ScienceDaily
For the first time scientists have looked at the net balance of the three major greenhouse gases -- carbon dioxide, methane, and nitrous oxide -- for every region of Earth's landmasses. They found surprisingly, that human-induced emissions of methane and nitrous oxide from ecosystems overwhelmingly surpass the ability of the land to soak up carbon dioxide emissions, which makes the terrestrial biosphere a contributor to climate change.
For the first time scientists have looked at
the net balance of the three major greenhouse gases--carbon dioxide,
methane, and nitrous oxide--for every region of Earth's landmasses.
They
found surprisingly, that human-induced emissions of methane and nitrous
oxide from ecosystems overwhelmingly surpass the ability of the land to
soak up carbon dioxide emissions, which makes the terrestrial biosphere
a contributor to climate change.
The results published in the March 10,
2016, Nature, revises our understanding of how human activity contributes to global warming.
Co-author Anna Michalak of Carnegie's Department of Global Ecology
remarked, "Typically we think of land as a net 'sink' of carbon dioxide.
But we found that the sign of the human-induced impact is reversed if
we also take into account methane and nitrous oxide."
The scientists looked at the so-called biogenic fluxes or flow of the
three greenhouse gases on land that were caused by human activities
over the last three decades and subtracted out emissions that existed
"naturally" during pre-industrial times.
Biogenic sources include gas
emissions from plants, animals, microbes, and the like. They were
interested in finding out how human activities have changed the biogenic
fluxes of these gases. Historically, such emissions have included
methane emissions from wetlands and nitrous oxide emissions from soil.
Human activity and human-caused climate change have changed the
magnitude of these fluxes, however, as well as added new categories of
biogenic fluxes such as those resulting from sewage, cattle, and
fertilizer use.
The scientists first added up all biogenic emissions of carbon
dioxide, methane, and nitrous oxide, then subtracted out those that
occurred naturally prior to human intervention to get to the net amount.
The study did not include non-biogenic gas emissions from activities
like fossil fuel burning or natural gas production.
The team discovered that the human impact on biogenic methane and
nitrous oxide emissions far outweighed the human impact on the
terrestrial uptake of carbon dioxide, meaning that humans have caused
the terrestrial biosphere to further contribute to warming.
In other
words, the terrestrial biosphere, through human action, is now
contributing to climate change rather than mitigating climate change.
This runs counter to conventional thinking based on previous studies,
which had focused only on carbon dioxide and had emphasized the climate
change mitigating effect of human impacts terrestrial carbon uptake.
The scientists found that greenhouse gas emissions vary considerably
by region. Interestingly, the human-induced emissions of the gases in
Southern Asia, including China and India, had a larger net warming
effect compared to other areas.
Southern Asia contains some 90% of the
world's rice fields and more than 60% of the world's nitrogen fertilizer
use. Thus, methane emissions in this region are largely from rice
cultivation and livestock, while human-made fertilizers are a major
source of nitrous oxide.
Lead author of the study, Hanqin Tain director of the International
Center for Climate and Global Change Research, School of Forestry and
Wildlife Sciences at Auburn University said, "This finding reveals for
the first time that human activities have transformed the land biosphere
to a contributor to climate change."
"This study should serve as a wake-up call to governments,
policymakers, and individuals around the world," said Michalak. "We must
expand our focus and devise strategies that target the biogenic
emissions of these other greenhouse gases if we are to change the course
of climate change."
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