Session: Warm Climates (Mid-Holocene, Last interglacial, Deep-time, Pliocene)
Author: Ran Feng / email@example.com / National Center for Atmospheric Research
Co-author: Bette Otto-bliesner, National Center for Atmospheric Research;
Esther Brady, National Center for Atmospheric Research;
Tamara Fletcher, University of Montana;
Ashley Ballantyne, University of Montana;
Clay Tabor, National Center for Atmospheric Research;
Polar amplification is key to understanding the stability of the cryosphere and sensitivity of the earth system to CO2 forcings. A potential analogue for estimating future Arctic warming is the Mid-Piacenzian Warm Period (3.264 – 3.025 Ma). During this time interval, the paleogeography and paleotopography were similar to present-day, except for a few gateway changes. The CO2 level was also similar to today, with a best estimate of 400 ppm. The MPWP shows strong Arctic amplification, which is epitomized by the existence of circum-Arctic boreal forests, a large reduction in Arctic sea ice, and absence of the western Greenland ice sheet. These findings suggest a precarious state of northern high latitude cryosphere at the 400 ppm CO2 level.
However, earth system models that participated in phase one of the Pliocene Model Intercomparison Project (PlioMIP1) had limited success in simulating Arctic amplification during the MPWP, raising doubts about model sensitivity and the analogue nature of the MPWP. Since PlioMIP1, several improvements have been made to both the MPWP boundary conditions and the models’ climate physics. These improvements allow us to simulate the northern high latitude warmth and Arctic low sea ice state that are broadly consistent with MPWP proxy records. Based on a series of sensitivity tests, we further identify that 1) the MPWP North Atlantic warmth is attributable to the closed Arctic Ocean gateways, 2) the Arctic low sea ice state is partly due to reduced cloud cover over circum-Arctic oceans under pristine atmospheric conditions, and 3) circum-Arctic terrestrial warming can largely be explained by a vegetation feedback from high density boreal forests. We argue that the latter two changes are relevant to future climate change at the centennial to millennial time scale. In this regard, the MPWP remains a potential analogue for the equilibrium climate state of the RCP2.6 world.