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Humans have been tinkering with greenhouse gas levels
in Earth's atmosphere for at least 2,000 years and probably
longer, according to a surprising new study of methane
trapped in Antarctic ice cores conducted by an international
research team.
The study showed wild gyrations of methane from biomass
burning from about 1 A.D. to present, said Dominic Ferretti,
lead study author and a University of Colorado at Boulder
researcher with a joint appointment at the National Institute
of Water and Atmospheric Research, or NIWA in Wellington,
New Zealand. Scientists had expected to see slowly increasing
concentrations of methane, a major greenhouse gas produced
primarily by burning and anaerobic activity from agriculture,
livestock and natural sources, up until the onset of the
Industrial Revolution in the late 1700s, he said.
For the first time, researchers were able to separate
"pyrogenic" and anaerobic methane sources using
a stable-isotope analysis of the ice cores, said James
White of CU-Boulder's Institute of Arctic and Alpine Research
and study co-author. They found methane emissions from
burning dropped about 40 percent from 1000 to 1700, likely
due in large part to decreased landscape burning by indigenous
populations in the Americas devastated by diseases brought
to the New World by European explorers.
Undertaken by a team from CU-Boulder, NIWA, Australia's
Commonwealth Scientific and Industrial Research Organization,
or CSIRO, Australia's Department of the Environment and
Heritage and the National Oceanic and Atmospheric Administration,
the study was published in the Sept. 9 issue of Science.
"The results frankly were a shock," said White.
"We can see human fingerprints all over atmospheric
methane emissions for at least the last 2,000 years. Humans
have been an integral part of Earth's carbon cycle for
much longer than we thought."
The researchers recorded a huge drop in methane levels
from biomass burning from 1500 to 1600, when anthropologists
say indigenous humans in South and Central America --
who had been expanding in population and range -- declined
by 90 percent. Since most forests in Europe and China
had been mostly cleared for agricultural or habitable
lands by 1 A.D., "the seemingly small indigenous
populations of the Americas would have had a disproportionate
influence on anthropogenic methane emissions from fires,"
the researchers wrote in Science.
The study is important because methane increases have
had the second highest impact on climate change over the
past 250 years behind carbon dioxide, accounting for about
20 percent of the warming from all greenhouse gas increases,
White said. Methane is more powerful than carbon dioxide
in slowing the release of radiated heat away from Earth,
he said.
About 60 percent of atmospheric methane is generated
from human-related activities, according to the International
Panel on Climate Change. Methane increases in the past
200 years are due to increased burning of grasslands,
forests and wood fuels, more intense livestock activity
and rice cultivation and gas leaked from fossil fuel production
and waste management. In addition, natural sources of
methane include wetlands, termites and wildfires.
Overall methane levels in the atmosphere increased about
2 percent from about 1 A.D. to 1000 and decreased by 2
percent from 1000 to 1700, according to the study. Since
the 1700s, the levels have increased by nearly 300 percent,
said White.
Surprisingly, the study showed the amount of methane
produced from burning was about the same 1,000 years ago
as it is today, said White. "There has been a naÔve
idea out there that humans were just passive, pastoral
passengers on the planet up until just a few hundred years
ago," he said. "We have shown that is not the
case."
The study also suggests that natural climate change has
played a role in changing methane levels in the atmosphere,
at least on a regional level, White said. During the Medieval
Warm Period from about 1000 to 1270, there appears to
have been a slight increase in biomass burning in Europe.
In cooler periods like the Little Ice Age from roughly
1300 to 1850, biomass burning in the Northern Hemisphere
appears to have decreased somewhat while anaerobic activity
by bacteria in bogs and swamps probably increased, he
said.
Involving the United States, New Zealand and Australia,
the international project focused on ice cores from Antarctica's
Law Dome, White said. "We could not have undertaken
this study without all three countries," he said.
"These types of projects are not cheap, and each
group brought a unique line of expertise."
White said the team hopes to look at methane levels going
back prior to 2,000 years ago. "The larger question
is when humans began influencing the climate and nutrient
system," he said. "We are in an unusually long
interglacial period right now, and another interesting
but unresolved question is whether humans, without forethought,
have inadvertently kept Earth out of the next ice age
by altering its energy budget."
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