture the spatial variability and indicate the minimum and maximum projections possible within the region.

Figure 1.5 also highlights that the projections for hot extremes show regional variations across different ecoregions in the southern part of the study domain. In Table 1.2, we summarize the mean increase in the number of days above 95˚F for seven different ecoregions ranging from coastal plains to the interior plateau. Compared to the high-elevation Central Appalachians ecoregion, the coastal ecoregion of Piedmont experiences significantly more hot days per year. More generally, the projections also indicate higher increase in the number of hot days in coastal (e.g., Piedmont, Middle Atlantic Coastal Plain) and interior (e.g., Western Allegheny Plateau) ecoregions compared to the mountainous middle ecoregions (e.g., Central Appalachians, Ridge and Valley).

Table 1.2: Ecoregion-level projections for the number of days per year above 95˚F for two periods in the future under the high emissions scenario, RCP8.5. The 5th to 95th percentile spread in projections is based on 9 climate models for area averaged values for each region.

References

Adapted from

Staudinger, M.D., A.V. Karmalkar, K. Terwilliger, K. Burgio, A. Lubeck, H. Higgins, T. Rice, T.L. Morelli, A. D'Amato. 2024. A regional synthesis of climate data to inform the 2025 State Wildlife Action Plans in the Northeast U.S. DOI Northeast Climate Adaptation Science Center Cooperator Report. 406 p. https://doi.org/10.21429/t352-9q86

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