Reversal of Extreme Precipitation Trend over the Northeast U.S. in Response to Rapid Reductions in Greenhouse Gas Concentrations

As the scientific community and policymakers have started to explore rapid reductions in greenhouse gas (GHG) concentrations through various climate mitigation efforts, understanding how extreme events on regional scales will respond to these reductions is critical for infrastructure design and resilience planning. Here we focus on changes in extreme precipitation over the Northeast U.S. in response to rapid reductions in GHG concentrations later this century.

The Northeast U.S., the most densely populated region in North America including the Boston to Washington, D.C. metro corridor, has faced the most rapid increase in extreme precipitation events within the US in the past few decades. As a consequence, severe flooding has struck urban areas. For example, the remnants of Hurricane Ida on September 1, 2021, poured record-breaking rainfall in the Northeast U.S., inundating cities like New York City and Philadelphia. Similarly, the Great Vermont Flood on July 10-11, 2023, caused widespread flooding, including the state capital, Montpelier, where the entire downtown district was inundated. Projections under SSP2-4.5 and SSP5-8.5 scenarios suggest that extreme precipitation frequency will continue to rise if GHG emissions remain high.

To study the impact of GHG reductions on extreme precipitation, we use an ensemble of simulations driven by the SSP5-3.4OS scenario from the fully-coupled 25-km GFDL SPEAR model. In this overshoot scenario, GHG emissions and other anthropogenic drivers rapidly decline after 2040, resulting in negative emissions after around 2070. Our results show that extreme precipitation frequency over the Northeast U.S. is projected to decrease following the reduction in GHG concentrations. However, the timing of this reversal of the extreme precipitation trend varies seasonally: in summer and fall, the reversal happens around 2065; while in spring and winter, the reversal is delayed until around 2075. These results suggest that the response of extreme precipitation to GHG reductions may depend on the underlying mechanisms driving these events. For example, cold-season extremes are more often associated with large-scale extratropical cyclones and/or atmospheric rivers, where dynamical processes play a significant role, indicating that the response may not be linear. Our study underscores the urgent need to better understand the timing, linearity, and reversibility of regional climate extremes to more effectively inform urban decision-making and infrastructure planning in the context of climate mitigation efforts.