As October wildfires blazed over much
of Southern California, few people realized that a toxic metal was
crackling out of the burning grass and trees, billowing up into the
sky to travel the world on stratospheric air currents.
Forest fires in the United States may release about as much mercury
into the atmosphere as coal-burning power plants — around 44
tons a year, with roughly 18 of those in the Western states —
say scientists at the National Center for Atmospheric Research in
Boulder, Colo. Their just-published research is now pushing
regulatory agencies to include fire-emitted mercury in their annual
mercury inventories – numbers that the researchers say have
been substantially underestimated for years.
When NCAR
scientists Hans Friedli and Christine Wiedinmyer began looking into
mercury emitted from wildfires several years ago, they were
surprised at how much of the volatile metal — which can cause
brain damage and severe birth defects when it reaches humans
through the aquatic food chain — was going up in smoke. Their
current study, published Oct. 17 on the Environmental Science &
Technology Web site, was the first attempt to estimate the
emissions on a national basis. “We knew that mercury was
being released by fires,” says Wiedinmyer. “The big
question was how much, and where, and when.” Although the
mercury levels in wildfire smoke are not high enough to directly
harm people, says Wiedinmyer, the increased mercury in the air
raises the likelihood that it will end up in waterways, where it
can be converted to methyl mercury, the toxic form that accumulates
in fish and other aquatic animals.
Wiedinmyer emphasizes
the fact that forest fires don’t create mercury. Rather, they
release the mercury that was already there — previously spewed
into the atmosphere by both humans and the earth. The heat of
wildfires converts carbon-bound mercury in both plants and soil
into its gaseous elemental form. Carl Lamborg, a mercury expert at
the Woods Hole Oceanographic Institute, estimates that two-thirds
of mercury in surface soils is deposited by human activities such
as fossil fuel burning, power plants and factories, while the other
third comes from natural sources including volcanic explosions and
ocean vents. As particulate mercury lands in the forests, it is
taken up by plants and tied up in the soil, essentially locking it
away in a relatively harmless form.
“Forests,
particularly those with thick organic soils, do a really good job
of storing the mercury we’ve been pumping out into the
environment,” says Merritt Turetsky, an ecologist at Michigan
State University.
But as climate change and years of fire
suppression lead to more frequent and intense fires, Turetsky and
others are concerned that more mercury will be released —
particularly from boreal forests that store large amounts of the
metal in their thick soils. In the NCAR study, Friedli and
Wiedinmyer found that wildfires in Alaska released an average of 12
tons of mercury a year — more than a third as much as
wildfires in the Lower 48 states combined. Over the years, Turetsky
says, northerly air currents have blanketed the Arctic with the
chemical byproducts of industry, and deep, ancient soils such as
peat and permafrost store more than their fair share of the metal.
Until recently, Alaskan forests rarely went up in flames.
“There’s a lot there to burn,” says Turetsky,
“(If fires keep increasing), wildfire in Northern ecosystems
could contribute a lot of mercury into the atmosphere.”
And the toxic element will not necessarily stay in one
place. “It’s an extremely moveable metal,”
explains Friedli. “If a fire is intense, it may go all the
way up into the stratosphere, and once it’s up there, it may
go all the way around the globe.”
The link between
wildfires and mercury is adding to the perception of mercury
pollution as a global rather than local issue. In 2005, the
Environmental Protection Agency implemented the Clean Air Mercury
Rule, which was the first federal attempt to regulate interstate
mercury emissions, but the agency is only now revising its protocol
to include mercury from wildfires. Meanwhile, Friedli is working
with the United Nations’ Mercury Programme to add the
wildfire data to its 2009 assessment. Because including mercury
from fires will increase the EPA’s annual mercury emissions
estimate by about 30 percent, reducing those emissions to
acceptable levels might mean requiring even sharper cuts for
industry.
The flow of mercury through the air, water and
land is a vicious cycle, Friedli says. “It doesn’t go
away. The only way mercury gets out of the biogeochemical cycle is
when it sediments at the bottom of the ocean — and
that’s rare.” As long as forests harbor the metal,
fires will continue to release it, and there’s not much that
anyone can do about it, says Turetsky: “The only way to
reduce emissions is to reduce industrial
emissions.”
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