During Earth’s recent history, there has been a relationship between carbon dioxide and global temperatures: when temperatures rise, the concentration of CO2 subsequently rises, and when temperatures fall, CO2 concentration thereafter declines. Ice cores from Vostok in Antarctica have been studied intensively to analyze temperatures and concentrations of various gases over the course of millennia. Euan Mearns reviews the literature as it relates to both CO2 and methane:
In their seminal paper on the Vostok Ice Core, Petit et al (1999) note that CO2 lags temperature during the onset of glaciations by several thousand years but offer no explanation. They also observe that CH4 and CO2 are not perfectly aligned with each other but offer no explanation. The significance of these observations [is] therefore ignored. At the onset of glaciations temperature drops to glacial values before CO2 begins to fall suggesting that CO2 has little influence on temperature modulation at these times. …The fit of CO2 to temperature is actually not nearly so tight as for CH4. There is a persistent tendency for CO2 to lag temperature throughout and this time lag is most pronounced at the onset of each glacial cycle “where CO2 lags temperature by several thousand years.”
This chart shows how the Vostok ice cores reveal temperature and CO2 concentration over the Earth’s recent history, i.e., the last 450,000 years. The present is on the left; click to enlarge:
Rising temperatures cause an increase in CO2 levels primarily because warmer oceans hold less CO2. (“The most likely source for most of the CO2 is considered to be the oceans where warming seawater can hold less CO2.”) At neither end of the glaciation cycle does CO2 concentration forecast temperature. The linked article contains much technical discussion of the mechanisms by which rising and falling temperatures cause (over a time period of millennia) rising and falling levels of CO2. These are the paper’s conclusions:
* Over four glacial cycles CO2, CH4 and temperature display cyclical co-variation. This has been used by the climate science community as evidence for amplification of orbital forcing via greenhouse gas feedbacks.* I am not the first to observe that CO2 lags temperature in Vostok and indeed Petit et al make the observation that at the onset of glaciation CO2 lags temperature by several thousand years. But they fail to discuss this and the fairly profound implications it has.* Temperature and CH4 are extremely tightly correlated with no time lags. Thus, while CO2 and CH4 are correlated with temperature in a general sense, in detail their response to global geochemical cycles are different. Again Petit et al make the observation but fail to discuss it.* At the onset of the last glaciation the time lag was 8,000 years and the world was cast into the depths of an ice age with CO2 variance evidently contributing little to the large fall in temperature.* The only conclusion possible from Vostok is that variations in CO2 and CH4 are both caused by global temperature change and freeze thaw cycles at high latitudes. These natural geochemical cycles makes it inevitable that CO2 and CH4 will correlate with temperature. It is therefore totally invalid to use this relationship as evidence for CO2 forcing of climate, especially since during the onset of glaciations, there is no correlation at all.
Based on Earth’s recent geologic history, claiming that CO2 concentration causes warming is like arguing that geese cause temperatures to fall by flying South.
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