New Paper in Science Magazine: Carbon Dioxide Did Not End The Last Ice Age

Published Online September 27, 2007
Science DOI: 10.1126/science.1143791

Southern Hemisphere and Deep-Sea Warming Led Deglacial Atmospheric CO2 Rise and Tropical Warming

Lowell Stott 1*, Axel Timmermann 2, Robert Thunell 3

1 Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA.
2 IPRC, SOEST, University of Hawaii, 2525 Correa Road, HI 96822, USA.
3 Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA.

Establishing what caused Earth’s largest climatic changes in the past requires a precise knowledge of both the forcing and the regional responses. Here we establish the chronology of high and low latitude climate change at the last glacial termination by 14C dating benthic and planktonic foraminiferal stable isotope and Mg/Ca records from a marine core collected in the western tropical Pacific. Deep sea temperatures warmed by ~2C between 19 and 17 ka B.P. (thousand years before present), leading the rise in atmospheric CO2 and tropical surface ocean warming by ~1000 years. The cause of this deglacial deep water warming does not lie within the tropics, nor can its early onset between 19-17 ka B.P. be attributed to CO2 forcing. Increasing austral spring insolation combined with sea-ice albedo feedbacks appear to be key factors responsible for this warming.

Summary at EurekAlert

Extract:

Deep-sea temperatures rose 1,300 years before atmospheric CO2, ruling out the greenhouse gas as driver of meltdown, says study in Science. Carbon dioxide did not cause the end of the last ice age, a new study in Science suggests, contrary to past inferences from ice core records. “There has been this continual reference to the correspondence between CO2 and climate change as reflected in ice core records as justification for the role of CO2 in climate change,” said USC geologist Lowell Stott, lead author of the study, slated for advance online publication Sept. 27 in Science Express. “You can no longer argue that CO2 alone caused the end of the ice ages.” Deep-sea temperatures warmed about 1,300 years before the tropical surface ocean and well before the rise in atmospheric CO2, the study found. The finding suggests the rise in greenhouse gas was likely a result of warming and may have accelerated the meltdown – but was not its main cause. The study does not question the fact that CO2 plays a key role in climate. I don’t want anyone to leave thinking that this is evidence that CO2 doesn’t affect climate,” Stott cautioned. “It does, but the important point is that CO2 is not the beginning and end of climate change.” While an increase in atmospheric CO2 and the end of the ice ages occurred at roughly the same time, scientists have debated whether CO2 caused the warming or was released later by an already warming sea. The best estimate from other studies of when CO2 began to rise is no earlier than 18,000 years ago. Yet this study shows that the deep sea, which reflects oceanic temperature trends, started warming about 19,000 years ago. “What this means is that a lot of energy went into the ocean long before the rise in atmospheric CO2,” Stott said. But where did this energy come from” Evidence pointed southward. Water’s salinity and temperature are properties that can be used to trace its origin – and the warming deep water appeared to come from the Antarctic Ocean, the scientists wrote. This water then was transported northward over 1,000 years via well-known deep-sea currents, a conclusion supported by carbon-dating evidence. In addition, the researchers noted that deep-sea temperature increases coincided with the retreat of Antarctic sea ice, both occurring 19,000 years ago, before the northern hemisphere’s ice retreat began. Finally, Stott and colleagues found a correlation between melting Antarctic sea ice and increased springtime solar radiation over Antarctica, suggesting this might be the energy source. As the sun pumped in heat, the warming accelerated because of sea-ice albedo feedbacks, in which retreating ice exposes ocean water that reflects less light and absorbs more heat, much like a dark T-shirt on a hot day. < > “The climate dynamic is much more complex than simply saying that CO2 rises and the temperature warms,” Stott said. The complexities “have to be understood in order to appreciate how the climate system has changed in the past and how it will change in the future.”