Energy-related carbon dioxide emissions

Overview

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Because anthropogenic emissions of carbon dioxide result primarily from the combustion of fossil fuels, energy consumption is at the center of the climate change debate. In the IEO2011 Reference case, world energy-related carbon dioxide emissions increase from 30.2 billion metric tons in 2008 to 35.2 billion metric tons in 2020 and 43.2 billion metric tons in 2035. Much of the growth in emissions is attributed to developing, non-OECD nations that continue to rely heavily on fossil fuels to meet fast-paced growth in energy demand. Non-OECD emissions total 28.9 billion metric tons in 2035, or about 73 percent above the 2008 level. In comparison, OECD emissions total 14.3 billion metric tons in 2035—only about 6 percent above the level in 2008 (Figure 110).

High world oil prices in 2008, compounded by the 2008-2009 global recession, resulted in a decrease in fossil fuel consumption for the OECD regions, with carbon dioxide emissions declining by 2.0 percent in 2008 and an estimated 6.3 percent in 2009. Non-OECD emissions, however, continued to increase in 2008 and 2009. As a result, total world emissions increased by 2.2 percent in 2008 and an estimated 0.3 percent in 2009. In the IEO2011 Reference case, the non-OECD share of overall energy-related carbon dioxide emissions increases from 55 percent in 2008 to about 67 percent in 2035.

The IEO2011 Reference case projections are, to the extent possible, based on existing laws and policies. Projections for carbon dioxide emissions may change significantly if laws and policies aimed at reducing greenhouse gas emissions are changed or new ones are introduced. Many countries have submitted emission reduction goals under the United Nations Framework Convention on Climate Change in conjunction with the Conference of Parties meetings in Copenhagen and Cancun (see section on "The Copenhagen Accord"); however, those goals are not considered in the IEO2011 Reference case. In addition, beyond energy-related carbon dioxide there are other gases (e.g., methane) and sources (e.g., deforestation) that contribute to greenhouse gas emissions. Other sources are not considered in IEO2011, but they could have significant impacts on national or regional shares of total global greenhouse gas emissions.

Emissions by fuel

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Worldwide energy-related carbon dioxide emissions from the use of liquid fuels, natural gas, and coal all increase in the Reference case projection, but the relative contributions of the individual fuels shift over time (Figure 111). Carbon dioxide emissions associated with the consumption of liquids accounted for the largest portion (43 percent) of global emissions in 1990. The liquids share fell to 37 percent in 2008, and it continues declining in the Reference case to 33 percent in 2035. The coal share follows an inverse pattern, accounting for 39 percent of total emissions in 1990, 43 percent in 2008, and 45 percent in 2035 in the IEO2011 Reference case. Coal, the most carbon-intensive fossil fuel, became the leading source of world energy-related carbon dioxide emissions in 2004 and remains the leading source through 2035. The natural gas share of carbon dioxide emissions remains relatively small by comparison, at 19 percent of the total in 1990 and a projected 21 percent of the total in 2035.

Global carbon dioxide emissions from coal use show the largest absolute increase in the Reference case, from 13.0 billion metric tons in 2008 to 19.6 billion metric tons in 2035. Coal is the largest contributor to emissions growth in the non-OECD economies, accounting for 54 percent of the projected non-OECD increase in total energy-related emissions. World coal-related carbon dioxide emissions grow at an average annual rate of 1.5 percent over the 27-year projection period, and the non-OECD countries account for nearly all of the increase. Although there is virtually no change in OECD coal-related emissions from 2008 to 2035, non-OECD coal-related emissions increase by 76 percent over the period. The world's top three national sources of coal-related emissions are China, the United States, and India, which remain at the top throughout the projection and in combination account for three-quarters of world coal-related carbon dioxide emissions in 2035.

In percentage terms, natural gas is the world's fastest-growing fossil fuel in the IEO2011 Reference case and, as a result, is also the world's fastest-growing source of energy-related carbon dioxide emissions. Nevertheless, emissions from natural gas combustion on a worldwide basis remain much smaller than emissions from combustion of coal or liquids. At an average annual growth rate of 1.6 percent, emissions from natural gas increase by more than 50 percent from 2008 to 2035 (Figure 112). Emissions from natural gas use account for about 90 percent of the projected increase in total OECD emissions. In contrast, only about 20 percent of the growth in total non-OECD emissions is expected to come from natural gas. Although non-OECD emissions from natural gas use grow by 79 percent from 2008 to 2035, the large absolute increases in non-OECD coal- and liquids-related emissions exceed the increase associated with natural gas.

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Carbon dioxide emissions from the consumption of liquids worldwide show the slowest growth over the projection period, at an average annual rate of 1.0 percent—a comparatively low growth rate that still results in an absolute increase of 3.3 billion metric tons of liquids-related carbon dioxide emissions from 2008 to 2035. As in the case of coal and natural gas, increases in liquids-related emissions are not distributed evenly across regions. In the OECD countries, liquids-related carbon dioxide emissions in increase on average by less than 0.1 percent per year. In the non-OECD countries, rising demand for transportation and industrial uses of liquids contributes to a much higher growth rate of 1.8 percent per year. As a result, the OECD share of carbon dioxide emissions from liquids declines from 55 percent in 2008 to 43 percent in 2035.

