"The Earth's carbon sinks are complex and there are some gaps in our understanding, particularly in our ability to link human-induced carbon dioxide emissions to atmospheric carbon dioxide concentrations on a year-to-year basis," said Corinne Le Quéré of the University of East Anglia, UK, and British Antarctic Survey. "But if we can reduce the uncertainty about the carbon sinks, our data could be used to verify the effectiveness of climate mitigations policies."

At the moment, it's not possible to work out annual global carbon emissions by measuring the change in concentration of carbon dioxide in the atmosphere because it's unclear exactly how much carbon dioxide is absorbed by land and ocean-based sinks – there's considerable variation in sink activity from year to year. But if these uncertainties were cleared up, the technique could provide a useful cross-check for calculations of global emissions from energy data.

Le Quéré and colleagues from seven countries around the world drew up global carbon dioxide budgets for each year from 1959 to 2008. To do this, they estimated carbon emissions from fossil-fuel combustion using national energy statistics; obtained a figure for carbon emissions from land-use change by looking at deforestation and other land-use data, as well as satellite observations of fire; collated measurements of atmospheric carbon dioxide concentrations; and modelled the response of the ocean and land sinks to meteorological conditions.

The team found that carbon dioxide emissions from fossil-fuel combustion were 8.7 Pg of carbon per year in 2008, 2% higher than the year before, 29% higher than in 2000 and a whopping 41% above 1990 emissions. Levels in 1990 are used as a baseline for emissions targets in the Kyoto Protocol. Emissions are now tracking the average of the most carbon-intensive family of scenarios put forward by the IPCC – the A1F1 scenario. Meanwhile, average emissions-per-person rose from 1.1 metric tons of carbon in 2000 to 1.3 metric tons in 2008, the highest ever.

Between 2000 and 2008, emissions increased by 3.4% a year whereas in the 1990s they increased at 1.0%. Le Quéré attributes most of this change to "the very, very fast growth in the economy in China". Developing countries now emit more greenhouse gases than developed countries but around one-quarter of the emissions growth in developing countries is due to the production of goods for export.

"The current trend that we are on for global carbon dioxide emissions from fossil fuels suggests that we are heading towards six degrees of global warming," said Le Quéré. "This comes from the analysis made by the International Energy Agency in the World Economic Outlook 2009 they just published. This is very different from the trend that we need to be on to limit global climate change to two degrees with a 75% probability."

Meeting this two-degree target requires that global carbon dioxide emissions from all sources peak between 2015 and 2020, she added, and that global per capita emissions be decreased to about 0.3 metric tons of carbon by 2050. For comparison, the US currently emits 5.1 metric tons of carbon per capita, while the UK produces 2.5 tons, China 1.5 tons, and India 0.4 tons.

The researchers found that emissions from land-use change have remained almost constant since 2000. In 2008 they made up 12% of total man-made emissions, whereas in 2000 they accounted for 20%.

It appears that carbon dioxide sinks are not keeping pace with the increase in man-made emissions, however; the fraction of carbon emissions remaining in the atmosphere increased by an average of 0.3% per year between 1959 and 2008. The team's models indicate that the sinks are responding to climate variability and climate change but the models do not yet include factors such as peat, buried carbon in permafrost soils, wild fires, ocean eddies and ocean acidification.

The researchers reported their work in Nature Geoscience.