The Big 3: CO2, CH4, N2O

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:

big3

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):

big3force

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:

big3rate

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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15 responses to “The Big 3: CO2, CH4, N2O

  1. “Lessons not learned, will be repeated”

  2. Reblogged this on Hypergeometric and commented:
    Greenhouse gases seen from the perspective of their marginal radiative forcings. This is a nice normalization of how much we should care about each.

  3. interesting that the rate of CO2 change dipped in the late 2000s. A side-effect of the Great Recession?

    [Response: No, it’s an artifact of the method for estimating the rate. This particular method imposes the “minimum acceleration” criterion at the endpoints, so the slope is forced to be constant within a small interval near the beginning and end of the time series. Its purpose is to prevent acceleration from “taking off” near the ends, which improves the estimated value but degrades the estimated rate (only near the endpoints).]

  4. Nice analysis, as usual!

    “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 bears repeating–loudly.

  5. Tamino,
    From what I’ve read from Natalie et al, the permafrost forcing seems much more likely to occur than clathrates. What are your thoughts on this?

  6. Do what common sense and Hansen say an put monetary cost on emissions!
    I just went by long distance bus and payed a voluntary climate offset fee that more or less amounts to 45 € / 50 US$ per tonne CO2. It’s ridiculously small. It’s invested in some climate friendly projects.
    Or buy some tonnes of CO2 out of the European ellowances Market, e.g. via sandbag.org. With the current low allowance price also only some beers less.

  7. You don’t mention that the forcing from CO2 is accelerating. This is another reason why we need to be very concerned about CO2.

  8. The post doesn’t touch on an additional reason to be fearful of our CO2 pollution. Of the three, CH4 requires continued topping-up to maintain its forcing. Without it, the forcing drops off quickly in a couple of decades. N2O is less needy of top-up. It also drops off although less quickly, hanging on a couple of centuries.
    CO2 levels do also drop off but more slowly and further, CO2 in dropping out of the atmosphere never really exits the system and leaves a large atmospheric residue. Where CO2 scores as the pollutant of choice is that it hangs around for millennia. If there were some vile swine hateful of our planet who truly wants to wreak havoc, CO2 is the one to use.

  9. I’m nitpicking a bit, but isn’t O3 a stronger greenhouse gas than N2O by forcing ? The temperature response is probably lower though, because of how short-lived O3 is.

  10. Al Rodger IIUC CH4 breaks down to CO2, so needs limiting too.

    • turboblocke,
      The CO2 resulting from the breakdown of our CH4 emissions is very small and doesn’t add greatly to our CO2 pollution. For instance, it doesn’t even feature in the “all sources” analyses. Our CO2 emissions (2014) from “all sources” was 10,900Mt(C) according to CDIAC. Our CH4 emissions are some 60% of a total global figure of about 500Tg(CH4). So that would set our emissions at some 300Tg(CH4) or 225Mt(C), so adding about 2% to our CO2 emissions, equal to half the input from cement production.
      (Although mentioning cement, when made into concrete cement does slowly re-absorb its CO2, only it takes a bit of time to do it – in the case of Portland cement its about 1,000 years.)
      The IPCC RCP2.6 sets out a possible course for our emissions. As this graphic shows, the CH4 emissions for RCP2.6 remain at about 50% of today’s emissions even by 2100 when CO2 emissions are below zero.

  11. The problem is that on a current basis, 1 unit of CH4 is the equivalent of 86 units of CO2, so the current atmospheric concentration of CH4 of 1.8 ppmv provides the forcing of CO2 at 155 ppmv for a total of 559 CO2e. However, the IPCC in its wisdom assumes a atmospheric half-life for CH4 of 12.5 years, so they set a unit of CH4 is the equivalent of 24 units of CO2. How DID they get to a half life of 12.5 years?

    If we look at the observational record, CH4 has gone from a pre-industrial level of 700 ppbv to the current level of ~1,800 ppmbv. We have good observational data for the last 27 years, and CH4 concentrations in the atmosphere have gone up. On an system observational basis the IPCC’s assumption of a 12.5 yr half-life seems very weak.

    When I was a kid, we learned that CH4 oxidation in the atmosphere was rate limited by the available OH- radicals and was thereby limited to a set quantity of CH4 per unit time, rather than to some sort of a half-life decay. With this analysis, in a time of AGW with higher emission rates, the half-life of CH4 in the atmosphere increased with the increasing concentration in the atmosphere. We were just chemists.

    Jay Forrester told us to look at the SYSTEM. Today, I suggest that the sources of CH4 are fossil fuels, free methane in geologic formations, biogenic (wetlands and ocean), thermogenic, and clathrate decomposition. The sinks are atmospheric oxidation, terrestrial metabolism, aquatic metabolism, solution in water, and formation of clathrates. The stores of methane are the atmosphere, dissolved in the oceans, geological formations, and sea floor clathrates.

    The terrestrial and aquatic metabolic sinks tell us that in time scales of thousands of years, CH4 can be converted to biomass, and need not be a greenhouse gas at all. Or, it can be converted to CO2. There is no real limit on how much, or even how fast, CH4 can be removed from the atmosphere. The use of a half-life for atmospheric CH4 installed a large number of assumptions into the climate models that seem not to be correct.

    There is a 2-way equilibrium between the concentration of CH4 in the oceans and in the atmosphere. There is a 2-way equilibrium between the concentration of CH4 in the oceans and the clathrate in the sea floor. Warming will cause the clathrates to decompose and the solubility of CH4 in the oceans will decline. AGW can cause CH4 to flow from clathrate and ocean stores into the atmosphere very rapidly. In a time of global cooling, with sea ice formation, CH4 can be sequestered out of the atmosphere as sea floor clathrates very rapidly, and the colder ocean waters can hold more CH4 in solution.

    If you account CH4 as the equivalent of 86 units of CO2, then this year, you have Forcing = ~3.6 W/m^2 plus the forcing from N2O. It is a larger number than what the IPCC admits at this time.

    For forcing calculations, it does not matter how long a particular molecule of CH4 persists in the atmosphere, what matters is the total concentration. If we put a little chemist out there whacking CH4 molecules, more CH4 will come out of the ocean to keep the concentration in the atmosphere in equilibrium with the concentration in the ocean at the then global temperature.

    • All the more important to limit methane leakage

    • Aaron, if you have questions about CH4, then might I recommend emailing Ed Dlugokencky at the NOAA? He’s been very helpful in answering the questions I’ve had.