Deglacial ice sheet meltdown: orbital pacemaking and CO<sub>2</sub> effects

Abstract

Abstract.One hundred thousand years of ice sheet buildup came to a rapid end ∼ 25-10 thousand years before present (ka BP), when ice sheets receded quickly and multi-proxy reconstructed global mean surface temperatures rose by ∼ 3-5 • C. It still remains unresolved whether insolation changes due to variations of earth's tilt and orbit were sufficient to terminate glacial conditions. Using a coupled three-dimensional climate-ice sheet model, we simulate the climate and Northern Hemisphere ice sheet evolution from 78 ka BP to 0 ka BP in good agreement with sea level and ice topography reconstructions. Based on this simulation and a series of deglacial sensitivity experiments with individually varying orbital parameters and prescribed CO 2 , we find that enhanced calving led to a slowdown of ice sheet growth as early as ∼ 8 ka prior to the Last Glacial Maximum (LGM). The glacial termination was then initiated by enhanced ablation due to increasing obliquity and precession, in agreement with the Milankovitch theory. However, our results also support the notion that the ∼ 100 ppmv rise of atmospheric CO 2 after ∼ 18 ka BP was a key contributor to the deglaciation. Without it, the presentday ice volume would be comparable to that of the LGM and global mean temperatures would be about 3 • C lower than today. We further demonstrate that neither orbital forcing nor rising CO 2 concentrations alone were sufficient to complete the deglaciation.

New Insights on the Physical Nature of the Atmospheric Greenhouse Effect Deduced from an Empirical Planetary Temperature Model

“…The current prevailing view mostly based on theoretical considerations and results from climate models is that the Pleistocene glacial-interglacial cycles have been caused by a combination of three forcing agents: Milankovitch orbital variations, changes in atmospheric concentrations of greenhouse gases, and a hypothesized positive icealbedo feedback [101,102]. However, recent studies have shown that orbital forcing and the ice-albedo feedback cannot explain key features of the glacial-interglacial oscillations such as the observed magnitudes of global temperature changes, the skewness of temperature response (i.e.…”

An ice sheet model of reduced complexity for paleoclimate studies

“…Although the ice sheets during the LGM were not in equilibrium (Clark et al, 2009;Heinemann et al, 2014), all simulations are forced until an equilibrium is achieved. It takes around 120 kyr with a constant climate forcing until a steady state of all ice sheets is reached.…”

“…The often prohibitive cost to run climate models over periods of several millennia has limited such attempts to use of either somewhat arbitrary methods to reduce simulation time (e.g., Herrington and Poulsen, 2011;Abe-Ouchi et al, 2013;Heinemann et al, 2014) or climate models of reduced complexity (Gallée et al, 1992;Smith et al, 2003;Charbit et al, 2007;Bonelli et al, 2009;Robinson et al, 2011;Ganopolski and Calov, 2011;Stap et al, 2014). However, in spite of their focus on numerical efficiency, the ice sheet models used in some of the latter studies rival their climate model counterparts in complexity and computational cost, which is not justified for all applications.…”

“…Thus, we consider the range provided here suitable to constrain the ensemble. Note that while the real-world ice sheets probably did not reach an equilibrium during the LGM (Clark et al, 2009;Heinemann et al, 2014), the simulations here are forced with an LGM climate until their volume equilibrates. We assume that this uncertainty does not exceed the range of reconstructions.…”

Brief communication: An Ice surface melt scheme including the diurnal cycle of solar radiation

“…Finally, we use that M = 0 in the moment when the sun passes Φ and formulate the instantanous energy balance anlogously 25 to Eq. (6) as…”