Periodic Rises In Atmospheric Carbon Dioxide and Methane

Studies of atmospheric gases from ice cores revealed that sharp increases in atmospheric carbon dioxide and methane occurred during Dansgaard-Oeschger events. The increases were so sudden and massive that the release of the gases must have been enormous. Paleoclimatologists think the increase in carbon dioxide was caused by changes in the deep circulation patterns of the ocean. Like the atmosphere,the ocean circulates in a regular pattern. Moreover, scientists have learned, the ocean's circulation is influenced by changes in the atmosphere. Researchers theorize that, as global temperatures rose during Dansgaard-Oeschger events, the vertical circulation of the ocean increased, carrying more carbon dioxide-rich water from the deep ocean up to the surface. At the surface, the carbon dioxide escaped into the air, raising global levels of atmospheric carbon dioxide.

The sudden increase in atmospheric methane during Dansgaard-Oeschger events may have been triggered by geologic activity, such as underwater earthquakes or landslides. Such incidents could have disturbed huge pockets of methane gas, produced by the decay of organic (from plants and animals) matter that had become buried beneath layers of sea-floor sediment. Another theory is that the rising temperatures melted huge deposits of methane hydrate, an icy substance formed when methane gas molecules are trapped in a “cage” of water ice. Huge deposits of methane hydrate exist today in the deep ocean and below the permafrost (a layer of permanently frozen soil) of Siberia.