Since 1979, scientists have generally agreed that a doubling of atmospheric carbon dioxide increases the earth’s average surface temperature by 3-8°F (1.5-4.5°C). More recent studies have suggested that the warming is likely to occur more rapidly over land than the open seas. Moreover, the warming in temperatures tends to lag behind the increase in greenhouse gases. At first, the cooler oceans will tend to absorb much of the additional heat and thereby decrease the warming of the atmosphere. Only when the ocean comes into equilibrium with the higher level of CO2 will the full warming occur.
The Intergovernmental Panel on Climate Change estimates that the concentration of CO2 will double from pre-industrial levels by the mid- to late 21st century. Currently, the panel projects a global average warming of 1.0-4.5°F (0.6-2.5°C) in the next fifty years and 2.5 to 10.4°F (1.4 to 5.8°C) by the year 2100, compared with the global average temperature in 1990. The wide range in projected temperatures is due to varying assumptions about future trends in greenhouse gas emissions and sulfate aerosols.
Source: IPCC Third Assessment Report (2001)