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is devoted to exploring this problem and its possible solutions, as well as to the nature and properties of fossil fuels, namely coal, petroleum, and natural gas. Carbon dioxide is produced when any carbon-containing substance undergoes complete combustion:

As we saw in Chapter 6, C02 is an important greenhouse gas, and the increase in its atmospheric concentration is responsible for the largest fraction of global warming of any anthropogenic factor.

Developed countries have accounted for about three-quarters of all carbon dioxide emissions from fossil-fuel combustion and cement manufacture since the beginning of the Industrial Revolution. The emissions from these sources from various countries in the more recent period (1980-2004) are illustrated by the bands in Figure 7-1. Notice that:

• The United States was the biggest emitter country, though recent data indicate China overtook it in 2007.

• The emission from the developed countries, taken together and defined by the top of the band for "Other developed countries," has been rising slowly, as has their total energy usage, discussed previously, and now amounts to about 60% of the total.

• Emissions from the countries of the former Soviet Union decreased significantly in the early 1990s, after the fall of communism; the decrease in the

FIGURE 7-1 C02 emissions from fossil-fuel combustion for different countries and regions since 1 980. [Source: M. Raupach et a I., Proceedings of the National Academy of Sciences 104 (2007): 10288.]

8000

5000

3000

5000

3000

2004

1990s roughly matched the increase in emissions from developing countries over that period.

• Emissions from China have risen sharply recently, with a spike that began in 2002.

Almost all the increase in C02 emissions discussed above arose from increases in usage of energy derived from fossil fuels. However, the ratio of carbon dioxide to energy varies among countries and over time because of differences in the fraction of energy produced by combustion and, as discussed later in the chapter, because different fossil fuels produce very different amounts of the gas per joule. Because of the importance of C02, the term carbon intensity, defined as the ratio of C02 emissions per dollar of GDP, has become widely discussed by policymakers.

The global carbon intensity declined over the last half of the twentieth century, especially over its last two decades, when it declined by about one-quarter. However, as the solid green line in Figure 7-2 indicates, the global intensity remained approximately constant from 2000 to at least 2005. Currently, the production of one dollar's worth of goods and services results, on average, in the emission of about 730 g of carbon dioxide into the atmosphere. Carbon intensities are usually expressed as the carbon content alone of the emitted C02, or about 200 g of carbon per dollar in the present case. (Notice that the curves in Figure 7-2 are not absolute, but relative to their year 2000 values.)

Year

FIGURE 7-2 Global C02 emissions (heavy black curve; includes both fossil-fuel combustion and cement production) and important factors that influence it. Dashed green curve = population; thin black curve = per capita GDP; solid green curve = carbon intensity of GDP; heavy black curve = C02 emissions. Note that all parameters are normalized to year 2000 values. [Source: M. Raupach (Global Carbon Project), Carbon in the Earth System: Dynamics and Vulnerabilities, Beijing, November, 2006.1

PROBLEM 7-2

Show that the carbon content of 730 g of C02 is 199 g.

Although they had very similar carbon intensity values in the early 2000s, the intensities for the world's two largest economies—the United States and China—evolved quite differently over time, as illustrated by the solid green curves in Figures 7-3a and 7-3b. American carbon intensity has fallen gradually and almost continuously, whereas that of China fell precipitously once major industrialization began, then reached a minimum in about 2000 and rose significantly at least in the half-decade that followed. Presumably, much of the decrease in the U.S. intensity is due to its continuing conversion from a manufacturing to a knowledge economy, whereas the increase in China arises from its development as an energy-intensive economy that produces large quantities of manufactured goods.

FIGURE 7-3 C02 emissions and important factors that influence it, from

(a) the United States and

(b) China. See Figure 7-2 for curve identification. [Source: M. Raupach (Global Carbon Project), Carbon in the Earth System: Dynamics and Vulnerabilities, Beijing, November, 2006.)

FIGURE 7-3 C02 emissions and important factors that influence it, from

(a) the United States and

(b) China. See Figure 7-2 for curve identification. [Source: M. Raupach (Global Carbon Project), Carbon in the Earth System: Dynamics and Vulnerabilities, Beijing, November, 2006.)

The driving forces behind the changes over the last few decades in global and regional CO? emissions can be understood by considering the various curves in Figures 7-2 and 7-3; all factors in these graphs are normalized to their year 2000 values. The continuous rise in world population over time is indicated by the dashed green line in Figure 7-2. As the world economy developed, the average GDP per person also rose, as shown by the thin black curve. The product of these two quantities is the global GDP, which rose in a faster-than-linear fashion (not shown) since each of these factors was rising more or less linearly with time. However, in the period until about 2000, the carbon intensity of the global GDP (solid green line in Figure 7-2) declined almost linearly with time, so the total carbon dioxide emission rate—which is a product of the three factors—rose only gradually and more or less linearly in that period (heavy black curve in Figure 7-2). From 2000 to 2005 at least, since the carbon intensity did not fall, the emission rate of carbon dioxide rose dramatically.

COz emission rate = population X per capita GDP X carbon intensity

The corresponding emission curve for the United States (Figure 7-3a) involves the same combination of factors, with post-2000 emissions steady due to the continuing decline in carbon intensity. In contrast, the continuing strong rise in per capita GDP in China, combined with the increasing carbon intensity, has produced sharply increasing emissions (Figure 7-3b).

