31 May 2013
AGU Release No. 13-24
New research links
of arid areas like
concentrations of atmospheric carbon dioxide.
Scientists have long suspected that a
flourishing of green foliage around the globe, observed since the
early 1980s in satellite data, springs at least in part from the
increasing concentration of carbon dioxide in Earth’s atmosphere.
Now, a study of arid regions around the
globe finds that a carbon dioxide “fertilization effect” has,
indeed, caused a gradual greening from 1982 to 2010.
Focusing on the southwestern corner of North America, Australia’s
outback, the Middle East, and some parts of Africa, Randall Donohue
of the Commonwealth Scientific and Industrial Research Organization
(CSIRO) in Canberra, Australia and his colleagues developed and
applied a mathematical model to predict the extent of the
carbon-dioxide (CO2) fertilization effect.
They then tested this
prediction by studying satellite imagery and teasing out the
influence of carbon dioxide on greening from other factors such as
precipitation, air temperature, the amount of light, and land-use
The team’s model predicted that foliage would increase by some 5 to
10 percent given the 14 percent increase in atmospheric CO2
concentration during the study period.
The satellite data agreed,
showing an 11 percent increase in foliage after adjusting the data
for precipitation, yielding “strong support for our hypothesis,” the
“Lots of papers have shown an average increase in vegetation across
the globe, and there is a lot of speculation about what’s causing
that,” said Donohue of CSIRO’s Land and Water research division, who
is lead author of the new study.
“Up until this point, they’ve
linked the greening to fairly obvious climatic variables, such as a
rise in temperature where it is normally cold or a rise in rainfall
where it is normally dry. Lots of those papers speculated about the
CO2 effect, but it has been very difficult to prove.”
He and his colleagues present their findings in an article that has
been accepted for publication in Geophysical Research Letters, a
journal of the American Geophysical Union.
The team looked for signs of CO2 fertilization in arid areas,
Donohue said, because,
“satellites are very good at detecting changes
in total leaf cover, and it is in warm, dry environments that the
CO2 effect is expected to most influence leaf cover.”
Leaf cover is
the clue, he added, because,
“a leaf can extract more carbon from the
air during photosynthesis, or lose less water to the air during
photosynthesis, or both, due to elevated CO2.”
That is the CO2
But leaf cover in warm, wet places like tropical rainforests is
already about as extensive as it can get and is unlikely to increase
with higher CO2 concentrations.
In warm, dry places, on the other
hand, leaf cover is less complete, so plants there will make more
leaves if they have enough water to do so.
“If elevated CO2 causes
the water use of individual leaves to drop, plants will respond by
increasing their total numbers of leaves, and this should be
measurable from satellite,” Donohue explained.
To tease out the actual CO2 fertilization effect from other
environmental factors in these regions, the researchers first
averaged the greenness of each location across 3-year periods to
account for changes in soil wetness and then grouped that greenness
data from the different locations according to their amounts of
The team then identified the maximum amount of
foliage each group could attain for a given precipitation, and
tracked variations in maximum foliage over the course of 20 years.
This allowed the scientists to remove the influence of precipitation
and other climatic variations and recognize the long-term greening
In addition to greening dry regions, the CO2 fertilization effect
could switch the types of vegetation that dominate in those regions.
“Trees are re-invading grass lands, and this could quite possibly be
related to the CO2 effect,” Donohue said. “Long lived woody plants
are deep rooted and are likely to benefit more than grasses from an
increase in CO2.”
“The effect of higher carbon dioxide levels on plant function is an
important process that needs greater consideration,” said Donohue.
“Even if nothing else in the climate changes as global CO2 levels
rise, we will still see significant environmental changes because of
the CO2 fertilization effect.”
This study was funded by,
CSIRO’s Sustainable Agriculture Flagship
Water for a Healthy Country Flagship
the Australian Research
Land & Water Australia
Notes for Journalists
Journalists and public
information officers (PIOs) of educational and scientific
institutions who have registered with AGU can download a PDF copy of
this accepted article by clicking on this link:
Or, you may order a copy of the final paper by emailing your request
to Peter Weiss at PWeiss@agu.org. Please provide your name, the name
of your publication, and your phone number.
Neither the paper nor this press release are under embargo.
Title: CO2 fertilization
has increased maximum foliage cover across the globe's warm,
Randall J. Donohue and Tim R.
McVicar - CSIRO Land and Water, Canberra, Australia;
Michael L. Roderick - Research School of Biology, The
Australian National University, Canberra, Australia;
Research School of Earth Sciences, The Australian National
University, Canberra, Australia; and Australian Research
Council Centre of Excellence for Climate System Science;
Graham D. Farquhar - Research School of Biology, The
Australian National University, Canberra, Australia.
Contact information for the author:
Randall Donohue, Email: Randall.Donohue@csiro.au, Phone: