Links between abrupt change in tropical hydroclimate, high-latitude climate change, and atmospheric greenhouse gases during the last ice age
Abstract:Development of very high-resolution data from polar ice cores over the last decade reveals a rich spectrum of greenhouse gas variability and its relationship to both tropical and subtropical hydroclimate and high-latitude abrupt climate change. The well-known atmospheric methane variations associated with Dansgaard-Oeschger events are now strongly linked to enhanced wetland emissions in the northern tropics based on recent work on the interpolar methane gradient. An increase in tropical rainfall associated with ITCZ migration is consistent with these observations. In addition, small, on order 5-10 ppm, changes in carbon dioxide accompany at least some Dansgaard-Oeschger events. Changes in terrestrial carbon storage, possibly in the tropics, are one explanation, but new stable isotope measurements indicate that this cannot be the only source for these events, and suggest that rising sea surface temperature must contribute.
Very detailed recent data reveal variability during Greenlandic stadial periods that add to the potential links between greenhouse gases and tropical hydroclimate. During the last ice age and deglaciation, small, but rapid increases in atmospheric methane during some “Heinrich Stadials” suggest increases in methane emissions from the southern tropics associated with Heinrich events, possibly due to extreme southerly migration of rainfall belts associated with the ITCZ. Abrupt increases in carbon dioxide occur at precisely the same time as many of these Heinrich Stadial methane events. Stable isotopic data related to two of these abrupt carbon dioxide changes (during HS1 and preliminarily for HS 4) implicate an isotopically depleted source. Rapid release of terrestrial carbon (possibly due to drying in the northern tropics) is a possible explanation, although release of respiratory carbon dioxide from an ocean source (for example, due to increases in southern ocean upwelling) is another plausible alternative, albeit one that requires a fast oceanic mechanism.