How long will global warming last?

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) [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.

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