Orbital forcing of Arctic climate: mechanisms of climate response and implications for continental glaciation

Progress in understanding how terrestrial ice volume is linked to Earth s orbital configuration has been impeded by the cost of simulating climate system processes relevant to glaciation over orbital time scales (10–10 years). A compromise is usually made to represent the climate system by models that are averaged over one or more spatial dimensions or by threedimensional models that are limited to simulating particular ‘‘snapshots’’ in time. We take advantage of the short equilibration time (310 years) of a climate model consisting of a three-dimensional atmosphere coupled to a simple slab ocean to derive the equilibrium climate response to accelerated variations in Earth s orbital configuration over the past 165,000 years. Prominent decreases in ice melt and increases in snowfall are simulated during three time intervals near 26, 73, and 117 thousand years ago (ka) when aphelion was in late spring and obliquity was low. There were also significant decreases in ice melt and increases in snowfall near 97 and 142 ka when eccentricity was relatively large, aphelion was in late spring, and obliquity was high or near its long term mean. These ‘‘glaciation-friendly’’ time intervals correspond to prominent and secondary phases of terrestrial ice growth seen within the marine dO record. Both dynamical and thermal effects contribute to the increases in snowfall during these periods, through increases in storm activity and the fraction of precipitation falling as snow. The majority of the midto high latitude response to orbital forcing is organized by the properties of sea ice, through its influence on radiative feedbacks that nearly double the size of the orbital forcing as well as its influence on the seasonal evolution of the latitudinal temperature gradient.