Another way of measuring the carbon dioxide emissions from different countries is to consider the per capita releases of this gas into the atmosphere. Currently the emissions of carbon dioxide amount to about 4 tonnes per person per year when averaged over the global population; this factor is usually expressed as 1 tonne of carbon, and it is this reference to carbon that will be used henceforth. The global average carbon emission remained remarkably steady, at about 1.1 tonnes, since rising to this value in the early 1970s, although it has increased slightly in recent years.

People in developed countries have much higher annual average emissions than do those in developing countries: 3 versus 0.5 tonnes of carbon per person. The United States leads in both total and per capita C02 emissions, according to the bar graph in Figure 7-4, where we have listed in order the top 20 carbon dioxide emitter countries as of 2003. The black bars indicate the country's percentage of total global emissions, and the green bars show the per capita emissions. Notice that, compared to the compact European countries and Japan, the United States, Canada, and Australia have the highest per capita C02 emission rates, in part due to the high transportation requirements of these vast lands. It is also true that, in these three countries, fossil-fuel energy is much cheaper than in European countries. The other developed countries have remarkably similar per capita annual carbon emissions—about 2 tonnes— which perhaps is the value currently developing countries will reach once they are fully developed. The per capita carbon emissions from the United States, the European Union (EU), and the world in total remained remarkably

FIGURE 7-4 Total (black bars) and per capita (green bars) emissions of carbon dioxide by top 20 emitter countries in 2003. I Source: Data from Carbon Dioxide Information Analysis Center.]

FIGURE 7-4 Total (black bars) and per capita (green bars) emissions of carbon dioxide by top 20 emitter countries in 2003. I Source: Data from Carbon Dioxide Information Analysis Center.]

constant over the last few decades of the twentieth century, the increases in GNP being matched by decreases in intensity of both energy and carbon.

Because populations of different countries vary so much, their greenhouse gas emissions per capita or per dollar of GNP are no guide to their total emissions. Thus in Figure 7-4, we see that China and India make substantial contributions to the total global emissions since their populations are so large, even though their per capita emission rates are still quite modest. Both countries generate most of their electricity by burning coal. China's total C02 emissions since the turn of the millennium have exceeded even its rapid growth of the 1980s and early 1990s. India's emissions have been growing almost linearly, currently by about 3-4% annually since 1980, though it still has the lowest per capita emissions of any of the top 20 emitter countries.

Patterns of Growth in C02 Concentrations

Because carbon dioxide has such a long lifetime in the atmosphere—a century or more on average—the gas accumulates in air. Thus almost all the C02 emissions from the 1990s, for example, that did not find a temporary sink will remain in the air for decades to come, adding to the bulk of the emissions from the 1980s, 1970s, and previous years. The actual carbon dioxide molecules that constitute this additional mass will change from year to year, as some C02 molecules leave the temporary sinks and enter the atmosphere while an equal number from the air enter one or another temporary sink.

Linearly increasing concentration,

Linearly increasing concentration,

Constant C02 emissions

Sharply increasing^ concentration

Sharply increasing^ concentration

FIGURE 7-5 C02 concentration related to its emission level, (a) Constant emissions of C02 produce a linearly increasing concentration of the gas. (b) Linearly increasing emissions produce a quadratically increasing concentration of the gas.

FIGURE 7-5 C02 concentration related to its emission level, (a) Constant emissions of C02 produce a linearly increasing concentration of the gas. (b) Linearly increasing emissions produce a quadratically increasing concentration of the gas.

The growth pattern of the C02 concentration in air is determined mainly by the pattern of C02 emissions. Suppose, for example, that the same amount of carbon dioxide emissions were added to the air each year and did not find a temporary sink (Figure 7-5a, black line). The total amount of C02 in air—and hence its concentration—would then annually increase by a constant amount. The carbon dioxide concentration increases linearly with time in this case (Figure 7-5a, green line). If the world could hold its carbon dioxide emissions constant at the year 2000 value, then the C02 concentration would presumably increase linearly, by the current 2-ppm yearly increment, and would consequently become about 500 ppm in 2060. Currently, global C02 emissions are growing slowly, increasing by only about 1% annually (see Figure 6-8), so the atmospheric carbon dioxide concentration is growing almost linearly (Figure 6-8 inset).

Another scenario, which at times has been more realistic than the situation just described, is that the C02 emissions were not the same each year but themselves increased linearly, that is, by a constant amount k each year. Thus, if the emissions one year amounted to A, the next year they were A + k, and the following year A + 2k, etc. (Figure 7-5b, black line). In this case, if the fraction of the gas that enters the oceanic sink each year is constant, the growth in C02 concentration will be quadratic, much sharper than linear: The resulting plot of C02 concentration curves upward, as illustrated in Figure 7-5b (green curve). Indeed, in the decades preceding the mid-1970s, the C02 concentration (Figure 6-8 inset) did increase quadratically since carbon dioxide emissions were increasing rapidly (Figure 6-9). However, since then, the C02 concentration has increased in an approximately linear manner with time, reflecting the slower rate of increase in emissions in this period, among other factors—-including the fact that the fraction of emissions that find a temporary sink varies with time (as was discussed in Chapter 6).

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