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5.1: How much of the CO2 Increase is Natural

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  • So the evidence, from both theory and observations (we have not even talked about models yet), suggests that the warming of the last century is unusual by the standards of the last few thousand years and almost certainly caused by increasing atmospheric CO₂ concentrations. But could the trends in CO₂ concentration themselves be natural?

    Almost certainly not. Figure \(\PageIndex{1}\) shows the history of atmospheric CO₂ and Antarctic temperature going back 800,000 years, thus covering many Milanković cycles. The CO₂ concentration was obtained from bubbles of air trapped in the ice cores.

    Clearly, the atmospheric concentration of CO₂ does vary naturally, in tandem with temperature, ranging from about 180 to about 280 parts per million by volume (ppmv). But the Milanković cycles cannot account for the enormous spike at the end of the record, a spike to 400 ppmv that humans put there. There is no evidence that it has been that large for many millions of years. If we do nothing, and there is no global economic meltdown, we may reach well over 1000 ppmv by the end of this century.

    A very close and careful analysis of the records of temperature and CO₂ in ice cores shows that during Milanković cycles, CO₂ mostly lags temperature, suggesting that the CO₂ variations were caused by the warming and cooling, not the other way around. In this case, the CO₂ was acting as a positive feedback, amplifying the Milanković oscillations. But in the last 100 years, the huge increase in CO₂ drove the temperature change. (The argument that one has to choose whether CO₂ is a forcing or a response is specious. The same agent can be a forcing in one circumstance and a response in another. Suppose you have a manual transmission car in first gear, pointed downhill, and you release the brake. The downhill motion of your car will spin up its engine. In fact, this is a good way to start your car if its battery is dead and you happen to be pointed downhill. But ordinarily, the engine powers the motion of the car.)

    Temperature and CO2 From Antarctic Ice Cores Over the Past 800,00 Years

    Figure \(\PageIndex{1}\): Atmospheric CO₂ (blue) and temperature (red) from Antarctic ice cores. The concentration of CO₂ in the year 2015 is shown by the star in the upper right. Data from Lüthi et al., 2008, Nature, 453, 379-382, and Jouzel et al., 2007, Science, 317, 793-797.