3.2: Our Effect on the Greenhouse

Much of the preceding, save for the details of the processes that control atmospheric CO₂, was understood by the end of the 19th century. In particular the Swedish chemist and Nobel laureate Svante Arrhenius understood the effect of greenhouses gases on climate and that CO₂ is the most important long-lived greenhouse gas. He also understood that we were beginning to emit prodigious amounts of CO2 into the atmosphere from industrial processes and was the first to worry that, owing to its long residence time in the atmosphere, we would perceptibly increase its concentration.

In 1896 Arrhenius published a paper predicting that if we ever managed to double the concentration of CO₂, the average surface temperature of the planet would rise between 5 and 6 K (9 and 11 °F), a number he revised downward to 4 K (7 °F) in a popular book he published in 1908. Arrhenius arrived at these numbers by performing up to 100,000 calculations by hand, and although he made several incorrect assumptions, the resulting errors partially cancelled each other. It is truly remarkable that his 4 K is within the range of the most recent estimates of 1.5–4.5 K (2.7–8.1 °F). Arrhenius also understood that the radiative effects of CO₂ increase nearly logarithmically (rather than linearly) with its concentration, so that increasing CO₂ by a factor of 8 would produce about three (rather than four) times more warming than doubling it would.

Global Mean Surface Temperature and CO₂

Figure \(\PageIndex{1}\): Annual, global mean surface temperature (green) and thenatural logarithm of atmospheric CO2 concentration (blue).

Arrhenius predicted that increasing CO₂ would warm the planet. How did his prediction fare? Figure \(\PageIndex{1}\) compares Arrhenius’s prediction based on atmospheric CO₂ concentrations with measured global mean surface temperature for the period from 1880 to 2010. The CO₂ content of the atmosphere was measured directly beginning in 1958; before that time (and going back for hundreds of thousands of years) we deduced its abundance by measuring its concentration in gas bubbles trapped in ice cores.

Over the period of record, the global mean temperature generally follows the natural logarithm of the concentration of CO₂, just as Arrhenius predicted. At the same time, there are obvious deviations from this correlation. The shorter-period deviations mostly reflect the natural, chaotic variability of the climate system (an example of which is El Niño), while longer excursions are mostly due to other influences on climate, such as volcanoes and manmade aerosols. A lawyer may pick apart all these wobbles, but it is hard to avoid the conclusion from Figure \(\PageIndex{1}\) that the data largely vindicate a prediction made more than a century ago, based on simple physics and hand calculations. It stands to reason that more warming will occur if we continue to increase the concentration of CO₂ in the atmosphere.

But what if we are fooling ourselves? Correlation is not causation, and perhaps the correspondence of temperature and CO₂ is a coincidence—maybe something else is causing the warming. Or perhaps the rising temperature is causing CO₂ concentrations to increase and not the other way around. How accurate is the green curve in Figure 2—can we really measure the global mean temperature? Climate is always changing, so what is so special about the last 100 years? Are there other predictions of climate science that are verified or contradicted by observations?

These are all legitimate questions and deserve serious consideration; indeed, we would not be good scientists if we did not constantly ask ourselves such questions.