Abstract
Abstract. A range of proxy observations have recently provided constraints on how Earth's hydrological cycle responded to early Eocene climatic changes. However, comparisons of proxy data to general circulation model (GCM) simulated hydrology are limited and inter-model variability remains poorly characterised. In this work, we undertake an intercomparison of GCM-derived precipitation and P − E distributions within the extended EoMIP ensemble (Eocene Modelling Intercomparison Project; Lunt et al., 2012), which includes previously published early Eocene simulations performed using five GCMs differing in boundary conditions, model structure, and precipitation-relevant parameterisation schemes. We show that an intensified hydrological cycle, manifested in enhanced global precipitation and evaporation rates, is simulated for all Eocene simulations relative to the preindustrial conditions. This is primarily due to elevated atmospheric paleo-CO2, resulting in elevated temperatures, although the effects of differences in paleogeography and ice sheets are also important in some models. For a given CO2 level, globally averaged precipitation rates vary widely between models, largely arising from different simulated surface air temperatures. Models with a similar global sensitivity of precipitation rate to temperature (dP∕dT) display different regional precipitation responses for a given temperature change. Regions that are particularly sensitive to model choice include the South Pacific, tropical Africa, and the Peri-Tethys, which may represent targets for future proxy acquisition. A comparison of early and middle Eocene leaf-fossil-derived precipitation estimates with the GCM output illustrates that GCMs generally underestimate precipitation rates at high latitudes, although a possible seasonal bias of the proxies cannot be excluded. Models which warm these regions, either via elevated CO2 or by varying poorly constrained model parameter values, are most successful in simulating a match with geologic data. Further data from low-latitude regions and better constraints on early Eocene CO2 are now required to discriminate between these model simulations given the large error bars on paleoprecipitation estimates. Given the clear differences between simulated precipitation distributions within the ensemble, our results suggest that paleohydrological data offer an independent means by which to evaluate model skill for warm climates.
Highlights
Considerable uncertainty exists in understanding how the Earth’s hydrological cycle will function on a future warmerthan-present planet
We initially summarise model skill in simulating preindustrial mean annual precipitation (MAP) to provide context for our Eocene model intercomparison and to identify which, if any, precipitation structures are unique to the Eocene and which are more fundamentally related to errors particular to a given general circulation model (GCM)
All of the EoMIP GCMs simulate the principal features of the observed preindustrial MAP distribution, errors occur in their position and strength
Summary
Considerable uncertainty exists in understanding how the Earth’s hydrological cycle will function on a future warmerthan-present planet. Other studies utilising leaf physiognomy and paleosols suggest that the North American continental interior became drier at the onset of the PETM or alternated between wet and dry phases (Kraus et al, 2013; Smith et al, 2007; Wing et al, 2005) These proxies collectively indicate an early Eocene hydrological cycle different to that of the present day, but only limited proxy–model comparisons have been made (Pagani et al, 2006; Speelman et al, 2010; Winguth et al, 2010). We refer to the simulations throughout this paper in terms of their atmospheric CO2 level relative to preindustrial conditions (i.e. an Eocene simulation with an atmospheric CO2 concentration twice that of preindustrial conditions is referred to as “x2” and one with a concentration four times that value is referred to as “x4”)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
