Modeling the influence of orbital forcing, greenhouse gases and Northern Hemisphere ice-sheets on Southern Hemisphere climate
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To understand climate change an obvious first step is to understand the major climate shifts in history. All over the world in sediments deposited on land and on the bottom of the oceans, as well as from ice core measurements a regular pattern of glacial cycles is found. They are dominated by periods caused by variations in the orbit of the Earth and in the orientation of Earth’s rotational axis, i.e., eccentricity (100,000 years (100 kyr)), obliquity (41 kyr) and precession (23 and 19 kyr). Next to climate variations directly forced by orbital induced forced insolation changes, Earth’s climate is also affected by variations in ice-sheets and greenhouse gas (GHG) concentrations. Both ice-sheet volumes as well as GHG-concentrations show large differences between glacials and interglacials. The timing of the mid-latitudinal glacials on the SH seems to be synchronous with the variations in NH ice-sheets. Still there is much debate about the relation between NH and SH climate on orbital timescales. Here this issue is addressed by performing transient climate simulations for the last 650,000 years with a climate model of intermediate complexity (CLIMBER-2) that includes prescribed varying NH ice sheets and greenhouse gas concentrations. Four different runs were performed to be able to study the influence of orbital forcing alone (with fixed NH ice sheets and GHG-concentrations) as well as the combined influence of orbital forcing and NH ice-sheets and/or GHG-concentrations. The studied SH climate variables are low-latitudinal monsoonal precipitation averaged over December-January-February (DJF), annual Surface Air Temperature (SAT) at low, mid- and high latitudes, annual Sea Surface Temperature (SST) at low and mid-latitudes and annual sea-ice. The results show that if climate is only forced by orbital induced insolation changes the variability is only determined by obliquity and precession with dominance of obliquity at high latitudes and precession at low latitudes. Adding GHG-concentrations and/or NH-ice sheets generally results in a dominant 95 kyr component for the annual SAT, SST and sea-ice at all latitudes. The exception is the seasonal monsoonal precipitation that is still dominated by precession and SAT at high latitudes that is still dominated by obliquity if only NH ice-sheets are introduced (i.e., GHG-concentrations are fixed). At low latitudes temperatures are equally affected by varying GHG-concentrations and NH ice-sheets while at high latitudes GHG-concentrations is the dominant forcing. At the precession band the monsoonal precipitation leads precession by about 700 years with orbital forcing only which is barely influenced by GHG and NH ice sheets. The lag of the annual SAT and SST at mid- and high latitude of about 1 kyr with orbital forcing only increases to about 3-4 kyr if GHG-concentrations and NH ice-sheets are introduced. At the obliquity band the phase difference vary from very small with orbital forcing only to about 4-5 kyr including NH ice sheets and GHG-concentrations. Comparison of Antarctic Deuterium measurements to the simulated Antarctic SAT from the model results showed that orbital forcing alone cannot explain the Antarctic temperature and that both NH ice-sheets and greenhouse gas concentrations are necessary with a larger influence of the GHG-concentrations compared to NH ice-sheets. In the fully forced simulation CLIMBER-2 is able to accurately simulate the (unfiltered) Antarctic temperature derived from Deuterium (correlation 0.87) which improves significantly if only obliquity and the 95 kyr components are taken into account (correlation ~0.97).