A Storyline Analysis of Hurricane Irma's Precipitation Under Various Levels of Climate Warming
Results from both observational and modeling studies demonstrate that precipitation from hurricanes is increasing due to anthropogenic climate change. A simple approximation for the rate of change in extreme precipitation is 7% per °C increase in atmospheric temperature, which comes from a thermodynamic principle known as the Clausius-Clapeyron equation; however, previous studies have found that precipitation from some hurricanes is increasing more than 7% per °C due to other changes in the hurricanes’ characteristics, such as their intensities. This study uses the “hindcast attribution” method with the Community Atmosphere Model (CAM) to simulate Hurricane Irma, which hit Florida in September of 2017, under various climate states. Irma is simulated in the observed climate of 2017 and in three warming scenarios where the climate is 2 K, 3 K, and 4 K warmer than the pre-industrial climate. With these simulations, we can quantify how much more precipitation Hurricane Irma would have produced had it happened in warmer climates that may happen in the future.
Simulating extreme weather events that have recently happened in the real world under various theoretical climate scenarios (known as the “storyline approach”) can be useful for scientific, communication, and risk analysis purposes. This approach works well for hurricanes because it is hard to determine if climate change caused a hurricane to form or not, but it is more straightforward to assess how a hurricane’s precipitation and intensity are impacted. Additionally, focusing on one hurricane facilitates the use of high-resolution models and ensemble simulations, since the computational cost of running a model for only a few days is relatively low compared to decadal climate simulations.
This work uses the storyline approach with the Community Atmosphere Model (CAM5) to simulate Hurricane Irma under a “current climate” scenario and three future warming scenarios representing climates that are 2 K, 3 K, and 4 K warmer than pre-industrial. Under these warming scenarios, the mean accumulated precipitation from Hurricane Irma over Florida increased by 24%–55%, the maximum precipitation within the storm increased by 43%–61%, and larger precipitation amounts are the result of more extreme precipitation rates. These increases in precipitation are due to both the increased atmospheric moisture in a warmer climate and an increase in Irma’s intensity. This work demonstrates the ability of the storyline approach to present versions of a real event had it happened under various levels of climate warming that are plausible to occur within this century.