Emissions by region

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World energy-related carbon dioxide emissions increase at an average annual rate of 1.3 percent from 2008 to 2035 in the IEO2011 reference case. OECD emissions increase by only 0.2 percent per year on average, but non-OECD emissions increase at 10 times that rate (Figures 113 and 114). OECD emissions fell in 2008 and in 2009—primarily because of the global recession and high oil prices in 2008. In the IEO2011 Reference case, OECD carbon dioxide emissions do not return to 2008 levels until after 2020.

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Among the OECD countries, Mexico/Chile and South Korea have the highest projected growth rates for energy-related carbon dioxide emissions, at 1.7 percent and 1.0 percent per year, respectively (Figure 113). From a combined 8 percent in 2008, their share of total OECD emissions increases to 10 percent in 2035. The two regions also have the highest projected rates of economic growth in the OECD over the period, with Mexico/Chile's GDP increasing by 3.7 percent per year in the IEO2011 Reference case and South Korea's by 2.9 percent per year. For the OECD region as a whole GDP growth averages 2.1 percent per year.

In Japan and OECD Europe, carbon dioxide emissions decline from 2008 to 2035. Japan emits 1.1 billion metric tons of energy-related carbon dioxide in 2035, or 11 percent less than in 2008. OECD Europe's emissions decline by 0.1 percent per year over the projection period; Japan's by 0.4 percent per year. In 2035, OECD Europe accounts for less than 10 percent of world emissions, as compared with about 14 percent in 2008.

Among the OECD regions, the United States continues to be the largest source of energy-related carbon dioxide emissions through 2035. U.S. emissions grow by an average of 0.3 percent per year—a lower rate of growth than in Mexico/Chile, South Korea, Australia/New Zealand, and Canada. However, in terms of absolute increases across the OECD regions, the United States contributes the most additional metric tons of energy-related carbon dioxide emissions in 2035 compared with 2008 levels. Still, the U.S. share of world emissions falls from 19 percent in 2008 to 15 percent in 2035, as fast-paced expansion of fossil fuel use, and thus energy-related carbon dioxide emissions, in non-OECD countries displaces the U.S. share.

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Non-OECD Asia accounts for about 74 percent of the growth in world carbon dioxide emissions from 2008 to 2035. China's emissions grow by an average of 2.6 percent per year (Figure 114) and account for more than two-thirds of the total increase in non-OECD Asia's emissions. India's carbon dioxide emissions increase by 2.7 percent per year, and emissions in the rest of non-OECD Asia increase by an average of 2.1 percent per year. In 2035, India is the fifth-highest emitter among the IEO2011 regions, following China, the United States, OECD Europe, and "other" non-OECD Asia (i.e., non-OECD Asia excluding China and India). Emissions increases in non-OECD Asia, particularly China, are led by coal-related carbon dioxide emissions—but emissions from natural gas and liquids use also increase substantially (Figure 115).

Non-OECD Europe and Eurasia exhibits the slowest growth in carbon dioxide emissions among the non-OECD regions, at 0.2 percent per year in the IEO2011 Reference case. Natural gas is the region's leading source of fuel emissions, accounting for 54 percent of total carbon dioxide emissions in Russia in 2008 and 39 percent in the other non-OECD Europe and Eurasia nations. Total carbon dioxide emissions in non-OECD Europe and Eurasia increase only slightly, from 2.8 billion metric tons in 2008 to 3.0 billion metric tons in 2035, in part because of Russia's projected population decline and increasing reliance on nuclear power to meet electricity demand in the future. Natural gas continues to be the region's leading source of energy-related carbon dioxide emissions throughout the projection, accounting for nearly 50 percent of total energy-related emissions in 2035.

Cumulative carbon dioxide emissions

The IEO2011 Reference case projects about 1 trillion metric tons of additional cumulative energy-related carbon dioxide emissions between 2009 and 2035. The pace of carbon dioxide emissions growth slows in the IEO2011 Reference case during the last 15 years of the projection period (2021-2035) in comparison with the previous 15 years (2006-2020). In the period from 2021 to 2035, cumulative emissions are 22 percent higher than those in the period from 2006 to 2020 (including historical emission years 2006-2008), and emissions in the period from 2006 to 2020 are 38 percent higher than the total from 1991 to 2005.

