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 . 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 . 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 , ocean methane clathrate deposits , and future evolution of glacial/interglacial cycles  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 . 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.
Even the present-day net flux tends to underestimate the real lifetime of global warming. The atmosphere contains about 160 Gt more carbon than it did then. If we divide this number by the CO2 invasion flux into the ocean of 2 Gt C/year, we get an apparent uptake time scale of 80 years. This result is shorter than model air/water equilibration time scales by a factor of four or so. I believe the problem is with the simple calculation. The CO2 concentration of the atmosphere is going up continuously, and so it invades the ocean as it equilibrates with warm surface waters. If atmospheric CO2 were not going up, the warm surface waters would saturate in a year or two, the overall ocean invasion rate would decrease, and the lifetime estimates by this method would increase. Different parts of the ocean equilibrate with the atmosphere on different time scales, ranging from a year for the tropical surface ocean to a millennium for the deep sea. Overall, model experiments show a CO2 equilibration time of a few centuries [5, 6, 11, 12]. The other problem with both of these conceptions is that they implicitly assure us that the CO2 concentration is going back to its initial concentration, which it will not.
Another source of short-lifetime bias in the community probably comes from a calculation used to compare the greenhouse consequences of different gases, called the Global Warming Potential (GWP) . Some trace gases such as methane have a stronger impact on the heat balance of the earth, per molecule, than CO2 does. However, to really compare them fairly one might want to factor in the fact that methane only lives about 10 years before it goes away (actually, it is oxidized to CO2, another greenhouse gas, but it is common to ignore that in GWP calculatons). Global warming potentials are calculated by integrating the radiative energy impact of a molecule of gas over its atmospheric lifetime. However, if the full lifetime of CO2 were considered, including that long tail, then methane would be by that calculation unimportant. On human time scales, methane is certainly an important greenhouse gas, and so what’s done is to arbitrarily limit the time horizon of the calculation to something like human timescales. Methane GWP is higher when considered on the 50-year time horizon than it is on the 500-year time horizon or it would be on a 500,000-year time horizon, if anyone bothered to do that calculation. Perhaps the adoption of time horizons for GWP calculations conditions scientists to believe that CO2 only persists for as long as this time horizon lasts. The table in the EPA document, for example, was associated with a discussion of global warming potentials.
It could also be that we-who-only-live-to-be-77.2-years-old don’t want to worry about climate impacts from fossil fuel CO2 release 100,000 or even 1,000 years from now. That would be a perfectly rational position, and I have no argument with it. Climate change negotiations are grounded in IPCC projections and scenarios to the year 2100, a far cry from the year 100,000, but even 2100 seems almost unimaginably remote given the pace of social and technological change in the world today. On the other hand, nuclear waste lasts for millions of years for some isotopes such as iodine 129. The public seems to find this information relevant, so the true longevity of anthropogenic climate change might be considered by some to be relevant to here-and-now decisions as well. At any rate, the facts as reported ought to be accurate, rather than judging in advance that no one cares about climate impacts that last thousands of years and more into the future.
- (with the subscript “No single lifetime can be defined for CO2 because of the different rates of uptake by different removal processes”) (EPA (2005), Inventory of U.S. Greenhouse Gas Emissions and Sinks Draft Report: 1990 -2003, U.S. Environmental Protection Agency, Office of Atmospheric Programs
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