Carbon Dioxide

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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 Quere 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 (20002008), Le Quere 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

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FIGURE 6.1 Estimated global CO2 emissions from fossil fuel sources, in gigatons (or billion metric tons). Based on data from Boden et al. (2009; available at http://cdiac.ornl.gov/trends/emis/tre_glob.html).

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FIGURE 6.1 Estimated global CO2 emissions from fossil fuel sources, in gigatons (or billion metric tons). Based on data from Boden et al. (2009; available at http://cdiac.ornl.gov/trends/emis/tre_glob.html).

in Hawaii, far away from major industrial and population centers, and analyzing the composition of these samples to determine whether CO2 levels in the atmosphere were increasing. Similar in situ measurements continue to this day at Mauna Loa as well as at many other sites around the world. The resulting high-resolution, well-calibrated, 50-year-plus time series of highly accurate and precise atmospheric CO2 measurements (Figure 6.2), commonly referred to as the Keeling curve, is both a major scientific achievement and a key data set for understanding climate change.

The Keeling curve shows that atmospheric CO2 levels have risen by more than 20 percent since 1958; as of January 2010, they stood at roughly 388 ppm, rising at an average annual rate of almost 2.0 ppm per year over the past decade (Blasing, 2008; Tans, 2010). When multiplied by the mass of the Earth's atmosphere, this increase corresponds to 15.0 ± 0.1 Gt CO2 added to the atmosphere each year, or roughly 45 percent of the excess CO2 released by human activities over the last decade. The remaining 55 percent is absorbed by the oceans and the land surface. The size of these CO2 "sinks" is estimated via both modeling and direct observations of CO2 uptake in the oceans and on land. These estimates indicate that the oceans absorbed on average 8.4 ± 1.5 Gt CO2 annually over the last decade (or 26 percent of human emissions), while the land surface took up 11.0 ± 3.3 Gt per year (29 percent), with a small residual of 0.3 Gt (Le Quere et al., 2009).

A careful examination of the Keeling curve reveals that atmospheric CO2 concentrations are currently increasing twice as fast as they did during the first decade of the record (compare the slope of the black line in Figure 6.2). This acceleration in the rate of CO2 rise can be attributed in part to the increases in CO2 emissions due to increasing

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FIGURE 6.2 Atmospheric CO2 concentrations (in parts per million [ppm]) at Mauna Loa Observatory in Hawaii. The red curve, which represents the monthly averaged data, includes a seasonal cycle associated with regular changes in the photosynthetic activity in plants, which are more widespread in the Northern Hemisphere. The black curve, which represents the monthly averaged data with the seasonal cycle removed, shows a clear upward trend. SOURCE: Tans (2010; available at http://www.esrl.noaa. gov/gmd/ccgg/trends/).

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FIGURE 6.2 Atmospheric CO2 concentrations (in parts per million [ppm]) at Mauna Loa Observatory in Hawaii. The red curve, which represents the monthly averaged data, includes a seasonal cycle associated with regular changes in the photosynthetic activity in plants, which are more widespread in the Northern Hemisphere. The black curve, which represents the monthly averaged data with the seasonal cycle removed, shows a clear upward trend. SOURCE: Tans (2010; available at http://www.esrl.noaa. gov/gmd/ccgg/trends/).

energy use and development worldwide (as indicated in Figure 6.1). However, recent studies suggest that the rate at which CO2 is removed from the atmosphere by ocean and land sinks may also be declining (Canadell et al., 2007; Khatiwala et al., 2009). The reasons for this decline are not well understood, but, if it continues, atmospheric CO2 concentrations would rise even more sharply, even if global CO2 emissions remain the same. Improving our understanding and estimates of current and projected future fluxes of CO2 to and from the Earth's surface, both over the oceans and on land, is a key research need (research needs are discussed at the end of the chapter).

To determine how CO2 levels varied prior to direct atmospheric measurements, scientists have studied the composition of air bubbles trapped in ice cores extracted from the Greenland and Antarctic ice sheets. These remarkable data, though not as accurate and precise as the Keeling curve, show that CO2 levels were relatively constant for thousands of years preceding the Industrial Revolution, varying in a narrow band between 265 and 280 ppm, before rising sharply starting in the late 19th century (Figure 6.3). The current CO2 level of 388 ppm is thus almost 40 percent higher

FIGURE 6.3 CO2 variations during the last 1,000 years, in parts per million (ppm), obtained from analysis of air bubbles trapped in an ice core extracted from Law Dome in Antarctica. The data show a sharp rise in atmospheric CO2 starting in the late 19th century., coin cident with the sharp rise in CO2 emissions illustrated in Figure 6.1. Similar data from other ice cores indicate that CO2 levels remained between 260 and 285 ppm for the last 10,000 years. SOURCE: Etheridge et al. (1996).

FIGURE 6.3 CO2 variations during the last 1,000 years, in parts per million (ppm), obtained from analysis of air bubbles trapped in an ice core extracted from Law Dome in Antarctica. The data show a sharp rise in atmospheric CO2 starting in the late 19th century., coin cident with the sharp rise in CO2 emissions illustrated in Figure 6.1. Similar data from other ice cores indicate that CO2 levels remained between 260 and 285 ppm for the last 10,000 years. SOURCE: Etheridge et al. (1996).

than preindustrial conditions (usually taken as 280 ppm). As discussed in further detail in the next section, data from even longer ice cores extracted from the hearts of the Greenland and Antarctic ice sheets—the bottoms of which contain ice that was formed hundreds of thousands of years ago—indicate that the current CO2 levels are higher than they have been for at least 800,000 years.

Collectively, the in situ measurements of CO2 over the past several decades, ice core measurements showing a sharp rise in CO2 since the Industrial Revolution, and detailed estimates of CO2 sources and sinks provide compelling evidence that CO2 levels are increasing as a result of human activities. There is, however, an additional piece of evidence that makes the human origin of elevated CO2 virtually certain: measurements of the isotopic abundances of the CO2 molecules in the atmosphere—a chemical property that varies depending on the source of the CO2—indicate that most of the excess CO2 in the atmosphere originated from sources that are millions of years old. The only source of such large amounts of "fossil" carbon are coal, oil, and natural gas (Keeling et al., 2005).

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