The important role played by CO₂ in the Earth’s energy balance has been appreciated since the late 19th century, when Swedish scientist Svante Arrhenius first proposed a link between CO₂ levels and temperature. At that time, humans were only beginning to burn fossil fuels—which include coal, oil, and natural gas—on a wide scale for energy. The combustion of these fuels, or any material of organic origin, yields mostly CO₂ and water vapor, but also small amounts of other by-products, such as soot, carbon monoxide, sulfur dioxide, and nitrogen oxides. All of these substances occur naturally in the atmosphere, and natural fluxes of water and CO₂ between the atmosphere, oceans, and land surface play a critical role in both the physical climate system and the Earth’s biosphere. However, unlike water vapor molecules, which typically remain in the lower atmosphere for only a few days before they are returned to the surface in the form of precipitation, CO₂ molecules are only exchanged slowly with the surface. The excess CO₂ emitted by fossil fuel burning and other human activities will thus remain in the atmosphere for many centuries before it can be removed by natural processes (Solomon et al., 2009).

A number of agencies and groups around the world, including the Carbon Dioxide Information Analysis Center at Oak Ridge National Laboratory and the International Energy Agency, produce estimates of how much CO₂ is released to the atmosphere every year by human activities. The most recent available estimates indicate that, in 2008, human activities released over 36 Gt (gigatons, or billion metric tons) of CO₂ into the atmosphere—including 30.6 ± 1.7 Gt from fossil fuel burning, plus an additional 4.4 ± 2.6 Gt from land use changes and 1.3 ± 0.1 Gt from cement production (Le Quéré et al., 2009). Emissions from fossil fuels have increased sharply over the last two decades, rising 41 percent since 1990 (Figure 6.1). CO₂ emissions due to land use change—which are dominated by tropical deforestation—are estimated based on a variety of methods and data sources, and the resulting estimates are both more uncertain and more variable from year-to-year than fossil fuel emissions. Over the past decade (2000–2008), Le Quéré et al. (2009) estimate that land use changes released 5.1 ± 2.6 Gt of CO₂ each year, while fossil fuel burning and cement production together released on average 28.2 ± 1.7 Gt of CO₂ per year.

Up until the 1950s, most scientists thought the world’s oceans would simply absorb most of the excess CO₂ released by human activities. Then, in a series of papers in the late 1950s (e.g., Revelle and Suess, 1957), American oceanographer Roger Revelle and several collaborators hypothesized that the world’s oceans could not absorb all the excess CO₂ being released from fossil fuel burning. To test this hypothesis, Revelle’s colleague C. D. Keeling began collecting canisters of air at the Mauna Loa Observatory