THE ENIGMA OF POLAR WARMTH IN GREENHOUSE WORLDS: INSIGHTS FROM THE MID-CRETACEOUS TERRESTRIAL PALEOCLIMATOLOGY OF NORTH AMERICA

Atmospheric general circulation models simulating warm periods in Earth History (4 x CO₂) have not yet reproduced the shallow equator-to-pole temperature gradients that are indicated by robust empirical evidence from paleoceanographic (foraminiferal d¹⁸O data) and terrestrial paleoclimatic (fossil leaf margin analysis) records. Efforts to correct this model-data misfit through postulation of increased Cretaceous ocean heat flow have not sufficiently reduced modeled latitudinal temperature gradients, and are difficult to defend on physical oceanographic principles. Alternative mechanisms for increased equator-to-pole heat transport need to be explored, and our studies of the mid-Cretaceous (Albian Stage) hydrologic cycle in North America support the idea that the role of latent heat has previously been largely underestimated. Our mass balance model of paleolatitudinal changes in the d¹⁸O of Cretaceous paleoprecipitation, as recorded by Albian pedogenic carbonates (Ufnar et al., 2002, Palaeogeography, Palaeoclimatology, Palaeoecology 188:51-71) suggest that Cretaceous precipitation rates were 156-220 % greater in the mid-latitudes, and 99 % greater at high latitudes. These higher rates of precipitation were sustained by a 76-96 % increase in evaporation fluxes, principally focused in the tropics and subtropics. The increased latent heat of evaporation released by increased condensation of water vapor at high latitudes is a plausible mechanism for sustaining Cretaceous polar warmth. Moreover, a latitudinal comparison between the meteoric sphaerosiderite lines (Ludvigson et al., 1998, Geology 26:1039-1042) from Cretaceous and Late Quaternary freshwater siderites shows that the Cretaceous siderites are significantly more enriched in ¹³C at higher latitudes, suggesting that increased methane fluxes from polar wetlands provided a positive feedback in Greenhouse World climate systems.