Note: This program first aired January 10, 2015.
Last week we introduced a key non carbon dioxide green house
gas, methane. There are several naturally occurring sources of methane, a gas
that is produced by anaerobic bacteria when organic matter decays.
Methane is a fossil carbon based fuel, and is the main
constituent of natural gas, so when we burn it, just like when we burn any
carbon based fuel, one of the waste products is carbon dioxide. Carbon dioxide
is a major green house gas, meaning as we increase concentrations of it in the
atmosphere, we create conditions that prevent the escape of infrared radiation
(also known as sensible heat) from Earth’s climate system (that being the
atmosphere and the ocean). When more heat is trapped than escapes, then we have
a system that is warming. That is the current climate change scenario in a nut
shell, and the combustion of methane plays a role.
Methane plays a second, and more significant role in climate
change by being a green house gas itself.
The methane molecule, just like the carbon dioxide molecule, the water
molecule, the nitrous oxide molecule and various other fluorine containing and
halogenated industrial compounds, absorbs specific wavelengths of infrared
radiation. Water is by far the most important green house gas, in that it traps
the widest range of infrared wavelengths emitted by the Earth back towards
space. And remember, the green house effect that results from these various
atmospheric gasses is what makes our planet livable, so it’s a good thing. The
issue currently is that the proportions of the green house gasses in the
atmosphere are changing, resulting in more radiation being trapped down here in
the lower atmosphere, so things are heating up on the surface of the earth. I
said that water vapor traps the widest range of infrared wavelengths, but it
doesn’t absorb all of them. There are gaps, or windows in the water vapor
absorption spectrum. In the absence of other green house gasses, it is through
these wavelength windows that some heat escapes the atmosphere. Some of these
gaps are partially filled by the other greenhouse gasses, including carbon
dioxide, and methane.
On a per molecule basis, methane absorbs a large amount of
infrared radiation, so you often hear that it is something like 40 times as
powerful as carbon dioxide gas in terms of its greenhouse warming potential.
Some studies have linked methane releases to major warming events like the PETM
(or Paleo Eocene Thermal Maximum) or mass extinctions like the end Permian. Methane
is different than most other non water vapor green house gasses in that it
breaks down relatively quickly, usually within a decade of emission. In the
atmosphere it is oxidized, which essentially does the same thing chemically as
combusting it, so when methane goes away, carbon dioxide and water are left,
and we already know those are both green house gasses.
In the big scheme of things, methane makes up a very small percentage
of green house gasses, and because it oxidizes so quickly, some scientists down
play its role in climate change. Others see it as a great place to start
mitigation efforts, because the effect of reducing emissions of methane can be
felt within a decade, due to its short lifespan. As we have learned, there are
many natural sources of methane. However, at this point there are many more
anthropogenic sources of methane emissions than natural ones. These sources are
industrial and agricultural and are somewhat unavoidable components of modern
society. Landfills, the end point for much of our waste stream, are a huge
source. When land fills are created and then capped, they become anaerobic
environments, full of organic matter—perfect incubators for the creation of
methane. Some forward thinking land fill operators actually capture that land
fill gas and burn it to create heat or electricity. Methane comes from a
variety of agricultural sources, including artificial wetlands where rice is
grown, and ruminant livestock as well as manure. The oil, gas and coal industry
itself is a large source of methane emissions, coming mainly from leaks in
infrastructure, and directly from wells and mines. The anthropogenic effects of
methane emissions can be compounded by a positive feedback loop that was
recently quantified; Methanogenic bacteria are strongly temperature dependent,
so the warmer it gets, the more methane they will make. In other words the
warmer climate gets, the more active wetland bacteria are, the more methane is
emitted, the warmer it gets. It is this effect that causes some scientists to
link methane emissions to specific rapid climate change events in the geologic
record.
The consensus in the climate science community seems to be
that issue is the carbon dioxide, and that is where we should be directing our
energy. At the same time however, methane needs to stay on the table, with a
particular eye to all those methane hydrates at the bottom of the ocean. Ocean
temperatures are warming, particularly deep ocean temperatures, which means
that things could get very weird very fast in the climate system. Methane
remains the wild card here, one that bears watching in the years to come.
References:
Science Daily digest here on the study that quantified the
“the warmer it gets the more methane we get” positive feedback loop: http://www.sciencedaily.com/releases/2014/03/140327111724.htm
Even high school students preparing for their school exams
need to know about combustion chemistry: http://www.gcsescience.com/o30.htm
Old archived science blog, but it has good info: http://www.oocities.org/marie.mitchell@rogers.com/GreenhouseEffect.html
Real Climate Science Blog on Methane: http://www.realclimate.org/index.php/archives/2012/01/much-ado-about-methane/
More about the nitty gritty of the bond chemistry that makes
green house gasses work: http://www.windows2universe.org/earth/climate/greenhouse_effect_gases.html
Google “sources of methane” and take a look at the images.
You will see a wide variety of pie charts, with huge variation in what they
show. Most show anthropogenic sources as nearly ¾ of the methane emissions, the methane hydrates that
make up the huge volume of fossil fuels on Earth aren’t being emitted, so they
aren’t included on these tallies (except for where they are emitted, in thus
far relatively small amounts in coastal oceans and some high latitude lakes).
Land fill gas: http://www.epa.gov/lmop/faq/landfill-gas.html
