Global temperatures are on a path to rise by an average of 6C by the end of the century as CO2 emissions increase and the Earth's natural ability to absorb the gas declines, according to a major new study.

Scientists said that CO2 emissions have risen by 29% 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.

The news will give greater urgency to the diplomatic manoeuvring before the Copenhagen summit. President Obama and President Hu of China attempted to breathe new life into the negotiations today by announcing that they intended to set targets for easing greenhouse gas emissions next month. Obama said that he and Hu would continue to press for a deal that would "rally the world".

The new study is the most comprehensive analysis to date of how economic changes and shifts in the way people have used 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 Corinne Le Quéré of the University of East Anglia who led the study with colleagues at the British Antarctic Survey.

"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 1 tonne of CO2 by 2050. Currently the average US citizen emits 19.9 tonnes per year and UK citizens emit 9.3 tonnes.

By studying 50 years of data on carbon emissions and combining with estimates of human carbon emissions and other sources such as volcanoes, the team was able to estimate how much CO2 is being absorbed naturally by forests, oceans and soil. The team conclude in the journal Nature Geoscience that those natural sinks are becoming less efficient, absorbing 55% of the carbon now, compared with 60% half a century ago. The drop in the amount absorbed is equivalent to 405m tonnes of carbon or around 60 times the annual output of Drax coal-fired power station, which is the largest in the UK.

"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 preindustrial levels in a smooth and organised way," said Le Quéré. "If the agreement is too weak or if the commitments are not respected, we will be on a path to 5C or 6C."

Le Quéré's work, part of the Global Carbon Project, showed that CO2 emissions from burning fossil fuels increased at an average of 3.4% a year between 2000 and 2008 compared with 1% a year in the 1990s. Despite the global economic downturn, emissions still increased by 2% in 2008. The vast majority of the recent increase has come from China and India, though a quarter of their emissions are a direct result of trade with the west. In recent years, the global use of coal has also surpassed oil.

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

But Le Quéré's conclusion on the decline of the world's carbon sinks is not universally accepted. Wolfgang Knorr of the University of Bristol recently published a study in Geophysical Research Letters, using similar data to Le Quéré, 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.

The amount of CO2 that natural carbon sinks can absorb varies from year to year depending on climactic and other natural conditions, and this means that overall trends can be difficult to detect. Le Quéré said 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 Niño 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. While the science has advanced rapidly, there are still gaps in our understanding."

The scientists agreed, however, that an improved understanding of land and ocean CO2 sinks was crucial, since it has a major influence in determining the link between human CO2 emissions and atmospheric concentration of the greenhouse gas. In turn, this has implications for CO2 targets set by governments at climate negotiations.

• The headline to this article was amended on Wednesday 18 November 2009 to make clear that the study said global temperatures could rise 6C by end of century, not that they will do so.

