Abstract
Future global warming is widely anticipated to increase the occurrence of extreme precipitation events, but such hydrological changes have received limited attention within paleoclimate studies. Several proxy studies of the hydrological response to the Paleocene–Eocene Thermal Maximum hyperthermal, ∼56 Ma, have recently invoked changes in the occurrence of extreme precipitation events to explain observations, but these changes have not been studied for the geologic past using climate models. Here, we use a coupled atmosphere–ocean general circulation model, HadCM3L, to study regional changes in metrics for extreme precipitation across the onset of the PETM by comparing simulations performed with possible PETM and pre-PETM greenhouse gas forcings. Our simulations show a shift in the frequency–intensity relationship of precipitation, with extreme events increasing in importance over tropical regions including equatorial Africa and southern America. The incidence of some extreme events increases by up to 70% across the PETM in some regions. While the most extreme precipitation rates tend to relate to increases in convective precipitation, in some regions dynamic changes in atmospheric circulation are also of importance. Although shortcomings in the ability of general circulation models to represent the daily cycle of precipitation and the full range of extreme events precludes a direct comparison of absolute precipitation rates, our simulations provide a useful spatial framework for interpreting hydrological proxies from this time period. Our results indicate that changes in extreme precipitation behaviour may be decoupled from those in mean annual precipitation, including, for example in east Africa, where the change in mean annual precipitation is small but a large increase in the size and frequency of extreme events occurs. This has important implications for the interpretation of the hydrological proxy record and our understanding of climatic, as well as biogeochemical, responses to global warming events.
Highlights
Projections of how Earth’s hydrological cycle will operate on a future warmer Earth indicate changes to the sub-annual character of precipitation including the frequency and intensity of extreme events (Emori and Brown, 2005; O’Gorman, 2015)
While the global precipitation rate is energetically constrained to ∼2% K−1 (Allen and Ingram, 2002), extreme events are predicted to scale with Clausius–Clapeyron, occurring where most atmospheric moisture is converted to precipitation (Dai and Trenberth, 2004)
Examples of hourly General Circulation Model (GCM) output for one year of the model simulations are shown for illustrative purposes in Fig. 1 for a number of model locations at which either Paleocene–Eocene Thermal Maximum (PETM) or early Eocene precipitation proxy data have been published
Summary
Projections of how Earth’s hydrological cycle will operate on a future warmer Earth indicate changes to the sub-annual character of precipitation including the frequency and intensity of extreme events (Emori and Brown, 2005; O’Gorman, 2015). Observational data indicate that in some mid-latitude regions, there has already been a disproportionate increase in heavy precipitation events relative to mean annual changes (Donat et al, 2013; Dittus et al, 2015). Changes in the occurrence of extreme precipitation are expected from thermodynamical effects of warming which increases the saturation vapour pressure of the atmosphere on the order of 7% K−1, according to the Clausius–Clapeyron rela-. Regional variations in the occurrence of extremes are more uncertain: dynamical changes in atmospheric circulation can lead to variations in moisture convergence, and observational studies have shown increases in extremes which exceed Clausius–Clapeyron scaling (Lenderink and van Meijgaard, 2008)
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