Between 40 and 60 percent of all anthropogenic emissions of carbon dioxide appear to remain in the atmosphere. The remainder of the excess (about 4 gigatonnes of carbon per year (GtC/year)) is sequestered within the global carbon cycle – where and by what processes is still not clear. A variety of studies into ocean carbon fluxes cautiously support earlier estimates for a net ocean sink in the order of 2 GtC/year, although estimates vary between 1.4 and 3 GtC/year. "Biological pumping" to deep oceans, and incorporation in forests and soils (carbon dioxide fertilization effect) provide for other carbon dioxide sinks. However, the geographical and seasonal distributions of ocean carbon sources and sinks are complex and can vary significantly with year-to-year climate variability.
The cost of afforestation to increase the rate of carbon dioxide uptake has been estimated to cost as low as $35 per ton of carbon sequestered in parts of the South and up to $1700 per ton in the North. A massive global afforestation programme would sequester at most about 1.2 gigatonnes of carbon per year. This is about one-fifth of current energy-related emissions, and just one-half to twice the estimated carbon releases through land-use changes, including tropical deforestation.