Plants fart too?

Methane is the simple organic compound in natural gas. In the US, we use it primarily as a fuel for cooking, heating, and generating electricity. When you fire up your gas stove or gas fireplace, the gas you are probably burning is methane. You may also use it to heat your water and even your home. In the US, about 20% of our electricity is generated by burning natural gas or methane.

Methane is also a greenhouse gas. When it is in the atmosphere it absorbs heat and contributes to global warming. In its publication Climate Change 2001: The Scientific Basis, the Intergovernmental Panel on Climate Change (IPCC) proposed that as a greenhouse gas methane is over 20 times more potent than carbon dioxide when you compare their heat-absorbing abilities over a hundred years. This suggests that from a global warming perspective, every ton of methane in the atmosphere is like having over 20 tons of carbon dioxide. That's like wearing 20 pairs of thermal underpants instead of just one.

If you've seen Al Gore's An Inconvenient Truth, you've seen how historical atmospheric carbon dioxide concentrations and global temperatures trend closely together. A similar chart from the IPCC's report shows how methane closely follows the same trend, implying a similar relationship.

Gore continued the historical carbon dioxide trend to the present day, suggesting that we have added so much to the atmosphere we are now "off the chart". IPCC's report shows how we are off the chart with methane concentrations as well. From the first graph you can see that over the last 400,000 years carbon dioxide concentrations cycled between 200 and 300 parts per million by volume (ppmv) and methane concentrations cycled between 350 and 700 parts per billion by volume (ppbv). From the second graph you can see that over the last 200 years, carbon dioxide (CO2) and methane (CH4) concentrations have gotten up to 360 ppm and 1750 ppb, respectively. Compared to the peaking concentrations over the past 400,000 years, current concentrations for carbon dioxide are 20% higher and methane concentrations are 150% higher. (IPCC also presents concentrations of nitrous oxide, another greenhouse gas, which have also increased over the last couple hundred years, but we'll tackle that in another entry.)

So where does all this methane come from? Methane is a product of anaerobic digestion, or digestion in the absence of oxygen. This happens in landfills where below the surface microbes break down the organic material in garbage. Anaerobic digestion also occurs inside animals as they process food. Because of our farming practices a significant source of methane comes from livestock. Cows and other ruminants burp up so much methane that the EPA estimates this makes up about 20% of anthropogenic (human caused) methane emissions.

There are also natural sources of methane, again from digestive processes including microbial breakdown of organic material in wetlands and breakdown of cellulose in termites. And yes, another source, a trivial source, is flatulence from other animals like you and me. (Please note, however, methane in its pure form is an odorless gas.)

These are all anaerobic processes, but another natural source of methane that caught scientists off guard was reported about a year ago by Keppler et. al. in their article in Nature. Their studies shook traditional assumptions showing that methane was produced by plants under aerobic (oxygenated) conditions. They also proposed that the amount of methane from plants was a significant contribution to the amount of methane emitted from natural sources.

Considering methane's bad boy rap and that the Kyoto protocol promoted reforestation to combat global warming, folks started questioning whether planting trees was a good or bad idea. Even in that same volume of Nature, David Lowe authored a commentary on the study and suggested that it may be possible that reforestation may do more harm than good with respect to global warming. He also suggested that the anthropogenic source of methane from ruminants could be equivalent to the methane generated if the land the livestock grazed were forested.

But don't pave the rainforest yet! Since the study and the commentary were published, several papers have been written to support the idea that despite their new found methane contribution, plants and forests do not contribute to global warming. Kelliher et. al. argued that emissions from grazing animals are about ten times higher than the emissions from that land if it were forested. Using Keppler's data, Kirschbaum et. al. calculated only a quarter of the global plant emissions Keppler had reported. The original authors themselves even responded with their own press release and again more recently in next month's issue (Feburary 2007) of Scientific American to clarify the interpretation of their findings. They calculated that the net global warming benefit from trees and forests are lessened by just 1 to 4% because of the methane they emit, even after accounting for methane's higher potential as a greenhouse gas.

Keppler et. al. reported on a frequently overlooked phenomenon in nature. There are still questions about the mechanism for how plants generate methane under aerobic conditions and what contributions plants made historically to atmospheric methane concentrations. But what Keppler's study has not changed that methane concentrations in the atmosphere have more than doubled over the last two hundred years thanks to anthropogenic sources like landfills, natrual gas systems, and livestock.

Carbon dioxide may be the single largest contributor to global warming, so it gets the most attention, but methane is not too far behind (no pun intended). To be clear, you can blame this on your furnace, but not on your ficus.

Graphics from IPCC website.
First graphic, Climate Change: The Scientific Basis, Chapter 2, Figure 2.22
First graphic, Climate Change: The Scientific Basis, Summary for Policy Makers, Figure 2a

Sources:
National Geographic article
EPA's Methane factsheet
Wikipedia article on methane
Wikipedia article on global warming potential or GWP
Wikipedia article on radiative forcing