PALEOCLIMATE, CO2, SEA LEVEL AND SEA-ICE RECORDS OF THE LAST 5 MA AND THEIR IMPLICATIONS

Paleoclimatology provides unique perspectives on Earth’s climate sensitivity to changes in atmospheric CO2 and other climatic forcings. Polar regions are of particular interest because recent declines in Arctic sea ice and mass balance changes in the Greenland and Antarctic Ice Sheets suggest elevated CO2 concentrations already are influencing high-latitude climate. We review recent discoveries from Pliocene-Quaternary paleoclimate records from the Arctic Ocean and Antarctica, reconstructed atmospheric CO2 concentrations and climate model simulations that provide new insight into earth’s climate. During mid-Pliocene warmth, global sea level was 15-25 meters above present, CO2 concentrations were about 400 ppmv, Greenland and West Antarctic Ice Sheet volume was reduced, and the Arctic Ocean was seasonally ice-free. Periodic collapses of the West Antarctic Ice Sheet during the mid-Pliocene occurred during 41-kyr orbital cycles. At 2.7 Ma, a major climatic threshold was crossed characterized by rapid global cooling, Northern Hemisphere ice sheet and sea-ice build-up, and amplified orbitally-induced glacial-interglacial cycles. Hypothesized causes of this transition include tropical Pacific ocean-atmosphere dynamics, opening and closing of ocean gateways, altered global precipitation and oceanic circulation patterns, and a decrease in CO2 concentrations. During orbital climatic cycles of the last 0.8 Ma, CO2 varied between 190 and 280 ppmv, sea level oscillated up to 160 meters, and seasonally ice free conditions existed in parts of the Arctic Ocean during interglacial periods. The current Holocene interglacial also experienced dynamic changes in Arctic sea-ice cover. We will discuss these paleoclimate records in the context of our understanding of the climate-CO2 relationship.