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Variations in Atmospheric CO2, Temperature and Global Ice Volume Derived from the Vostok Ice Core
Reference
Mudelsee, M. 2001. The phase relations among atmospheric CO2 content, temperature and global ice volume over the past 420 ka. Quaternary Science Reviews 20: 583-589.

What was done
Using proxy data, the author performed a statistical analysis (lagged, generalized least-squares regression and bootstrap resampling) to estimate the phase relations (leads/lags) of atmospheric CO2 concentration, air temperature and global ice volume over the past 420,000 years as derived from the Vostok ice core.

What was learned
Variations in atmospheric CO2 concentration were found to lag behind variations in air temperature by 1.3 to 5 ka (thousand years). Phase relations between CO2 and global ice volume were not as clear cut. When CO2 values were compared with global ice volume data derived from a delta 18O record of a marine sediment core, it was shown that between 420 and 196 ka years ago, variations in CO2 lagged behind changes in global ice volume by 1.4 3.7 ka, whereas from 150 ka to the present they lead by 6.2 2.7 ka. A more uniform phase relationship was obtained when comparing the Vostok CO2 record with a Vostok delta 18O record. Although considerable scatter existed in the data, atmospheric CO2 concentration consistently led global ice volume by an average of 3.9 0.5 ka.

What it means
The results of this study, along with those of many others we have described (see CO2-Temperature Correlations in our Subject Index), should put to rest the notion that atmospheric CO2 is a major driver of climate change. Throughout the greatest temperature transitions experienced by the planet over the past 420,000 years, atmospheric CO2 concentration has been proven to have been a follower, and not a leader, of climate change, rising from one to five thousand years after major increases in air temperature, and falling in similar manner throughout the course of the past four glacial/interglacial cycles.

Also evident from this study is the even longer delayed response of global ice volume to changes in air temperature. Because this lag stretches an additional 4,000 to 6,000 years beyond the lag in CO2, it suggests that any present decline in global ice volume may be more related to the warm temperatures experienced during the Holocene Maximum - 4,000 to 7,000 years ago, when global temperatures were around 2C warmer than present - than it is to 20th Century warming.