Natural
wetlands are the single largest source of methane, a potent greenhouse gas estimated
to account for about one-quarter of Earth's current excess warming. Now, in
an odd twist that highlights the complex nature of interactions between Earth's
systems, researchers have found that acid rain is significantly suppressing
the global emission of methane from wetlands.
Vincent Gauci collects air samples to analyze methane from a wetland area in
the eastern highlands of Moray, Scotland. Researchers now say that acid rain
may reduce the emission of methane from wetland areas. Courtesy of Vincent Gauci.
Acid rain contains sulfate derived from the burning of coal, particularly "dirty
coal" with high sulfur content. In wetland environments, the additional
sulfate input gives sulfate-reducing microbes a competitive advantage over their
methane-producing kin. The process "results in a reduction in methane production
and therefore a reduction in emission," says Vincent Gauci, a biogeochemist
at Open University in Milton Keynes, United Kingdom, and lead author of a recent
study on acid rain in wetlands.
"This is actually far from the first time we've seen links between microbial
and global processes — most of the important processes that regulate the
global system are microbial," says Josh Schimel, a biogeochemist at the
University of California, Santa Barbara. "What we often don't do is make
an explicit quantitative link between the small and large scales."
In field experiments, Gauci and colleagues found that high rates of sulfate
deposition suppressed methane emissions in wetlands by 30 to 40 percent. The
researchers also modeled the process using global sulfur emissions and sulfate
deposition from 1960 to 2080.
The model showed that 8 percent less methane is currently being emitted from
natural wetlands than would be emitted without acid rain. Using estimates from
the Intergovernmental Panel on Climate Change of future sulfur emissions, the
model showed a reduction in wetland methane emissions due to acid rain of 15
percent by 2030.
Although this particular microbial process was previously known — for example,
saltwater marshes with sulfate from seawater are not significant methane sources,
whereas freshwater marshes are — it had not previously been quantified
in a way that made its global significance understood.
That quantification of the link between the methane and sulfur budgets likely
will prove important for future climate models. "It may be an order of
magnitude estimate of what the impacts are likely to be," Schimel adds,
but "it is a much stronger quantitative link than we'd been able to develop
before."
Another novel aspect of the study, which was published in the Aug. 24 Proceedings
of the National Academy of Science, is that the researchers were able to
assess "whether or not an interaction at the microbial scale was significant
at the global scale," Gauci says. "Our results show that it is."
The researchers say that the effect of sulfate deposited by acid rain had been
overlooked as an important component of the methane budget. "The impact
of sulfate hadn't been fully examined until now," Gauci says, "whereas
components of the methane budget, such as termites and oxidation in dry soils,
have been very well-studied."
The study also suggests that global warming in wetlands is not responsible for
the rise in atmospheric methane since the late 19th century, because acid rain
is offsetting the effects, Gauci says. "Overwhelmingly, the increase is
likely due to anthropogenic output of methane,” he says — “emissions
from rice agriculture, cattle, exploitation of natural gas reserves and landfills."
Nonnatural wetlands, such as rice paddies, are a significant and growing source
of methane. How methane emissions from rice paddies will respond to acid rain
is an important question, Gauci says. "Asia is forecast to increase rice
production to feed a growing population, which should mean increases in methane
emission," he notes, "but Asia is also forecast to be impacted by
more and more acid rain as the region develops economically."
This is not the first time sulfur pollution has been found to counterbalance
elements of climatic warming. Sulfate aerosols reflect the sun's rays, thus
acting to cool the planet. However, people should not embrace the notion that
sulfate emissions and acid rain are good things, researchers say.
"It has long been known that acid rain damages natural ecosystems,"
Gauci says. "But our findings suggest that small amounts of pollution may
also have a positive effect in suppressing this important greenhouse gas."
Sara Pratt
Geotimes contributing writer
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