[OANNES Foro] Last glaciation: Ocean-circulation changes produced much of the causative variation in greenhouse gases

[Mario Cabrejos]

Nature 456, (20 November 2008)
Published online 19 November 2008

331-333 pp
doi:10.1038/456331a
Palaeoclimate: Greenhouse-gas fingerprints

Thomas F. Stocker & Adrian Schilt

Thomas F. Stocker and Adrian Schilt are in the Division of Climate and Environmental Physics, and the Oeschger Centre for Climate Change Research, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland. Email: stocker en climate.unibe.ch

Abstract
Short episodes of warming and cooling occurred throughout the last glaciation. An innovative modelling study indicates that ocean-circulation changes produced much of the causative variation in greenhouse gases. 

Much of what we know about abrupt climate change and tipping points in the climate system comes from polar ice cores1. But these unique data archives provide only a narrow view of the richness of climate dynamics and impacts.

________________________________________________



373-376 pp
doi:10.1038/nature07531
Glacial greenhouse-gas fluctuations controlled by ocean circulation changes

Andreas Schmittner1 aschmitt en coas.oregonstate.edu & Eric D. Galbraith2

  1.. College of Oceanic & Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA 
  2.. Atmospheric and Ocean Sciences, Princeton University, Princeton, New Jersey 08544, USA 
Earth's climate and the concentrations of the atmospheric greenhouse gases carbon dioxide (CO2) and nitrous oxide (N2O) varied strongly on millennial timescales during past glacial periods. Large and rapid warming events in Greenland and the North Atlantic were followed by more gradual cooling, and are highly correlated with fluctuations of N2O as recorded in ice cores. Antarctic temperature variations, on the other hand, were smaller and more gradual, showed warming during the Greenland cold phase and cooling while the North Atlantic was warm, and were highly correlated with fluctuations in CO2. Abrupt changes in the Atlantic meridional overturning circulation (AMOC) have often been invoked to explain the physical characteristics of these Dansgaard-Oeschger climate oscillations1, 2, 3, but the mechanisms for the greenhouse-gas variations and their linkage to the AMOC have remained unclear4, 5, 6, 7, 8. Here we present simulations with a coupled model of glacial climate and biogeochemical cycles, forced only with changes in the AMOC. The model simultaneously reproduces characteristic features of the Dansgaard-Oeschger temperature, as well as CO2 and N2O fluctuations. Despite significant changes in the land carbon inventory, CO2 variations on millennial timescales are dominated by slow changes in the deep ocean inventory of biologically sequestered carbon and are correlated with Antarctic temperature and Southern Ocean stratification. In contrast, N2O co-varies more rapidly with Greenland temperatures owing to fast adjustments of the thermocline oxygen budget. These results suggest that ocean circulation changes were the primary mechanism that drove glacial CO2 and N2O fluctuations on millennial timescales.