Methane, the second largest human-caused contributor to global warming, often doesn’t get any respect. The constant focus on “carbon” leads people to believe that our only climate change issues are issues with fossil fuels and carbon dioxide. A key source of methane is livestock (basically, belching cattle); in fact, livestock agriculture is also a significant source of carbon dioxide as well.
This is a rather lengthy post, so I am going to summarize my basic argument up front, and then go into detail on each point.
UPDATE 4/18/2016: this blog is actually more about aerosols and their effect on climate change than it is about methane. The title, I now realize, is quite misleading. I’m not an expert, I’m just reading stuff and trying to figure out what it means, and because it’s not my field I go over things in a laborious way to make sure I’m not missing something. So please make comments or e-mail me privately.
The gist of this blog: if aerosols emitted by burning fossil fuels are now having a large impact in blunting global warming effect of carbon dioxide (“negative radiative forcing”), then if we stop burning fossil fuels for whatever reason, we may experience a temporary rapid warming effect. The aerosols and their negative radiative forcing will drop out fairly quickly, but the carbon dioxide and its positive radiative forcing will hang around for decades or centuries. Conclusion: climate change is actually substantially worse than it appears today, because the aerosols have been masking the true warming effect of the carbon dioxide all along. As soon as we stop burning fossil fuels we will find this out as the aerosols are going to go away in a hurry as the real effects of the carbon dioxide will be “unmasked” and fully revealed. I find this extremely scary. If I were to rewrite this blog today, I would just mention this fact and not discuss methane at all. What was I thinking?
We can calculate the total “global warming effect” of the various human-caused greenhouse gas emissions in terms of “watts per square meter.” (Hereafter I will abbreviate this as “w/m2.”) This is the equivalent of 1 watt over each square meter of the earth’s surface. This doesn’t seem like much, but the overall impact would be significant.
1. Methane doesn’t seem to be that significant. If you calculate the total effect of all the human-caused greenhouse gases, you get approximately this.
Carbon dioxide — +1.6 w/m2
Methane — +0.5 w/m2
Other gases* — +0.9 w/m2
(*Nitrous oxides, ozone, CFCs, water vapor)
Air pollution (“aerosols”) — -1 to -2 w/m2
TOTAL — about +1 w/m2 to +2 w/m2
Note that the effect of air pollution is negative (global cooling) and is highly uncertain. When you add the totals, therefore, the exact end result is also uncertain, although it is clear that there is warming.
2. But, air pollution changes the equation by increasing the importance of methane. Air pollution (an “aerosol”) is associated with burning of fossil fuels (coal, oil, etc.) but not with methane (livestock methane, landfills, rice production, etc.). So, the net effect of fossil fuels is somewhat offset by air pollution. This means that the relative importance of methane (which is not offset by pollution) is increased.
Note that air pollution is generally a bad thing, with all kinds of bad results for human health. In terms of climate change, though, it is a relatively good thing, although this is its only redeeming feature, and even this is a mixed blessing, as we will see below.
3. Indirect effects of methane further increase the importance of methane. Some of the ozone, water vapor, and even some of the carbon dioxide is a result of breakdown of methane. The indirect effects of methane are about as much as the direct effects, in effect doubling the practical effect of methane emissions.
4. Methane breaks down much more quickly than carbon dioxide. In a century, most methane will be gone, but most carbon dioxide will still be around. So, if you needed quick results on the climate change front, your best bet would be to try to reduce methane.
The bottom line is that methane is more important than initially appears. When you count the indirect as well as the direct effects of methane, that essentially doubles the global warming effect of methane. Air pollution, though, decreases the immediate importance of the carbon dioxide produced by burning fossil fuels.
While the situation is complex, the practical effects of methane emissions are roughly comparable to those of burning of fossil fuels. In fact, in some ways methane may be more important; targeting methane will get quicker results than targeting fossil fuels.
This doesn’t mean that we can just ignore fossil fuels, or that there’s no problem; quite the contrary. We have a big problem with fossil fuels. It just means that we also need to address methane, and that in fact that the best way to address the carbon dioxide problem is by addressing methane simultaneously.
Let’s go over these points in more detail.
1. Global Warming Effects of Carbon Dioxide and Methane
If you look at a standard chart of human-caused greenhouse emissions, it looks like the effect of carbon dioxide is about three times the effect of methane.
Climate scientists don’t talk about “global warming,” they talk about “radiative forcing.” Why is this? Basically these two terms are equivalent. “Forcing” or “radiative forcing” is the same thing as “global warming.” The key difference is that “forcing” can be either positive or negative, whereas “warming” is always positive. Otherwise, scientists would need to constantly refer to “negative global warming” which would sound strange.
