Methane, Plants and Climate
Change By Frank Keppler and Thomas Rockmann FEBRUARY 2007
The
surprising recent finding that living plants produce methane does not throw
doubt on the cause of global warming.
Human
activities-not plants-are the source of the surge in this and other greenhouse
gases
Overview/Nature's Surprise
∎ The
established view has been that methane (natural gas) is produced by microbes
that thrive without oxygen, but experiments by the authors' team unexpectedly
revealed that living plants also manufacture this potent greenhouse gas.
∎ Although
this startling finding can explain many previously puzzling observations, a
number of scientists are still skeptical, in particular about the amount of
methane that plants generate. Knowing the sources of methane and how much they
emit is important because of methane's role in trapping heat.
∎ An
early misinterpretation of the finding suggested that forests might actually be
contributing to global warming, but the authors emphasize that plants do not
contribute to the recent increase in methane and global warming.
What
do you do as a scientist when you discover something that clearly contradicts
the textbooks? The two of us faced this problem head-on when experiments we
were running in 2005 showed that living vegetation produces the greenhouse gas
methane. The established view held that only microbes that thrive without
oxygen (anaerobic bacteria) can manufacture this gas. But our tests
unexpectedly revealed that green plants also make methane-and quite a lot of
it.
The
first thing we did was look for errors in our experimental design and for every
conceivable scenario that could have led us astray. Once we satisfied ourselves
that our results were valid, though, we realized we had come across something
very special, and we began to think about the consequences of our findings and
how to present them to other researchers. Difficult as this discovery had been
for us to accept, trying to convince our scientific peers and the public was
almost impossible-in large part because we had to explain how such an important
source of methane could have been overlooked for decades by the many able
investigators studying methane and puzzling over climate change.
Natural Gas
MOST
PEOPLE KNOW methane (often written as the chemical formula CH4 as
natural gas. Found in oil fields and coal beds as well as in natural gas
fields, it has become an important source of energy and will most likely remain
so given the limited reserves of oil on the planet. Approximately 600 million metric
tons of it-both anthropogenic (from human activities) and natural-rise into the
atmosphere every year. Most of
these emissions have been thought to come from the decay of nonfossil
organic material as a result of activity by anaerobic bacteria. Wetlands such
as swamps, marshes and rice paddies provide the greatest share. Cattle, sheep and
termites also make methane, as a by-product of anaerobic microbial digestion in
their gut.
Nevertheless,
some observations were difficult to explain. For instance, large fluctuations
of atmospheric methane during the ice ages and warm ages, which have been
reconstructed from air bubbles trapped in ice cores, remained a mystery. But no
scientist in 2001 would have factored in direct emissions of methane by plants,
because no one suspected that biological production of methane by anything
other than microbial anaerobic processes was possible.
Knowing
the sources of methane and how much they emit is important because methane is an
extremely efficient greenhouse gas. Much more carbon dioxide is spewed into the
atmosphere every year, but one kilogram of methane warms the earth 23 times
more than a kilogram of carbon dioxide does. As a result of human activities,
the concentration of methane in the atmosphere has almost tripled over the past
150 years. Will it continue to increase into the 21st century? Can emissions be
reduced? Climate scientists need to answer such questions, and to do so we must
know the origin and fate of this important gas.
Startling Findings
THE
IDEA OF INVESTIGATING plants as methane emitters grew out of research we had
been conducting on chloromethane, a chlorinated gas that destroys ozone and was
thought to come mainly from the oceans and forest fires. A few years ago, while
working at the Department of Agriculture and Food Science in
To
satisfy our curiosity, we collected 30 different kinds of tree leaves and
grasses from tropical and temperate regions and placed them in small chambers
with typical concentrations of atmospheric oxygen. To our amazement, all of the
various kinds of leaves and plant litter produced methane. Usually a
gram of dried plant material releases between 0.2 and three nanograms
(one billionth of a gram) of methane an hour. These relatively tiny amounts
were difficult to monitor, even using our highly sensitive state-of-the-art
equipment.
The
task was made still more challenging because we had answers about the mechanism
of how they did this, although we suspect that pectin, a substance in the walls
of the plant cells, is involved. We decided that this question would have to
await further research, which is currently under way. Because of methane's role
in climate change, however, we realized it was crucial to begin to take into
account the quantity of gas released into the atmosphere by this newly
discovered source.
How
much might plants be contributing to the planet's
methane totals? It was immediately obvious to us that even though a single leaf
or plant made only tiny amounts of methane, these small bits would add up
quickly because plants cover a substantial part of the globe. We were
nonetheless astounded by the figure generated by our calculations: between 60
million and 240 million metric tons of methane come from plants every year-this
constitutes 10 to 40 percent of annual global emissions. Most of it, about two
thirds, originates in the vegetation-rich tropics. We knew, of course, that
extrapolating global estimates from a limited sample of laboratory measurements
was open to error. Still, the final number seemed extremely large-and if it
surprised us, it would be heresy to many of our scientific peers.
