Which came first, the carbon dioxide or the heat?

A glacier on the coast of Greenland
Image:Basheer Tome

The topic of anthropogenic climate change always seems to be a "hot" issue, and an article in an upcoming issue of Science will look at the relation of two components of climate change. The authors examine both the timing of CO2 increases relative to temperature increases, and verify a prediction made by climate models. HangZhou Night Net

Over the course of glacial and interglacial cycles, CO2 and temperature have been highly correlated, but many increases in CO2 appear to occur after increases in temperature, rather than vice versa. Most climate scientists have accepted that the climate system is complicated and other forces may have initiated warming, while CO2 merely acted as a positive feedback. Others, however, have used this evidence to suggest that CO2 may not cause increased temperatures.

The current study looks at individual spikes in the Greenland temperature record and compares those to global CO2 concentrations. They find that, from 30 to 90 thousand years ago, major warming events in Greenland (known as Dansgaard Oeschger or D-O events) have been preceded by increases in CO2. For the major D-O events, CO2 typically begin to increase two to four thousand years earlier.

This alone is an important result, but the authors take it a step further by looking at ocean circulation patterns during these D-O events and compare that to climate model predictions. Climate models indicate that specific oceanic events—maximum North Atlantic Deep Water (NADW) shoaling and minimal Southern Ocean stratification—should correspond to maximum rates of CO2 increase.

As with other studies, this one uses δ13C data off the coast of Iberia as a proxy for NADW circulation patterns, and δ15N off the coast of Chile as a proxy for stratification of the Southern Ocean. The authors show that these data have the pattern predicted by climate models, which serves as a further verification of the efficacy of climate models.

Science, 2008. DOI: 10.1126/science.1160832

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