We seem to know less about methane emissions than we thought. After
a decade of stability, methane concentrations in the atmosphere have
been rising strongly in the last 18 months.
Early research work
suggested that this rise was concentrated in the northern latitudes of
the northern hemisphere and was consistent with greater emissions from
decaying organic matter in melting permafrost or from the melting of
Arctic sea ice.
Now this result has been called into question
by the publication of a new study showing the concentrations of methane
are rising almost everywhere. Since methane takes some time to diffuse
around the globe, the later work suggests that the rise in methane may
not be directly due to enhanced emissions from biological sources.
is the second most important greenhouse gas, producing about 20% of the
radiative forcing of all the main gases. Its concentration in the
atmosphere has risen about two and half times since the industrial
revolution to about 1750 parts per billion.
Although it is
present in very much smaller concentrations than CO2, each molecule has
a more powerful global warming effect. It also lasts much less long in
the atmosphere, typically reacting with the hydroxyl radical (*OH) to
form carbon dioxide and water. The average life of a molecule of
methane in the atmosphere is about 8 years compared to about a century
for carbon dioxide.
The growth rate of methane concentrations in
the atmosphere slowed in the second half of the last century. The
period between 1999 and early 2007 showed virtually no increase,
leading to optimism that methane emissions were under control.
coal mines were an important source and much of the industry was closed
down in the northern hemisphere in the 1980s and 1990s. Rice farming
practices, which floods vegetated areas, allowing plant matter to rot
anaerobically and produce methane, were changed in some parts of Asia
to reduce emissions. Natural gas is largely methane and pipeline leaks
were also reduced.
The recent rise surprised many researchers. In
the past, methane concentrations have tended to increase in periods of
marked El Niño (high sea temperatures in the eastern Pacific),
resulting in greater dieback of vegetation and more methane production
from the rotting plant matter.
But the last year or so has been
a period of lower than average temperatures in the Pacific (La Niña
rather than El Niño). So this isn't a good explanation for the sudden
jump of about 10 parts per billion, or about 0.5% increase in the
Dr Rebecca Fisher of Royal Holloway
College, University of London, published work with colleagues early
this year showing that the rise in methane was particularly great in
the Arctic. Some measuring stations saw increases of twice the average
global rise. Since methane takes time to diffuse around the world, this
suggested a regionally specific source. It could be from the sea or
from rotting vegetation exposed by melting.
Dr Fisher's work
was, in a sense, comforting. It suggested we might have an explanation
for why methane concentrations were rising. In particular, she showed
that increased methane concentrations were associated with rises in the
percentage of gas containing the lighter carbon isotope, C12, which is
associated with emissions from methane-producing bacteria. It looked as
though we could be reasonably confident that at least part of the
source of increased emissions was rotting plant matter.
recent work has dented this belief. MIT scientists have just published
work with scientists from Australia and elsewhere that shows that the
rise in methane levels has been quite uniform across the globe. This
shouldn't happen if methane was produced by plant sources, since there
is far more organic matter in the northern hemisphere. Concentrations
should be temporarily higher in the north in the months and years it
takes methane to spread uniformly across the globe.
The MIT team
speculate that the rise in methane may be a function of decreasing
concentrations of hydroxyl, the scavenger radical that mops up methane,
perhaps as well as increasing emissions. But we don't yet have good
monitoring of *OH concentrations and it will be some time before we are
able to tell if this hypothesis is correct or, indeed, what is causing
The scientific debate about the cause of increased
methane is important because it suggests that we do not yet have a good
model for what determines changes in concentrations.
One of the
primary worries about global warming is that it will eventually trigger
the eruption of untold millions of tonnes of methane from deep sea
water. (This is usually known as the 'clathrate gun' hypothesis.) The
gas is currently locked into a stable bond with the extremely cold
waters in the deep oceans.
Continued world temperature
increases will eventually cause the methane to burst from its chemical
locks within the cold liquid and rise to the surface. This probably
happened at times of rapid warming in the far-distant past.
fact that we cannot immediately know today why the methane rise is
occurring, and whether it is likely to continue, raises worries that
our understanding of methane's role in the global carbon cycle is
simply not very good.