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Climate Change

More than cow burps: the many sources of methane

 

What is methane and why should we care?

When you think of greenhouse gases, CO2 is probably the first that comes to mind; however, it isn’t the only player out there. Methane (CH4) is a chemical compound found in the atmosphere and is the second most important greenhouse gas. In fact, on a 100 year timescale it has 25 times the warming potential of CO2 (IPCC Fifth Assessment Report). Pre-industrial (before 1750) atmospheric concentrations were 700 ppb (parts per billion) but rose to 1803 ppb in 2011. Methane is an interesting greenhouse gas because for almost a decade it appeared to have stabilized. There was no appreciable growth from 1999-2006. It started increasing again in 2007 and has become a head scratcher for scientists. There are many environmental and economic possibilities to explain this but there is no consensus yet. What we do know is that from 1999-2006 there was a balance between sources and losses (sinks) that is now off kilter.

Where does it come from?

Figure 1: Cows are not the only source of methane! (Source: Jean-Luc Bailleul (own work), via Wikicommons)

Figure 1: Cows are not the only source of methane! (Source: Jean-Luc Bailleul (own work), via Wikicommons)

When you think about methane sources I bet there is one source that comes to mind for us all. Their burps have been the brunt of our jokes regarding methane for years. Who am I thinking of? Cows. Cows are known methane emitters but methane comes from a variety of sources. Let’s put our cow jokes aside and consider the wide range of methane sources to the atmosphere.

First, it is important to know how scientists divide methane sources. From 2000-2010, natural emissions sources comprised 35-50% of the total emission sources and the rest (50-65%) was anthropogenic in origin. Why is there a range of numbers? There are two scientific approaches to calculating global emission sources of any chemical. The first, called top-down, measures total emissions on a global level and then estimates how much of that total are from different sources. The second, called bottom-up, measures emissions from different sources (e.x. cows) and then estimates global emissions using inventories (e.x. cattle farming in different countries).

Methane can be broken down into source type as well. Thermogenic sources make methane as a result of the thermal (heat from deep within the Earth) decomposition of organic matter (e.g. natural gas, coal, oil, etc). Biogenic sources are biologically formed methane sources (e.g. wetlands, landfills, rice paddies, fresh water sediments, etc). Pyrogenic sources make methane from fire (e.g. biomass burning and biofuels). Some sources, such as hydrates and seeps, are considered combinations because they are natural geological sources (biogenic and thermogenic).

Wetlands rule the day

Figure 2: Wetlands are the major natural methane source. (Source: Dr. Andreas Hugentobler (own work), via Wikicommons)

Figure 2: Wetlands are the major natural methane source. (Source: Dr. Andreas Hugentobler (own work), via Wikicommons)

Wetlands come in all shapes and sizes and are found from the tropics to high latitudes. Common wetland types considered are swamps, bogs, wet soils, tundra (Figure 2), and peatlands.  This diversity makes it hard to model wetlands and predict future methane emissions. Wetlands are important because from 2000 – 2010 they were the highest natural methane emission source. Scientists are still uncertain how much methane emissions from wetlands will increase in a warmer climate. It is dependent on temperature and precipitation which will vary drastically in the future for different regions.  It is known that wetland methane emissions increase because of higher net primary production thanks to higher temperatures and CO2 levels.

Rice is rising

Figure 3: Rice paddy in Guilin, China (Source: Dariusz Jemielniak (own work), via Wikicommons)

Figure 3: Rice paddy in Guilin, China (Source: Dariusz Jemielniak (own work), via Wikicommons)

Rice is a delicious and important food source globally. Currently, over half of all rice paddies in the world are in China and India. As the world moves towards more rice production, it will be important to reduce methane emissions.  As a result, studies of Chinese rice paddies increased over the last decade. A link was found between crop yields and methane emissions. Scientists found that reducing crop yield by removing flowers increased methane production. The hypothesis is that less photosynthetically formed carbon can be stored and the carbon ends up in the soil and then is converted to methane by bacteria. Traditionally, farmers flood the fields during growing season which makes prime conditions for microbes to produce methane as they eat up organic matter. Chinese farmers started draining the fields in the middle of the growing season because it improved crop yields and saved water. This draining also significantly reduces methane emissions! The downside is more nitrous oxide (another greenhouse gas) is produced but this is a start in the right direction. Scientists and farmers working together can improve rice crop yields AND reduce methane emissions.

Natural gas – the way of the future?

Natural gas is becoming a popular fuel source in the United States. What is natural gas? It is a mixture of several chemicals but over 90% is methane. Natural gas is praised as the “clean” fuel alternative to coal but it is not perfect. Loss of natural gas is the largest contributor to fossil fuel leakage related emissions in the US. There are miles and miles of natural gas pipelines in the US (305, 954 miles as of 2008) and if they are not properly maintained leaks become an issue. There are also safety concerns because it is highly flammable. Between 2008 and 2012, there were 51 deaths (and 681 safety incidents overall) reported to the Department of Transportation. A lot of these are attributed to equipment issues. These same safety issues also lead to methane leaking into the atmosphere. EPA regulations and inspections help prevent methane leaks and also protect workers. Dr. Townsend-Small found that methane thermogenic emissions (coming from natural gas processing) are decreasing in Colorado. One proposed cause is the increase in regulations preventing leaks.  Technology is constantly improving to find new equipment that is less likely to leak and, along with proper training, could reduce unintended methane emissions (and profit loss).

Information overload on the internet

If you want to learn more about climate change but don’t know where to start, the Intergovernmental Panel on Climate Change (IPCC) is a great (free!) resource. Every few years they write a new assessment synthesizing relevant scientific peer reviewed work relating to climate change. The Summary for Policy Makers at the beginning of the physical science book is a great place to start because it is written in a Q&A style. The IPCC report is a jumping off point to answer any of your climate change related questions!

Now what?

Methane has a variety of natural and anthropogenic sources. Throughout Earth’s history methane natural sources and sinks have been balanced but now the scale has tipped. As we move towards a warmer world with more uncertainties it will be important to curb our methane emissions. The United States is one of the four top emitters (along with China, India, and Brazil). In 2014, 11% of the US’s greenhouse emissions were from methane. The EPA has several plans in place to combat anthropogenic methane emissions in the US. This includes regulating and upgrading equipment used in natural gas extraction, transportation, and storage. There are also livestock management and landfill methane capture programs. Methane has an interesting and complex origin story that includes way more than cow burps!

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