Fortunately
for us, support for our work soon came from an unexpected source. A group of
environmental physicists in
Recently
further support has come from Paul J. Crutzen, a 1995
Nobel Prize winner, and his colleagues. After our findings were published in
January 2006, they reanalyzed measurements made in 1988 of air samples from the
Venezuelan savanna and concluded that 30 million to 60 million metric tons of
methane could be released from vegetation in these regions. Crutzen
said that "looking back to 1988, we could have made the discovery, but
accepting the general wisdom that methane can only be produced under anaerobic
conditions, we missed the boat."
Despite
this support for our work, many scientists are still skeptical about methane
emissions from plants, especially about our estimate of how much methane comes
from vegetation. A number of our scientific colleagues are therefore
recalculating the budget for the plant source, using different methods from
ours but applying our emission rates. Of course, we keenly await an independent
verification of our laboratory findings.
Solving an Old Puzzle
OUR
FINDINGS WOULD EXPLAIN a trend that has puzzled climate scientists for years:
fluctuations in methane levels in parallel with changes in global temperatures.
Ice cores serve as natural archives that store information about atmospheric
composition and climate variability going back almost a million years. Tiny
bubbles of air trapped in the ice reveal the relative concentrations of atmospheric
gases in the past [see box on next page]. We see in the ice cores, for example,
that variations of past carbon dioxide levels are
closely linked to changes in global temperatures. During ice ages, carbon
dioxide concentrations are low; during warm spells, levels increase.
In
general, methane concentrations follow the same trend as carbon dioxide, but
the reason has been unclear. Scientists have tried to use models of wetlands
(the only major natural source of methane previously believed to exist) to reconstruct
the curious variations of past methane levels. Yet they found it difficult to
reproduce the reported differences in atmospheric methane levels between
glacial and interglacial periods.
Another
explanation that has been suggested involves the gas in a form known as methane
hydrates. These develop at high pressure, such as that found on the ocean
floor. An unknown but possibly very large quantity of methane is trapped in
this form in ocean sediments. The sudden release of large volumes of methane from
these sediments into the atmosphere has been suggested as a possible cause for
rapid global warming events in the earth's distant past. Yet recent results
from polar ice core studies show that marine methane hydrates were stable at
least over the past 40,000 years, indicating that they were not involved in the
abrupt increases of atmospheric methane during the last glacial cycle.
We
know that terrestrial vegetation is very sensitive to environmental changes,
and thus the total amount of vegetation on the planet varies as the climate
cools down and warms up during glacial cycles. In light of our findings, such
variations should now be seriously considered as a possible cause of declines
in methane levels during glacial periods and rises during the interglacials. During the last glacial maximum-around
21,000 years ago-the plant growth of the Amazon forests was only half as
extensive as today, and tropical vegetation might thus have released much less
methane. Since that time, global surface temperature and carbon dioxide
concentrations have risen, leading to enhanced plant growth and, we would
expect, to more and more methane released from vegetation.
Similar
climate scenarios may have occurred during other periods of the earth's
history, particularly at mass extinction events, such as the Permian-Triassic
boundary (250 million years ago) and the Triassic-Jurassic boundary (200
million years ago). Extremely high atmospheric carbon dioxide concentrations as
well as rising temperatures could have resulted in a dramatic increase in
vegetation biomass. Such global warming periods could have been accompanied by
a massive release of methane from vegetation and by more heating. Though
speculative, the assumption that emissions may have been as much as 10 times
higher than at present is not totally unreasonable. If this is so, methane
emissions from vegetation, in addition to emissions of the gas from wetlands
and perhaps from the seafloor, could be envisaged as a driving force in
historic climate change.
Media Misinterpretations
WHEN
YOU SEE A REPORT on your scientific work on the BBC World News immediately
following news about bird flu and the situation in Iraq, on the very day your
work has first been published, you realize that you have found something with
great societal relevance. This realization was reinforced the next day as our
research appeared in newspapers around the world, often in front-page
headlines.
Unfortunately,
extensive media coverage can lead to exaggerations, and in our case it resulted
in the misinterpretation of our results. In particular, many reports claimed
that plants may be responsible for global warming; in one instance, we saw the
headline "Global Warming-Blame the Forests" on the front page of a
reputable newspaper.
When
you then receive many e-mails and phone calls from individuals asking whether
they should cut down all the trees in their garden to fight global warming, you
realize that something has gone badly wrong in the communication to the public.
We felt compelled to issue another press release to address the
misinterpretations.
