How much carbon is being “absorbed” by forests in the
Northern Hemisphere?

NASA-funded Earth Science researchers, using high-resolution
maps of carbon storage derived from NASA-developed satellite
data sets, suggest that forests in the United States, Europe
and Russia have been storing nearly 700 million metric tons
of carbon a year during the 1980s and 1990s.

Scientists hope to understand to what extent carbon is stored
in the Earth’s forests because of the need to account for the
fate of the carbon released into the Earth’s atmosphere in
the form of carbon dioxide from fossil-fuel combustion.
NASA’s research will further understanding of the role that
such “sinks” play in sequestering carbon and the impact
climate change has on agriculture, rangelands and forests.

The accumulation of carbon dioxide in the atmosphere is
considered to be the primary forcing agent for global climate
change, so forecasts of future climate require that the fate
of carbon dioxide released into the atmosphere be understood.

Right now, scientists have inferred that there is a sink of 1
to 2 billion tons of carbon into the land regions of the
northern hemisphere, which corresponds to some 15 to 30
percent of the global annual industrial carbon emissions.

“A critical challenge for Earth scientists is to reduce the
uncertainty in this estimate and to derive its geographical
variation,” said Compton Tucker of NASA’s Goddard Space
Flight Center, Greenbelt, Md.

The researchers exploited high-resolution spatial coverage
data of northern forests available only through NASA-built
weather satellites in their study.

Using detailed forest-inventory data from 171 provinces in
six countries, they derived and validated relationships
between forest “greenness” measured from satellites and the
“hands-on” measurements of the amount of carbon contained in
the woody biomass of forests. The robust relationship thus
obtained allowed them to produce high-resolution maps of
carbon stocked in about one-and-a-half billion hectares
(roughly, a little more than one-and-a-half times the size of
the United States) of northern forests located above the 30th

“We identified carbon sinks, areas where forests were storing
carbon, and sources, areas where forests were losing carbon,
by comparing the most recent carbon-stock maps of the late
1990s with those from the early 1980s,” said Ranga Myneni of
Boston University.

U.S. forests soaked up 140 million tons of carbon a year.
With the exception of Canada’s boreal forests, which were
found to be losing carbon, most northern forests were storing
carbon. Russia, the country with most forests in the Northern
Hemisphere, accounted for almost 40 percent of the biomass
carbon sink.

The researchers suggest a longer growing season from climate-
warming in the north, fire suppression and forest re-growth
in the United States, better forest management in the Nordic
countries and declining harvests in Russia as possible
reasons why some forests are storing carbon.

They also suggest increased incidence of fires and
infestations as possible reasons why some Canadian forests
are losing carbon. “The fact that the forests in different
areas appear to be behaving differently means that there is
no substitute for monitoring them from space, in view of the
global coverage provided by satellite observations,” said
Jiarui Dong of Boston University.

These results of a NASA-funded study will be published in the
December 18 issue of the Proceedings of the National Academy
of Sciences (PNAS). An electronic version of this article
will appear in an early edition of PNAS December 11 at:

The work was funded by NASA Headquarters’ Earth Science
Enterprise, a long-term research program dedicated to
understanding how human-induced and natural changes affect
our global environment.

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