That figure is roughly equivalent to the sink provided by geographic Europe but less than the value for the US. It means that China's terrestrial ecosystems absorbed 28–37% of its cumulated fossil carbon emissions during the 1980s and 1990s. For 2001 to 2005, however, the land sink absorbed just 16–22% of the country's rapidly growing carbon emissions. China is now the world's largest emitter of carbon.

"Projections for the year 2030, produced by the International Energy Agency, place Chinese fossil-fuel carbon dioxide emissions at 3.1 Pg of carbon per year, of which only 6–8% will be offset by biospheric uptake should China's biosphere continue to remove carbon at its current rate (a questionable prospect)," said Kevin Robert Gurney of Purdue University, who was not involved in the research, writing in a "News and Views" article in Nature.

Southern China contributed more than 65% of the carbon sink. The northeast of China, on the other hand, acted as a net source of carbon because of overharvesting and forest degradation. The scientists reckon that the factors behind the large sink in the south include regional climate change – the south of China has become wetter in summer, which has probably boosted plant productivity, the large-scale plantation programmes in place since the 1980s to reduce soil erosion and dust problems, and shrub recovery boosted by people moving to the cities and using less biomass as fuel. Changes in agricultural practice have also increased carbon sequestration in the soil in recent years.

Forest provided roughly half of the sink, with on average 75 teragrammes of carbon per year stored in forest biomass and 4 Tg entering the soil in forest areas. The researchers calculate that shrub, meanwhile, provided up to 30% of the sink, with 22 teragrammes of carbon absorbed by shrub biomass and 39 Tg absorbed by shrubland soil. Forest makes up roughly 14% of China's surface, with shrubland covering 20%.

To carry out their study, the scientists from Peking University, China; LSCE, France; the Biogeochemical Isotope Laboratory, France; Chinese Academy of Sciences; and Met Office Hadley Centre, UK, used three different methods – a bottom-up estimate of carbon incorporated in all of the major vegetation and soil types using field data and satellite greenness measurements; five different ecosystem models; and atmospheric inversions, which look at atmospheric gradients of carbon dioxide to estimate uptake by sinks.

The researchers reported their work in Nature.