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

If you said it was May, I'd believe you

It's about 65 degrees this morning and I'm sipping my caffeinated beverage of choice, a chai tea latte, thinking maybe I should have gotten an iced coffee. I mention this to Guy, who's working the counter at the local coffee shop I go to. Nodding his head, he says he had already gotten some orders for iced drinks today. "Global warming," chimes in another worker behind the counter, "I did my run in just shorts today!"

On my walk home from the coffee shop, I overhear a conversation about cherry trees in Brooklyn blooming. Later I go for my own run in Central Park where I see a small crowd taking pictures of a tree that thinks it's spring.

New York City is abuzz about the weather. And it's no wonder, with the temperature hovering around 70F and the humidity above 80%, it feels more like May than January. It's hard to be down about it. A couple of days ago I was talking with a contractor who works predominately outdoors. He said, "If this is global warming, bring it on!"

The unseasonably hot weather seems to make the idea of global warming real. But if public acceptance of its validity is dependent on the local weather forecast, it might not last. I can recall during the serious snow storms from the last couple of winters folks holding their shovels saying just the opposite, "What global warming?" or "Global warming, my ass!" Who's to say that if we see temperatures in February that put Antarctica to shame, the public opinion of global warming won't end back up in the, "yeah, right" pile.

One problem with believing or disbelieving in global warming based on current outdoor temperatures is that the phenomenon is geographically global not local. The northeast US is having a mild winter, but Denver is getting clobbered by snow storms. Another problem is that global warming is also a long term trend, not something proven in a season, a year, or even a decade. It is not about sporadic temperature rises of 40 degrees F in New York City. It's about the average temperature of the entire planet creeping up by a couple of degrees over the next couple of decades, and about 10 degrees in the next century.

Attributing local heat waves solely to global warming could jeopardize the credibility of the theory, which in turn could jeopardize sustained support to address the problem.

Picture: 6Jan07 - Folks in NYC's Central Park taking pictures of a tree in full bloom.
Sources:
Wikipedia on Global Warming
UNEP Climate Statistics
International Herald Tribune 4Jan07

Lighting the Way

In today's Business section, the New York Times highlighted Wal-mart's efforts to push the not-so-popular compact fluorescent light bulbs or CFLs as part of its broader initiative to get green. The bulb's advocates claim it is environmentally superior to the more familiar incandescent bulb because CFLs use a third to a quarter of the energy for the same amount of light and the bulb itself lasts up to ten times longer.

Aesthetics aside, there are tradeoffs. The main environmental strike against the energy efficient bulbs is that each one contains about five milligrams of mercury; incandescents don't contain any mercury. Mercury is a heavy metal that bioaccumulates, which means that once it gets into an animal it tends to stay there. This results in increased concentrations of mercury in animals higher in the food chain (eg. humans). Mercury can damage the nervous system and causes birth defects. For these reason, the Environmental Protection Agency (EPA) has strict regulations about mercury emissions from chemical and power plants. It is also why there has been a push for years to get mercury out of common household items like thermometers and thermostats.

So why is the mercury in compact fluorescents acceptable? The EPA claims that because the CFLs consume less energy, less electricity has to be generated to power the bulb. Less electricity means less coal is burned to produce the electricity. Less coal means less mercury is emitted from coal power plants. The result is that the amount of "mercury savings" from the power plant is greater than the mercury contained in the bulb. According to the numbers given by the EPA, it's about a 30% savings.

But an analysis of the energy efficiency to mercury tradeoff is not that simple. With the additional complexity of a CFL bulb, how much more energy is required to make them? With the increase in weight per bulb how much more energy is required for transport? How much mercury, if any, is emitted in the manufacturing of CFLs? How much mercury does coal burning emit? Even this last question is not simple to answer. It depends on factors such as the type of coal burned and how well the mercury released is captured by control devices. The type and effectiveness of the control device may depend on the age of the power plant and the federal and state environmental regulations it is subject to. Also, according to the Energy Information Administration (EIA) only about half our electric power in 2005 came from coal. It is not apparent that these considerations have been included in EPA's 30% mercury savings estimates.

Not all the mercury in CFLs will necessarily find a way directly to the environment. The EPA recommends that consumers bring expired CFLs to a recycling center for mercury recovery. But if it is taking Wal-mart, a giant in retail marketing, so much energy to get folks to simply buy the bulbs, what kind of heroic effort would be required to get folks to go to their recycling centers with the old bulbs? It is likely that some percentage of these bulbs and the mercury in them will end up in a landfill or in an incinerator, giving the mercury a potential pathway into the food chain.

This is not to say CFLs are bad, or that Wal-mart's intentions are ill placed. With the energy saved with CFLs, there is the added benefit of fewer emissions of the other pollutants generated from power plants - nitrous oxides, sulfur oxides, carbon monoxide, particulate matter, and carbon dioxide from non renewable sources, to name a few.

So what is the answer? A technical analysis, such as a life cycle assessment (LCA), can be used to compare the environmental impacts of the two bulb technologies. However, the results will be limited to the analysis' scope and will be as accurate as the data and the assumptions used to build it. Then there are social and commercial impacts that are more difficult to quantify. In their current incarnation and use, compact fluorescent bulbs are more energy efficient than their incandescent cousins, but they are not yet sustainable. Getting to sustainability, however, is an ongoing process and CFLs could be a transitional technology along the path to sustainability. For instance, Wal-mart's goal of increasing CFL sales may promote industry competition, generating even more research into energy efficient technologies. This research could lead to other types of lighting that contain less hazardous materials. With the public awareness of energy efficiency created by the promotion of CFLs, these next generation technologies could be adopted more quickly and the next step toward sustainability taken. So there is no clear and single answer, there is a path. And the next time you walk down the lighting isle at a Wal-mart store looking to replace a bulb, what you'll find is a fork in the road.

Sources:
Wikipedia Compact Fluorescent Lamp entry
EPA on mercury
EPA's Energy star on CFLs
Mercury Technology Services website