In
our second press release we emphasized that if our finding is true, plants have
been emitting methane into the atmosphere for hundreds of millions of years.
Those emissions have contributed to the natural greenhouse effect, without
which life as we know it would not be possible. Plants are not responsible,
however, for the dramatic increase in methane concentrations since the start of
industrialization. This surge was brought about by human activities.
Our
discovery also led to intense speculation that methane emissions by plants
could diminish or even outweigh the carbon storage effect of reforestation
programs. If that were correct, it would have important implications for
countries attempting to implement the Kyoto Protocol to minimize global carbon
emissions, because, under the protocol, tree-planting programs can be used in
national carbon dioxide mitigation strategies. But our calculations show that
the climatic benefits gained by establishing new forests to absorb carbon
dioxide would far exceed the relatively small negative effect of adding more
methane to the atmosphere (which may reduce the overall carbon uptake of the
trees by 4 percent at most). The potential for reducing global warming by
planting trees is most definitely positive.
In
the heat of this debate, people forgot a crucial fact: plants are the green
lung of our planet-they provide the oxygen that makes life as we know it
possible. They perform many other beneficial tasks as well. As just two crucial
examples, they provide a natural environment that fosters biodiversity, and
they control the tropical water cycle. The problem is not the plants; it is the
global large-scale burning of fossil fuels.
A
more legitimate concern is whether the methane produced by vegetation can have
an impact on climate in the near future. Although plants are not responsible
for the massive increase of methane in the atmosphere since pre-industrial
times, they do tend to grow faster. As we can expect methane emissions from
vegetation to increase with temperature, this would lead to even more warming.
This vicious cycle would be a natural phenomenon except for its speed, which is
accelerated mainly by anthropogenic activities such as burning fossil fuels.
The large plant feedback to global climate change that most likely happened in
the past, however, is probably unlikely today because so many forests have been
cut down.
Although
it is too early to say exactly how our revelation might influence predictions
for climate change in the more distant future, it is clear that all new
assessments should consider emissions of methane by plants.
SCIENTIFIC
AMERICAN THE TEXTBOOK VIEW
In
the past 150 years, methane emissions into the atmosphere have roughly tripled
(graph), and today some 600 million metric tons are sent into the air annually.
That rise is a concern because methane, like carbon dioxide, traps heat in the
earth's atmosphere and therefore contributes to global warming.
Until
the authors and their colleagues published their recent discoveries,
traditional thinking held that all natural releases of methane resulted from
the activity of bacteria that thrive in wet, oxygen-poor environments. Such
environments include swamps and rice paddies as well as the digestive systems
of termites and ruminants. And analyses of the sources of the gas in the
environment (pie charts) indicated that the dramatic rise in methane
concentrations
since the mid-1800s has stemmed from human industrial activities
(such as the use of fossil fuels for energy) and increased rice cultivation and
breeding of ruminants (because of population growth). The authors' work casts
no doubt on the explanation for why methane concentrations in the atmosphere
have increased, but estimates of the relative contributions to methane levels
from natural sources will have to be revised.
THE
NEW VIEW
The
authors' team scrutinized the gases emitted by plant debris and by living
plants. To their surprise, the scientists found that both plant debris and
growing vegetation produce methane. This important source of emissions had been
overlooked until the team performed experiments in chambers that had been
flushed of methane, which allowed the researchers to measure the minute amounts
of the gas that plants give off.
The
new view could explain puzzling fluctuations in methane levels that mirror
changes in levels of carbon dioxide
and in global temperatures (graphs). Scientists have tracked
these changes by studying ice cores, in which trapped bubbles preserve information
about the composition of the atmosphere going back almost a million years;
concentrations of deuterium in the ice provide information about temperature.
High atmospheric carbon dioxide concentrations and rising temperatures most
likely led to a large increase in vegetation, which could have been accompanied
by correspondingly large releases of methane.
SATELLITE
IMAGES of the earth's atmosphere provided support for the authors'
controversial finding. In 2005 environmental physicists observed clouds of
methane over tropical forests. Although the standard model of methane
production cannot explain this observation, the authors' discovery made sense
of the curious clouds: the abundant green vegetation of the tropics was
emitting the methane.
The Changing Atmosphere. Thomas E. Graedel
and Paul J. Crutzen in Scientific American, Vol. 261,
No. 3, pages 58-68; September 1989.
Climate
Change 2001: The Scientific Basis. Edited by J. T. Houghton,
Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, X. Dai, K. Maskell
and C. A. Johnson.
Methane Emissions from Terrestrial Plants underAerobic
Conditions.
Frank Keppler, John T. G. Hamilton, Marc Brass and
Thomas Rockmann in Nature, Vol. 439, pages 187-191;
January 12, 2006.
Methane Finding Baffles Scientists. Quirin
Schiermeier. Ibid., page
128.
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