Early Eocene latitudinal isotope gradients in precipitation and implications for global latent heat transport: new data from British Columbia, Canada and a global data-model comparison

Abstract:

The Early Eocene Climatic Optimum (EECO), occurring roughly 52 million years ago, represents an Earth system response to elevated atmospheric CO₂ levels and potentially serves as an analog for the Earth system response to anthropogenic CO₂ emissions over the next several centuries. During EECO, global temperatures were 12-15 ºC warmer than modern, with the majority of warming occurring at high latitudes, effectively reducing the Earth’s latitudinal temperature gradient. Despite a wealth of studies, there remain discrepancies between proxy records and global climate models as to the magnitude of this reduction in temperature gradients. In particular, proxy records of isotopes in precipitation appear to indicate shallower temperature gradients than models are able to simulate; however, many of these studies utilize empirical relationships between precipitation isotopes and temperature gradients that likely do not hold under different climate regimes. Here we present 5 new proxy records of EECO precipitation isotopes from British Columbia (BC), Canada spanning 49 – 55 ºN, a crucial latitudinal gap in existing records. We observe similar to slightly increasing δ¹⁸O values with increasing latitude, likely representative of a topographic highland in southern BC with decreasing elevations to the north. We incorporate this new data into a global compilation of proxy records of precipitation isotopes and compare these to a one dimensional, isotope-enabled mechanistic model of latitudinal water vapor transport. Using this model, we test the sensitivity of latitudinal isotopic gradients to varying latitudinal temperature gradients, mid-latitude eddy strengths, and latitudinal extents of vapor recharge zones via surface ocean evaporation. Finally, we analyze the implications for pole-ward latent heat transport in a high-CO₂, globally warmer climate.