Are Carbon Sinks Turning Into Carbon Sources?

Ten-year warming window closing

Sydney Morning Herald, May 12, 2007

Climate change change may have passed a key tipping point that could mean temperatures rising more quickly than predicted and it being harder to tackle global warming, research suggests.

Scientists at Bristol University say a previously unexplained surge of carbon dioxide levels in the atmosphere in recent years is due to more greenhouse gas escaping from trees, plants and soils. Global warming was making vegetation less able to absorb the carbon pollution pumped out by human activity.

Such a shift would worsen the gloomy predictions of the UN's Intergovernmental Panel on Climate Change, which warned last week that there is less than a decade to tackle rising emissions to avoid the worst effects of global warming.

The prediction came as an equally stark warning was issued that global warming was contributing to increased conflict over dwindling resources.

At the moment about half of human carbon emissions are re-absorbed into the environment, but the fear among scientists is that increased temperatures will reduce this effect. Wolfgang Knorr, a climate researcher at Bristol, said: "We could be seeing the carbon cycle feedback kicking in, which is good news for scientists because it shows our models are correct. But it's bad news for everybody else."

Measurements of carbon dioxide in samples of air show a sharp increase since the turn of the century, with unusually high levels in four of the past five years. The spike does not seem to match the pattern of increased emissions from fossil-fuel burning, and can only be partly explained by natural events such as fires and weather phenomena including El Nino.

Dr. Knorr's team compared the high carbon dioxide measurements in the atmosphere for 2002-03 with simulations of how soils and plants, including trees, behave under different conditions. They found the extra carbon dioxide in the atmosphere could be accounted for by plants taking up less carbon because of unusually dry and hot conditions.

Writing in Geophysical Research Letters, they say: "We find that the remarkable feature of the 2002-03 anomaly seems to be that climate fluctuations - not only related to El Nino and occurring across all latitudes - acted together to create an unusually strong out-gassing of CO2 of the terrestrial biosphere. Further research will be required to investigate if this fluctuation carries features of projected future climate change."

The British Foreign Secretary, Margaret Beckett, warned on Thursday that climate change could spawn a new era of conflicts over water and other scarce resources. She said climate-driven conflicts were already under way in Africa. Underlying the Darfur crisis was a struggle between nomadic and pastoral communities for resources made more scarce through a changing climate.

Speaking at the Royal United Services Institute in London on Thursday, Mrs. Beckett quoted evidence that a similar conflict was brewing in Ghana where Fulani cattle herdsmen are reportedly arming themselves to take on local farmers in a confrontation over water and land as climate change expands the Sahara Desert.

The Foreign Secretary said the Middle East - with 5 per cent of the world's population but only 1 per cent of its water - would be particularly badly affected, with Saudi Arabia, Iran and Iraq hard-hit by a drop in rainfall.

"Resource-based conflicts are not new, but in climate change we have a new and potentially disastrous dynamic."

Her speech echoed a similar warning from the European Commission in January that global warming could trigger regional conflicts, poverty, famine, mass migration and the spread of infectious diseases such as malaria and dengue fever.

The British Government has this year tried to focus global attention on climate change as a security threat, and Mrs. Beckett used the British chairmanship of the United Nations security council in April to convene the council's first debate on the issue.

GEOPHYSICAL RESEARCH LETTERS, VOL. 34, No. 9, May 5, 2007

Impact of terrestrial biosphere carbon exchanges on the anomalous CO₂ increase in 20022003

Abstract

Understanding the carbon dynamics of the terrestrial biosphere during climate fluctuations is a prerequisite for any reliable modeling of the climate-carbon cycle feedback. We drive a terrestrial vegetation model with observed climate data to show that most of the fluctuations in atmospheric CO₂ are consistent with the modeled shift in the balance between carbon uptake by terrestrial plants and carbon loss through soil and plant respiration. Simulated anomalies of the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) during the last two El Niño events also agree well with satellite observations. Our model results suggest that changes in net primary productivity (NPP) are mainly responsible for the observed anomalies in the atmospheric CO₂ growth rate. Changes in heterotrophic respiration (R_h) mostly happen in the same direction, but with smaller amplitude. We attribute the unusual acceleration of the atmospheric CO₂ growth rate during 20022003 to a coincidence of moderate El Niño conditions in the tropics with a strong NPP decrease at northern mid latitudes, only partially compensated by decreased R_h.

Received 6 December 2006; accepted 3 April 2007; published 5 May 2007.

W. Knorr

Quantifying and Understanding the Earth System Research (QUEST), Department of Earth Sciences, University of Bristol, Bristol, UK

N. Gobron

Institute for Environment and Sustainability, European Commission Joint Research Centre, Ispra, Italy

M. Scholze

Quantifying and Understanding the Earth System Research (QUEST), Department of Earth Sciences, University of Bristol, Bristol, UK

T. Kaminski

FastOpt, Hamburg, Germany

R. Schnur

Max-Planck-Institute for Meteorology, Hamburg, Germany

B. Pinty

Institute for Environment and Sustainability, European Commission Joint Research Centre, Ispra, Italy

Keywords: carbon cycle; climate change; terrestrial biosphere.

Index Terms: 1615 Global Change: Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912); 3305 Atmospheric Processes: Climate change and variability (1616, 1635, 3309, 4215, 4513); 3360 Atmospheric Processes: Remote sensing.