Depending on how you look at it, something good can always come out of
something bad. That's actually the case in a new study on greenhouse
gases by NASA scientists and others. The researchers discovered that
acid rain inhibits a swampland bacteria from producing methane, a
greenhouse gas.
Animation above:
This movie from the U.S. Environmental Protection Agency highlights the
science of acid rain, and its effects. Click arrow on bottom right to
move to next image. Credit: U.S. EPA
Methane, a gas that contributes to warming our planet, is produced by
natural processes and human activities. Increased amounts of methane and
other greenhouse gases in our atmosphere are warming the Earth beyond
its average temperature.
Carbon, heat and moisture are known to influence methane production by
members of the Archaea, single-celled creatures. Under normal
conditions, these microbes consume organic carbon in the soil for energy
and release methane as a byproduct. Wetlands provide an ideal
environment for these microbes. When acid rain drops sulfate onto
wetlands, another type of bacteria, ones that reduce sulfate are able
to outcompete the Archea, limiting the total production of methane.
Wetlands may produce as much as 320 million tons of methane annually but
only about half of that, or 160 million tons, is ultimately released to
the atmosphere. The other 160 million tons never makes it to the
atmosphere because it is destroyed via oxidation as it moves from wet
soils below the water table through dry soil to the surface. Despite
substantial oxidation, natural wetlands remain the single largest source
of methane emission accounting for about one third of the global annual
total methane.
Image to right:
Inland wetlands are most common on floodplains along rivers and
streams. Scientists have discovered that acid rain actually inhibits a
bacteria found in swamplands from producing methane, a greenhouse gas.
Inland wetlands include marshes and wet meadows dominated by herbaceous
plants, swamps dominated by shrubs, and wooded swamps dominated by
trees. Credit: U.S. EPA Region 1/Leo Kenney
"It's a complicated process because multiple factors at microscopic to
global scales interact in these processes," said Elaine Matthews, a
scientist at NASA's Goddard Institute for Space Studies (GISS), New
York. Matthews is co-author of the study on acid rain and methane in
wetlands. "The maximum emission of methane from wetlands occurs when
conditions are warm and wet, while the biggest reduction in methane
emissions is achieved when the location of wetlands, sulfates contained
in acid rain, high temperatures and substantial precipitation all come
together, to reduce optimal methane emissions from wetlands." These
factors vary over time and space.
According to Matthews, by 1960 these counteracting processes probably
reduced methane emission from wetlands to pre-industrial levels.
However, methane emission is predicted to rise in response to 21st
century climate change faster than sulfate suppression increases,
meaning that wetland emissions of methane will begin to rise above those
occurring before industrial sulfate pollution began.
In order to determine how the acid rain interacts with methane in
wetlands, lead author of the study, Dr. Vincent Gauci of Open
University, United Kingdom and his colleagues took to the field. In the
U.S., Britain and Sweden they attempted to determine if low levels of
sulfate, like those in acid rain, affected methane emissions in
wetlands. They applied several quantities of sulfate, similar to the
amounts found in acid rain, to the wetlands they were studying. The
results, acquired over several years, showed that these low doses of
sulfate suppressed methane emissions between 30-40 percent.
Image to left:
Coastal wetlands in the United States, as their name suggests, are
found along the Atlantic, Pacific, Alaskan, and Gulf coasts. They are
closely linked to our nation's estuaries, where sea water mixes with
fresh water to form an environment of varying salinities. The salt water
and the fluctuating water levels (due to tidal action) combine to
create a rather difficult environment for most plants. Credit: U.S. EPA
Region 8/Paul McIver
Matthews and climate experts expect methane emissions to increase over
the 21st century in response to climate change. They also predict that
sulfate levels in rainfall will increase, especially in Asia. The
authors have attempted to predict how this ecological balancing act will
turn out for the 21st century.
"When we used all the field data with the NASA computer models and
applied it to a global scale, it shows that the effect of acid rain from
1960 to 2030 actually reduces methane emissions to below pre-industrial
levels," said Gauci. The effect more than compensates for the increase
in methane emission that would be expected as wetlands become warmer. In
this way, acid rain acts like a temporary lid on the largest methane
source.
Gauci is cautious about the image presented by acid rain. "We wouldn't
want to give the impression that acid rain is a good thing - it has long
been known that acid rain damages natural ecosystems such as forests,
grasslands, rivers and lakes. But our findings suggest that small
amounts of pollution may also have a positive effect in suppressing this
important greenhouse gas. Moreover, they point to how complex the Earth
system is," he noted.
Image to right:
Wetlands are among the most productive ecosystems in the world,
comparable to rain forests and coral reefs. An immense variety of
species of microbes, plants, insects, amphibians, reptiles, birds, fish,
and mammals can be part of a wetland ecosystem. Physical and chemical
features such as climate, landscape shape (topology), geology, and the
movement and abundance of water help to determine the plants and animals
that inhabit each wetland. The complex, dynamic relationships among the
organisms inhabiting the wetland environment are referred to as food
webs. Credit: U.S. EPA/ Mark Sharp
Most attention has been given to the negative aspects of pollution but
if scientists want to understand all of Earth's complexities and make
better predictions of future climate we need to understand interactions
among a suite of processes that are not always well understood. "That's
not to say that acid rain is a good thing. Rather this study illuminates
really well how we have to work to understand relationships among
microscopic-to-global processes, at the same time that we attempt to
represent them in relatively simple ways," Matthews said.
While sulfate deposition results almost exclusively from human
activities, it may serve to delay impacts from the increase of at least
one greenhouse gas, methane, in the short term. The study recently
appeared in the Proceedings of the National Academy of Sciences.
NASA's Science Directorate works to improve the lives of all humans
through the exploration and study of Earth's system, the solar system
and the Universe.
