Precipitation changes

Key takeaways

• The Northeast U.S. (NEUS) 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 extreme precipitation over the historical period.  

• Average and extreme precipitation are projected to increase this century but high variations in year-to-year projections remain. Due to high variability, precipitation projections for different emissions scenarios overlap completely throughout this century, indicating more attention should be paid to precipitation variability than any particular emissions scenario.

• Climate projections reported here are consistent with those reported previously, providing reliable trajectories for risk assessment and management decision-making.

The NEUS receives abundant and relatively uniform precipitation throughout the year, but there can be large variations from one year to the next. The region has experienced a modest increase in total annual precipitation (Marvel et al., 2023; Easterling et al., 2017) with a relatively strong increasing trend in the warm season, and with a dramatic increase in very heavy rainfall (top 1% of events; Whitehead et al., 2023; Hoerling et al., 2016; Wuebbles et al., 2017) over the last 60 years. Figure 1.6a shows a steady increasing trend in annual mean precipitation since 1895 in the NEUS. The 1970s and 2000s also feature multi-year periods during which the region experienced significantly wetter conditions than the 20th century average. The spatial pattern of observed precipitation change (Fig. 1.6b) highlights the increase in precipitation in every part of the NEUS, in line with precipitation increases experienced in the eastern half of the United States. There are clear indications that its distribution throughout the year has also changed. For one, seasonal changes in precipitation in the last three decades relative to the first half of the twentieth century indicate that the wetting trend in the NEUS is pronounced in summer (June, July, August; JJA) and fall (September, October, November; SON), and is weakest in winter.

Figure 1.6: (a) The annual mean precipitation 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) Precipitation changes over the historical period across the contiguous US. Source: NCA5; Marvel et al., 2023.

A significant portion of the wetting trend in summer and fall is related to an increase in the intensity of heavy precipitation events related to tropical (Barlow, 2011) and extratropical storms (e.g., Nor’easters, Kunkel et al., 2012). Many extreme precipitation indices—including the 5-year maximum precipitation, and the total precipitation falling in the top 1% of all days with precipitation—indicate increases since 1901 with substantial increases in the last five decades (Easterling et al., 2017). Other indices such as the number of 2-day extreme precipitation events also show large increases over the eastern half of the U.S. (Easterling et al., 2017). This trend of increasing mean and extreme precipitation is projected to continue with warming in the future (Maloney et al., 2014; Rawlins et al., 2012; Fan et al., 2015; Ning et al., 2015).

Figure 1.7: Projected changes in the total annual precipitation. (a) The end-of-century (2071-2100 mean) projections, in inches, 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 precipitation in inches 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.

Consistent with the observed wetting trend, most climate models suggest a steady increase in future precipitation in the region (Fig. 1.7). The trend, however, is small compared to high seasonal and interannual variability in precipitation. High variability means that despite an overall wetting in the future, the region will remain susceptible to short-term drought conditions. Indeed, despite an overall increase in precipitation in recent decades, the region has experienced both short-term (seasonal, e.g., 2016) as well as long-term (multi-year, e.g., 1960s, early 2000s) droughts with significant impacts on human and natural systems (Peterson et al., 2013). A case study of how drought has led to cascading effects in forest ecosystems in the NEUS is discussed in Chapter 5.

Table 1.3: Northeast and state-level projections of annual total precipitation for two periods in the future under the medium emissions scenario, RCP4.5. The 5th to 95th percentile spread in projections is based on 9 climate models and spatial variability within the region under consideration.