How long will global warming last?

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) [13]. 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.

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