How long will global warming last?

Guest commentary from David Archer (U. Chicago)

The notion is pervasive in the popular and scientific literature that the lifetime of anthropogenic CO2 released to the atmosphere is some fuzzy number measured most conveniently in decades or centuries. The reality is that the CO2 from a gallon out of every tank of gas will continue to affect climate for tens and even hundreds of thousands of years into the future.

The U.S. Environmental Protection Agency Inventory of U.S. Greenhouse Gas Emissions and Sinks (2005) has the CO2 lifetime listed as 5-200 years, for example [1]. I have seen “hundreds of years” in scientific manuscripts and in environmentalist literature. David Goodstein in his excellent book The End of the Age of Oil states, “If we were to suddenly stop burning fossil fuel, the natural carbon cycle would probably restore the previous concentration in a thousand years or so.” I assume that Goodstein is conservatively applying several century-long e-folding times to derive his thousand years, but he implicitly assumes that the CO2 will relax toward its 1750 concentration. The point is that it does not.

When you release a slug of new CO2 into the atmosphere, dissolution in the ocean gets rid of about three quarters of it, more or less, depending on how much is released. The rest has to await neutralization by reaction with CaCO3 or igneous rocks on land and in the ocean [2-6]. These rock reactions also restore the pH of the ocean from the CO2 acid spike. My model indicates that about 7% of carbon released today will still be in the atmosphere in 100,000 years [7]. I calculate a mean lifetime, from the sum of all the processes, of about 30,000 years. That’s a deceptive number, because it is so strongly influenced by the immense longevity of that long tail. If one is forced to simplify reality into a single number for popular discussion, several hundred years is a sensible number to choose, because it tells three-quarters of the story, and the part of the story which applies to our own lifetimes.

However, the long tail is a lot of baby to throw out in the name of bath-time simplicity. Major ice sheets, in particular in Greenland [8], ocean methane clathrate deposits [9], and future evolution of glacial/interglacial cycles [10] might be affected by that long tail. A better shorthand for public discussion might be that CO2 sticks around for hundreds of years, plus 25% that sticks around forever.

The sticking-around-forever idea is not new, and the picture has not changed by very much since the effect was first predicted back in 1992 [2]. You can estimate the magnitude of the effect pretty well just using CO2 thermodynamics and the back of an envelope. It could be argued (by someone with a cruel heart) that since we don’t understand why CO2 was lower during the last ice age, we ought not go around making forecasts for the future. Well, OK, but I would point out that CO2 in the past appears to act as an amplifier for orbitally forced climate change, so if anything, we might expect the carbon cycle in the future to amplify our own climate forcing, rather than counteract it. If the past is any guide, CO2 surprises in the future, in the long run, seem unlikely to help us out.

A long lifetime for CO2 adjustment is also consistent with an isotopic event in the deep sea sedimentary record from 55 million years ago, the Paleocene/Eocene Thermal Maximum event. The record tells the story of the sudden release of an isotopically light source of carbon, triggering a fast warming in the deep sea of about 5 degrees C. Both the carbon isotope signal and the temperature (inferred from oxygen isotopes) then relaxed back toward their initial values in about 100,000 years. If the released carbon were initially in the form of methane, it would have been oxidized to CO2 within a few decades, but as CO2 it apparently stuck around, warming the deep ocean, for a long time before it went away.

The shortest lifetime estimates, such as EPA’s 5-years, derive from the exchange flux of CO2 between the atmosphere and ocean, which is about 200 Gt C/year (1 Gt C is 1012 kg of carbon) in each direction. Because the exchange flux is back-and-forth, it has nothing to do with the net uptake by the ocean of new CO2 to the system, which relies on the imbalance between the upward and downward exchange fluxes. That imbalance is only about 2 Gt C/year.

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