“Forcing” means a change in the warming effects since the beginning of the industrial revolution (about 1750). There are some human-caused pollutants that actually have a negative effect on global warming, that is, “negative forcing.”
The basic unit of “forcing” (“global warming”) is watts per square meter. A single watt is a single Christmas bulb on a Christmas tree. So “1 w/m2” is the equivalent of one Christmas bulb over each square meter of the earth’s surface.
The “radiative forcing” (i. e. warming effect) of carbon dioxide is about 1.5 w/m2 (watts per square meter), while that for methane is 0.5 w/m2, according to the IPCC. There are some other “minor” gases and all the positive forcings add up to about 3 w/m2.
But there is a rather large uncertainty concerning the effect of aerosols which have a negative forcing value (i. e. contribute to global cooling). This “negative forcing” counteracts the warming trend, and brings the total forcing (the total global warming effect) down to about 1 to 2 w/m2. James Hansen says (Storms of My Grandchildren, p. 9) that the net radiative forcing since pre-industrial times is from 1.5 to 2 w/m2, but with an uncertainty of 1 watt.
This would make methane’s effect (0.5 w/m2) about 1/3 or 1/4 of the net total effect. This isn’t an amount to be sneezed at, but isn’t as important as carbon dioxide. However, there are reasons to suspect that the methane is likely a bigger problem than we thought.
2. Effects of aerosols.
Here is another piece of scientific terminology: climate scientists don’t talk about “air pollution,” they talk about “aerosols.” Technically, an aerosol is any suspension of tiny solids or liquids in a gas. Fluffy white clouds are aerosols. But human-caused aerosols, like sulfates from burning coal, are typically air pollution.
There are two important facts we need to keep in mind about aerosols. First, most of them have a negative forcing effect; they tend to counteract global warming. Second, aerosols tend to disappear very quickly. They leave the atmospheric scene in a short period of time. However, our industrial system is constantly supplying the atmosphere with a new batch of aerosols, so they are ubiquitous.
Methane may be a bigger problem because atmospheric methane, unlike carbon dioxide from coal or fossil fuels, is not associated with aerosols.
This sounds odd, because most people think of air pollution as a bad thing, and now are we saying that air pollution is actually good? Let’s be clear: air pollution is generally a bad thing. It creates serious human health problems. However, it has one redeeming feature; it tends to counteract global warming.
Burning of fossil fuels like coal and oil not only produces carbon dioxide, but also aerosols. They have a negative radiative effect by blocking out some of the sun’s light. We wouldn’t want to block out all of the sun’s light, obviously, or plants wouldn’t grow and everything would die. But the slight amounts of sunlight that aerosols block out is just enough to mask some of the effect of the carbon dioxide produced.
The greater the effect of aerosols, the more they will cancel out global warming, and the greater the relative importance of methane. Let’s suppose that when we burn fossil fuels, we put carbon dioxide in the air so that it creates 1.5 w/m2 of forcing (warming). But at the same time, we create aerosols that have -1.0 w/m2 of forcing (cooling). The net total effect of burning the fossil fuels is actually only 0.5 w/m2, which is the same amount of forcing that methane has.
This is just an example; we don’t know what the exact amount of negative forcing that aerosols have. But the greater the value of the negative forcing, the more it will tend to cancel out the burning of fossil fuels.
In fact, it’s conceivable (though unlikely) that the aerosols would cancel most or all of the warming effects of carbon dioxide. Climate skeptics sometimes make the argument that we don’t have to worry because air pollution will cancel out the global warming effects. Well, this is false, because we observe that the climate has been getting steadily warmer for the last 50 to 100 years. But it is true that aerosols cancel out part of the warming effect.
But there’s another disturbing problem: when we ever do get around to getting rid of fossil fuels (or when we finally deplete them) it might actually cause a temporary increase in global warming. This is because aerosols dissipate very quickly, whereas carbon dioxide hangs around in the air for decades or centuries. Hansen explains:
. . . if the net forcing [total of all positive and negative effects] is only 1 watt, that is, if aerosol forcing is -2 watts, that means aerosols have been masking most of the greenhouse warming. In that case, if humanity reduces particulate pollution by even half, the net climate forcing would double. [Storms of My Grandchildren, p. 99; emphasis added.]
Net forcing would double? Oops!
