Rising temperatures in the past decade have reduced the ability of the world's plants to soak up carbon from the atmosphere, scientists said today.

Large-scale
droughts have wiped out plants that would have otherwise absorbed an
amount of carbon equivalent to Britain's annual man-made greenhouse gas
emissions.

Scientists measure the amount of atmospheric carbon
dioxide absorbed by plants and turned into biomass as a quantity known
as the net primary production. NPP increased from 1982 to 1999 as
temperatures rose and there was more solar radiation.

But the
period from 2000 to 2009 reverses that trend – surprising some
scientists. Maosheng Zhao and Steven Running of the University of
Montana estimate that there has been a global reduction in NPP of 0.55
gigatonnes (Gt). In comparison, the UK's contribution to annual
worldwide carbon dioxide emissions was 0.56Gt in 2007, while global
aviation industry made up around 0.88Gt (3%) of the world total of
29.3Gt that year, according to UN data.

The researchers used data from the moderate resolution imaging spectroradiometer (Modis) on board Nasa's Terra satellite, combined with global climate data to measure the change in global NPP over the past decade.

"The past decade has been the warmest since instrumental measurements began, which could imply continued increases in NPP," wrote Zhao and Running in the journal Science.

But
instead of helping plants grow, these rising temperatures instead
caused droughts and water stresses, particularly in the southern
hemisphere and in rainforests, which contain most of the world's plant
biomass. The growth there has been curtailed by lack of water and
increased respiration, which returns carbon to the atmosphere. These
problems counteracted any increases in NPP seen at the high latitudes
and elevations in the northern hemisphere.

Reduced plant matter
not only reduces the world's natural ability to manage carbon dioxide in
the atmosphere but could also lead to problems with growing more crops
to feed rising populations or make sustainable biofuels.

"Under a
changing climate, severe regional droughts have become more frequent, a
trend expected to continue for the foreseeable future," said the
researchers. "The warming-associated heat and drought not only decrease
NPP, but also may trigger many more ecosystem disturbances, releasing
carbon to the atmosphere. Reduced NPP potentially threatens global food
security and future biofuel production and weakens the terrestrial
carbon sink."

The researchers conclude that further monitoring
will be needed to confirm whether the decrease in NPP they have observed
in the past decade is an anomaly or whether it signals a turning point
to a future decline in the world's ability to sequester carbon dioxide.