Changes in extreme precipitation across the Northeast U.S. under different climate scenarios

The Northeast United States has experienced the most rapidly increasing occurrences of extreme precipitation within the U.S. over recent decades, particularly during the warm season. This historical trend is primarily linked to events associated with tropical cyclones. Understanding the drivers leading to long-term trends in regional extreme precipitation under different future climate scenarios is critical to adaptation and mitigation planning.

New simulations from the fully-coupled 25-km GFDL SPEAR model and its 10 ensemble members, present a unique opportunity to study changes in regional extreme precipitation and relevant physical processes. Under the SSP5-8.5 scenario, SPEAR projects that the frequency of events exceeding the historical top-1% precipitation threshold in the Northeast U.S. will increase by up to 2.4% by the end of the 21st century. The projected increase is driven by higher anthropogenic radiative forcing and is distinguishable from natural variability by the mid-century. From the meteorological perspective, the occurrences of warm season extreme precipitation related to both atmospheric rivers and tropical cyclones are projected to increase, even though the frequency of tropical cyclones in the North Atlantic is projected to decrease in the model.

The SSP5-8.5 scenario, however, represents a highly unlikely trajectory, prompting the scientific community to explore scenarios with rapid reductions in greenhouse gas (GHG) concentrations through various climate mitigation efforts. Using the SSP5-3.4OS overshoot scenario from the SPEAR model—where GHG emissions decline sharply after 2040 and reach net-negative levels by 2070—we assess the impact of mitigation on extreme precipitation over the Northeast U.S. Our results show that extreme precipitation frequency over the Northeast U.S. is projected to decrease as GHG concentrations decline. However, the timing of this reversal exhibits pronounced seasonality. In the warm season, extreme precipitation frequency begins to decline shortly after GHG drawdown begins. In the cold season, on the other hand, the frequency continues rising for roughly a decade after the peak global mean warming and exhibits hysteresis behavior. These results highlight the benefit of climate mitigation in reducing extreme precipitation events, but also the complexity of regional climate responses, which can be modulated by seasonality, local-scale effects, and other factors.