The four greenhouse gases with the strongest effect on climate through their climate forcing are water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) (I’m omitting halocarbons, which come in a wide variety). We don’t control the concentration of water vapor, temperature does that. But the CO2, CH4, and N2O load is directly due to us.
All three are on the rise. The question I’d like to address is, how are we changing, not their concentrations, but their climate forcing? Is one or the other dominant? Let’s take a look.
IPCC reports give formulae for computing the climate forcing for a given concentration of greenhouse gases. A complicating factor is that they can affect other chemical properties in the atmosphere, which can in turn affect climate forcing. Methane in particular affects ozone and statospheric water vapor, which magnifies its climate forcing considerably. I’ll include this effect in the methane forcing, so we know how this might impact the future unfolding of climate.
Here are the concentrations of the “big three” since 1990, from observations at Cape Grim:
CO2 dominates by a wide margin — note that the units for CO2 are parts per million (ppmv) while units for the others are parts per billion (ppbv). CO2 concentration is over 200 times as great as CH4, and 1000 times that of N2O. One might get the impression that “it’s all about the CO2.”
But those two less-concentrated, as greenhouse gases, are more potent per molecule than CO2. Here are their climate forcings since 1990 (including the impact of CH4 on ozone and water vapor):
CO2 forcing is biggest, but CH4 is substantial and even N2O isn’t negligible. And they’re all three on the rise.
But CO2 forcing is rising faster than the others, apparently a lot faster. I fit a smooth curve to each, which also estimates for me the rate of change, and got this:
The rate at which we’re changing CO2 forcing is quite a lot faster than the rates for the others; for CO2 it’s rising at .033 W/m^2/yr, for CH4 it’s .005 W/m^2/yr, and for N2O .003 W/m^2/yr. We can also see that during the early 2000s, the slower rise of CH4 meant that at that time, N2O forcing was rising faster.
But the bottom line is that carbon dioxide forcing is rising 6 times as fast as methane forcing, 10 times as fast as nitrous oxide forcing, and 4 times as fast as those two combined. Clearly they’re not negligibe — we need to reduce emissions of those other greenhouse gases too — but just as clearly it’s carbon dioxide that’s the biggest problem. That’s the one we most need to stop dumping in the atmosphere.
One other worrisome fact is that we don’t control the atmospheric concentration of these gases completely. Global warming itself threatens to bring the house down around our heads by feedbacks in the carbon cycle. In particular, the melting of permafrost and/or clathrates might add massive quantities of carbon dioxide and methane to our already overloaded atmosphere. That could bring disaster.
Let’s not find out … the hard way.
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