LONDON The Earth's natural ability to absorb carbon dioxide is declining and global temperatures are on course to rise by 6C by the end of the century as atmospheric levels of the greenhouse gas continue to rise, according to a study.

Scientists said that CO2 emissions have risen by 29 per cent in the past decade alone and called for urgent action by leaders at the UN climate talks in Copenhagen to agree drastic emissions cuts in order to avoid dangerous climate change.

By studying 50 years of data on carbon emissions from human and natural sources, such as volcanoes, a team of international researchers was able to estimate how much CO2 is being absorbed naturally by forests, oceans and soil.

They concluded, in the journal Nature Geoscience, that those natural sinks are becoming less efficient, absorbing 55 per cent of the carbon now, compared with 60 per cent half a century ago.

The drop in the amount absorbed is equivalent to 405m tonnes of carbon, or 60 times the annual output of the largest coal-fired power station in the UK.

The CO2 absorbed by a natural carbon sink can be adversely affected by annual variations in weather and rising concentrations of the gas in the atmosphere.

Corinne Le Quere of the University of East Anglia, England, who led the study with colleagues at the British Antarctic Survey, suggested that rapidly rising human emissions of CO2 might have initiated a feedback mechanism in the climate system, whereby natural sinks become even more inefficient as the amount of the greenhouse gas increases in future.

The research gives greater urgency to the diplomatic manoeuvring ahead of the Copenhagen summit. Earlier this week President Obama acknowledged that time had run out to secure a legally binding climate deal next month and said he would support plans to delay a formal pact until next year at the earliest.

The study is the most comprehensive analysis to date of how economic changes and shifts in the way people use the land in the past five decades have affected the concentration of CO2 in the atmosphere.

“The global trends we are on with CO2 emissions from fossil fuels suggest that we're heading towards 6C of global warming,” said Le Quere.

“This is very different to the trend we need to be on to limit global climate change to 2C [the level required to avoid dangerous climate change].” That would require CO2 emissions from all sources to peak between 2015 and 2020 and that the global per capita emissions be decreased to one tonne of CO2 by 2050. The average US citizen emits 19.9 tonnes a year.

Le Quere added “Based on our knowledge of recent trends in CO2 emissions and the time it takes to change energy infrastructure around the world and on the response of the sinks to climate change and variability, the Copenhagen conference is our last chance to stabilise climate at 2C above pre-industrial levels in a smooth and organised way.

“If the agreement is too weak or if the commitments are not respected, we will be on a path to 5C or 6C.”

Le Quere's work, part of the Global Carbon Project, showed that CO2 emissions from burning fossil fuels increased at an average of 3.4 per cent a year between 2000 and 2008 compared with 1 per cent a year in the 1990s. Despite the global economic downturn, emissions still increased by 2 per cent in 2008. Most of the recent increase has come from China and India, though a quarter of their emissions are a result of trade with the west.

Based on projected changes in GDP, the scientists said emissions for 2009 were expected to fall to 2007 levels, before increasing again in 2010.

But Le Quere's conclusion on the decline of the world's carbon sinks is not universally accepted. Wolfgang Knorr, of the University of Bristol, England, recently published a study in Geophysical Research Letters, using similar data to Le Quere, where he argued that the natural carbon sinks had not noticeably changed. “Our apparently conflicting results demonstrate what doing cutting-edge science is really like and just how difficult it is to accurately quantify such data,” said Knorr.

Overall trends can be difficult to detect because of the climactic and other natural conditions affecting absorption of sinks. Le Quere said, however, her team's analysis had been able to remove more of the noise in the data that is associated with the natural annual variability of CO2 levels due to, for example, El Nino or volcanic eruptions. “Our methods are different - Knorr uses annual data, we use monthly data and I think we can remove more of the variability.”

Jo House, of the University of Bristol, who worked on the Nature Geoscience paper, said “It is difficult to accurately estimate sources and sinks of CO2, particularly in emissions from land use change where data on the area and nature of deforestation is poor, and in modelled estimates of the land sink which is strongly affected by inter-annual climate variability ... there are still gaps in our understanding.”

The scientists agreed, however, that an improved understanding of land and ocean CO2 sinks was crucial because it had implications for CO2 targets set by governments at climate negotiations.—Dawn/The Guardian News Service