A common argument of global warming deniers goes like this:
Analysis of Greenland, Siberian and Antarctic ice cores shows that, although temperature and carbon dioxide correlate over long periods of time, temperature appears to lead CO2 by an average of 800 years. Cause and effect can't reach back in time, so clearly temperature is the cause and CO2 the effect, not vice versa. Therefore, we don't have to worry about carbon dioxide causing global warming.
Sounds plausible, doesn't it? And the temperature lead time in the ice cores is definitely there. Have the global warming deniers finally found a clinching argument?
Let's examine what causes ice ages. Climatologists agree that these are initiated by the so-called "Milankovic Cycles." These are periodic variations in the Earth's orbit and axial tilt that affect the distribution of sunlight over the Earth's surface:
These cycles alter the distribution of sunlight over the Earth's surface, regularly increasing and decreasing the relative amount that falls on the planet's ice-covered poles. Serbian astronomer Milutin Milankovic proposed in 1930 that these variations might account for the Quaternary ice ages that have governed Earth's climate for about the last 3,000,000 years. By 1970 or so his explanation was generally accepted.
But detailed calculation shows that these variations are not enough to account for the swings in temperature experienced in the ice ages, another variable measured from ice core data. The swing in mean global annual surface temperature (M-GAST) across the ice ages is about 5-6 K. Milankovic cycles only account for about 3 K.
Something amplifies the temperature swing. The most likely candidate is carbon dioxide, which is a greenhouse gas. More CO2 in the air raises the surface temperature, all else being equal. For an explanation as to why, try here.
Let's examine what happens in a natural deglaciation. Milankovic cycles start to warm the poles, and the Earth's temperature slowly rises. Carbon dioxide exists in large amounts in solution in the world's oceans. But the warmer the water gets, the less CO2 it can hold. CO2 bubbles out of the oceans, and it in turn raises the temperature more. The temperature does not run away because the feedback between warming and CO2 release is a converging series, not a diverging one.
In short, it's a feedback. Temperature raises CO2, which raises temperature. Carbon dioxide is both a cause and an effect.
But is this always what happens when the Earth warms? Does it account for the present global warming? No and no.
There have been other episodes in Earth's geological history where carbon dioxide increased first, e.g. during episodes of excessive volcanism, and temperature followed. One example is the Paleocene-Eocene Thermal Maximum (PETM), which occurred about 55 million years ago and caused a mass extinction.
And another example is the present global warming. The new carbon dioxide in the air is NOT coming out of the oceans. We know this for three reasons:
There is also a clue from the famous 800-year time lag. If this were a natural deglaciation, we would expect to see a lag centuries long between the temperature increase and the carbon dioxide increase. Instead, the two are highly correlated (r = 0.87) in the same year:Correlation
So, in short, the argument "temperature leads CO2, so it's the cause and CO2 is the effect" fails on several grounds. In brief:
And that's that. Carbon dioxide is the main cause of the present global warming, and it's a serious problem.