BUILDING AN EQUABLE CLIMATE: PUTTING THE PIECES TOGETHER

Understanding how past greenhouse climates functioned strengthens our grasp of the complex interactions in the Earth system. The early Eocene, and especially the climatic ‘aberration’ of the LPTM, are blue-prints of climatic change. They challenge us synthesise what is known about each of the Earth system’s building blocks--atmospheric, oceanic, cryospheric, ecological, biogeochemical, lithospheric--to try to reconstruct equable paleoclimates. While controversial, the equable climate blue-print generally suggests that with higher greenhouse gas (GHG) concentrations, the poles, deep ocean, and continental interior winter temperatures warm much more (2-3x) than tropical temperatures do. We try to build from this blue-print by coupling interactive atmosphere-ocean-sea ice climate models and carrying out experiments with realistic Eocene topography, bathymetry, and terrestrial vegetation distributions and GHG concentrations.

I present results from a suite of long, fully coupled climate model simulations of Eocene (55-35 mya) climate, focusing on ocean-atmosphere interactions and past climatic warmth. Interestingly, results show that the deep ocean, North Pacific, and the most of the Southern Ocean are warmer than modern by ~10C, whereas equatorial Pacific sea surface temperatures (SSTs) remain within 1C of modern. The SST reconstructions of Pearson et al. 2001 and those of Bralower et al. 1997 might be reconciled as a sampling issue. Increased ocean heat transport and increased atmospheric latent heat transport are neither produced in equilibrium with nor necessary for warm high latitude and deep ocean SSTs. Existing building blocks (GHGs, albedo) are sufficient to construct ‘low gradient’ ocean climates.

Too cold terrestrial winter temperatures prove, again, to be the chief obstacle to building a working model of equable climates. Teleconnections to the tropics and El Nino are shown to be critical. High latitude ocean currents, GHG concentrations, and terrestrial albedo have lesser but non-negligible impacts on land temperatures. The implication is that the structure of equable climate requires some missing interaction between (1) tropical temperature changes and the distribution of plankton or stratus clouds, or (2) terrestrial vegetation albedo, water vapor and GHG concentrations.