This is the key element that the climate change skeptics are overlooking. It is the scenarios in which aerosols cancel out most, or even all, of the carbon dioxide warming effect that are the scariest. We are not going to burn fossil fuels forever, either because of policy changes or because we will start depleting them. We are likely at or past “peak oil” (the maximum point of oil production) already. Coal and natural gas are not far behind. (Richard Heinberg’s book Blackout argues that coal depletion is a lot closer than many people think.) When fossil fuel use does decline, we are in for a major climate disaster, an event that will make everything that has occurred to climate so far look like a pleasant Sunday picnic by comparison.
Aerosols are “good news” and “bad news.” If aerosols are masking a lot of the CO2 forcing, then it’s good news — our problem isn’t as bad as we thought. But it’s really bad news when we try to deal with global warming by halting burning of fossil fuels, or when fossil fuel use starts to decline anyway due to depletion. Getting rid of all the coal plants could mean that for a long time we’ll have clean air and a hotter planet.
We do not know that aerosols are this significant. But they might be, and to the extent they are a factor, getting rid of them will make the short-term situation worse, with potentially disastrous results. While we wait a century or two for CO2 levels to slowly dissipate, the extra forcing might be just enough to trigger the disappearance of arctic ice, melting of the permafrost, melting of methane hydrates, or some other awful result. (Methane hydrates are frozen pieces of methane deep in the oceans. Scientists think that they melted during the Permian-Triassic extinction 251 million years ago, which wiped out 95% of all species then on earth.) By the time carbon dioxide levels do come down, it may be too late and we may be in a climate permanently changed for the worse.
3. Indirect Effects of Methane.
There is evidence that methane has indirect forcing effects in addition to its direct effects. A recent paper by Isaksen, et. al. looks at various “runaway” greenhouse gas scenarios in which methane emissions increase to 2.5 times, 4 times, or 7 times current levels of about 1.8 parts per million. They find that at these levels, the direct “forcing” (warming) effects of methane are only part of the problem — the indirect effects essentially double these effects. So if methane levels get a lot higher than they are now, we could be looking at a runaway greenhouse effect. It’s very scary.
But is it possible that some of these indirect effects are already occurring? When methane breaks down, it forms ozone, water vapor, and carbon dioxide. According to “Head in the Cloud” blog, stratospheric water vapor (due to methane breakdown) forms just 0.07 w/m2 (watts / square meter), and tropospheric ozone has a significant warming effect averaging about 0.35 w / m2. Isaksen et. al. states:
In the current atmosphere the indirect RF [radiative forcing] is approximately the same as the direct methane RF, taking into account the effect on its own lifetime, on ozone, and on stratospheric water vapor [Forster et al., 2007].
This doesn’t add to the number of known greenhouse gas emissions. It just means that we now know where the ozone, water vapor, and even some of the carbon dioxide come from. They come from the breakdown of methane. Counting both direct and indirect effects, methane could have a radiative forcing of 0.9 or 1.0 w/m2, or over half of the net total human-caused forcing effects.
4. Half-life of methane.
Molecule for molecule, methane is much worse for the climate than CO2, but it doesn’t hang around as long; it has a short half-life, of about eight years. (“Half-life” is the time during which one-half of the substance will dissipate or break down.)
What this means is that if we are aiming for quick results, methane is a target more likely to yield results. If we (somehow) stopped all CO2 emissions tomorrow, even assuming that aerosols have little masking effect, it will still be perhaps a century before we would start to see results. But if we (somehow) stopped methane emissions tomorrow, there are no associated aerosols which would disappear and we would see measurable results in as little as a decade or two.
If we’re near a tipping point in climate, quick results wouldn’t be a bad idea. Dr. Chris Jardine, author of a major report on methane emissions in 2005, argues that we are we are wasting precious time by not acting on methane: “We have been dithering around with Copenhagen and not achieving very much and while that is happening CO2 emissions are going up. You could actually do a whole load on methane emissions and get them down really quickly.”
The Role of Livestock
By drawing attention to methane I am also hoping to draw attention to the sources of methane. One of the key sources of methane is livestock. The FAO estimated (in Livestock’s Long Shadow) that livestock is the source of about 37% of all methane emissions. Rice production and landfills are also major sources of methane.
But this 37% figure seriously undercounts livestock methane, according to Goodland and Anhang in their article “Livestock and Climate Change.” Goodland and Anhang show that not only have methane emissions from cattle been undercounted, a lot of carbon dioxide is actually from animal agriculture as well. When you add the undercounted livestock methane and the misallocated carbon dioxide, livestock agriculture is responsible for just over half of all human-caused greenhouse gas emissions. This further strengthens the need to pay attention to livestock agriculture as a key source of greenhouse gas emissions.
We need to pay more attention to methane, and not focus solely on carbon dioxide.