3 February 2007Ian Sample
Predictions by international scientists that global warming will lead to a sharper rise in temperatures than previously thought made sobering reading yesterday. But what is the major factor that has driven their gloomy conclusion?
Dramatic flips in the way ecosystems absorb carbon dioxide will see oceans and vast swaths of land falter in their ability to draw up the greenhouse gas, allowing it to build up in the atmosphere and cause more warming. The phenomenon is known as a positive feedback - where global warming drives changes in ecosystems that themselves cause more heating.
The warning came in a major report on climate change published yesterday that suggests average temperatures could rise more than expected - by as much as 6.4C by 2100, unless greenhouse gas emissions are reined in. The report, from the UN's Intergovernmental Panel on Climate Change, has upgraded its 2001 estimate that temperatures would rise by at most 5.8C, because at the time the feedback mechanisms were either unknown or poorly understood.
The latest report states that the predicted temperature rise for 2100 was raised because "the broader range of models now available suggests stronger climate-carbon cycle feedbacks".
Early climate change predictions were calculated predominantly by anticipating levels of carbon dioxide and other greenhouse gases in the atmosphere. The gases allow radiation from the sun to warm the planet but block it as it is radiated back off the surface, forming a virtual blanket around the globe.
But scientists have steadily uncovered ecological feedback mechanisms driven by climate change that complicate the outcome. In some cases global warming triggers feedbacks that act to cool the planet, but others exacerbate the warming.
One of the earliest feedback mechanisms identified was the melting of ice sheets and sea ice. The vast sheets of bright white ice reflect nearly 80% of sunlight that falls on them. But as they melt they reveal dark waters or soils beneath that absorb sunlight, warm up and cause yet more melting.
The latest IPCC report for the first time includes climate models that take into account two other ecological feedback mechanisms that accelerate global warming: the ability of the oceans and land to absorb carbon.
"The oceans and the soils and trees absorb a half of the carbon dioxide released into the atmosphere from human activity. With climate change, they will get worse and worse at doing that, so more of our human emissions of carbon dioxide will remain in the atmosphere," said Corinne Le Quéré, an IPCC author and expert on the carbon cycle at the University of East Anglia Climatic Research Unit.
As the world warms up the oceans become less able to dissolve carbon dioxide. Warmer oceans are also having an adverse effect on carbon-absorbing marine phytoplankton, the organisms that lie at the very bottom of the aquatic food chain. As warming continues scientists fear that phytoplankton will begin to die off, creating a positive feedback cycle where warmer oceans release more carbon which in turn leads to more warming.
At the same time carbon dioxide which now fertilises soils and boosts the growth of forests and other plants will reach saturation point, so the land's ability to soak up carbon dioxide will stall. As temperatures rise even further many plants will become stressed by drought conditions and microbes in the soil will start breaking down organic matter from dead plants faster, meaning large areas of land will begin emitting carbon dioxide instead of acting as an overall sink for the gas.
Signs that soils were beginning to become part of the problem of global warming emerged in 2005 when researchers discovered that a vast expanse of western Siberia was undergoing an unprecedented thaw. The region, the largest frozen peat bog in the world, covering an area the size of France and Germany combined, had begun to melt for the first time since it formed 11,000 years ago at the end of the last ice age.
The team, from Tomsk State and Oxford universities believe the million square kilometre peat bog could begin to release billions of tonnes of methane locked up in the soils. Methane is a greenhouse gas 20 times more potent than carbon dioxide.
The team found that even if methane seeped from the peat bog over the next 100 years it would add 700m tonnes of carbon into the atmosphere every year, roughly the same that is released annually from the world's wetlands and agriculture. It would effectively double the atmospheric levels of the gas, leading to a 10% to 25% increase in global warming.
Last year Peter Cox, a climate modeller at Exeter University, found a similar feedback mechanism and warned that warmer temperatures could force soils around the world to release their stocks of carbon into the atmosphere, potentially driving temperatures up by a further 1.5C. He called for poorer countries to be paid not to cut down their forests as a possible solution.
Earlier this month Jim Hansen, director of the Nasa Goddard Institute for Space Studies and one of the first scientists to warn of climate change in 1988, said greenhouse gas emissions were beginning to trigger dangerous positive feedbacks.
"Previously these feedback mechanisms weren't well known about, and they have only recently been taken into account," said Dr Le Quéré. "We are very likely to find more of these feedbacks because now we are looking for them. At the moment we are not seeing their effects too strongly, but these are going to become a big part of the picture."
http://environment.guardian.co.uk/print/0,,329704994-121568,00.html