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Non-OECD Asia is the dominant source of cumulative emissions growth in the 30 years preceding 2035 (Figure 116). In the last 15 years of the projection, cumulative emissions from non-OECD Asia are 44 percent of total cumulative emissions, up from a 38-percent share between 2006 and 2020 and a 23-percent share between 1991 and 2005. Cumulative emissions from the OECD Americas region also grow between 1991 and 2035, but in the 15 years ending in 2035, they are 13 percent higher than those from the earliest period shown in Figure 116 (1991-2005). In contrast, non-OECD Asia's cumulative emissions from 2021 to 2035 are more than three times its cumulative emissions between 1991 and 2005. The second-largest increase, after non-OECD Asia, is projected for the Middle East; but because it is starting from a much smaller total, its contribution to cumulative emissions between 2021 and 2035 remains small, at 6 percent of the total for the period.

Factors influencing trends in energy-related carbon dioxide emissions

There are many factors that influence a country's level of carbon dioxide emissions. Two key measures provide useful insights for the analysis of trends in energy-related emissions:

• The carbon intensity of energy supply is a measure of the amount of carbon dioxide associated with each unit of energy used. It directly links changes in carbon dioxide emissions levels with changes in energy usage. Carbon emissions vary by energy source, with coal being the most carbon-intensive fuel, followed by oil and natural gas. Nuclear power and some renewable energy sources (i.e., solar and wind) do not directly generate carbon dioxide emissions. Consequently, changes in the fuel mix therefore alter overall carbon intensity. Over time, declining carbon intensity can offset increasing energy consumption to some extent. If energy consumption increased and carbon intensity declined by a proportional factor, carbon dioxide emissions would remain constant. A decline in carbon intensity can indicate a shift away from fossil fuels, a shift towards less carbon-intensive fossil fuels, or both.
• The energy intensity of economic activity is a measure of energy consumption per unit of economic activity, as measured by GDP. It relates changes in energy consumption to economic output. Increased energy use and economic growth generally occur together, although the degree to which they are linked varies across regions and stages of economic development.

As with carbon intensity, regional energy intensities do not necessarily remain constant over time. Energy intensity can be indicative of the energy efficiency of an economy's capital stock (vehicles, appliances, manufacturing equipment, power plants, etc.). For example, if an old power plant is replaced with a more thermally efficient unit, then it is possible to meet the same level of electricity demand with a lower level of primary energy consumption, thereby decreasing energy intensity.

Energy intensity is acutely affected by structural changes within an economy—in particular, the relative shares of its output sectors (manufacturing versus service, for example) or its trade balance. Higher concentrations of energy-intensive industries, such as oil and gas extraction, will yield higher overall energy intensities, whereas countries with proportionately larger service sectors will tend to have lower energy intensities. For example, the Middle East had a relatively high energy intensity of 10.6 thousand Btu per dollar of GDP in 2008, in part because of the important role played by hydrocarbon production and exports in most Middle East economies.

When carbon intensity and energy intensity components are multiplied together, the resulting measure is carbon dioxide emissions per dollar of GDP (CO2/GDP)—that is, the carbon intensity of economic output. Carbon intensity of output is another common measure used in analysis of changes in carbon dioxide emissions, and it is sometimes used as a standalone measure for tracking progress in relative emissions reductions. However, when the goal is to determine the relative strengths of forces driving changes in carbon intensity, disaggregation of the components helps to determine whether a change in carbon intensity is the result of a change in the country's fuel mix or a change in the relative energy intensity of its economic activity.

The Kaya decomposition of emissions trends

The Kaya Identity provides an intuitive approach to the interpretation of historical trends and future projections of energy-related carbon dioxide emissions. It can be used to decompose total carbon dioxide emissions as the product of individual factors that explicitly link energy-related carbon dioxide emissions to energy consumption, the level of economic output as measured by gross domestic product (GDP), and population size.

The Kaya Identity expresses total carbon dioxide emissions as the product of (1) carbon intensity of energy supply (CO2/E), (2) energy intensity of economic activity (E/GDP), (3) economic output per capita, and (4) population:

CO2 = (CO2/E) (E/GDP) x (GDP/POP) x POP .

Using 2008 data as an example, world energy-related carbon dioxide emissions totaled 30.2 billion metric tons in that year, world energy consumption totaled 505 quadrillion Btu, world GDP totaled $65.8 trillion, and the total world population was 6,731 million. Using those figures in the Kaya equation yields the following: 59.8 metric tons carbon dioxide per billion Btu of energy (CO2/E), 7.7 thousand Btu of energy per dollar of GDP (E/GDP), and $9,773 of income per person (GDP/POP). Appendix H delineates the Kaya factors for all IEO regions over the projection period.

Of the four Kaya components, policymakers generally focus on the energy intensity of economic output (E/GDP) and carbon dioxide intensity of the energy supply (CO2/E). Reducing growth in per-capita output may have a mitigating influence on emissions, but governments generally pursue policies to increase rather than reduce output per capita in order to advance objectives other than greenhouse gas mitigation.

Policies related to energy intensity of GDP typically involve improvements to energy efficiency. However, the measure is also sensitive to shifts in the energy-intensive portion of a country's trade balance, and improvements may simply reflect a greater reliance on imports of manufactured goods, which may decrease one country's energy intensity but, if the country producing the imported goods is less energy efficient, could lead to a worldwide increase in energy consumption and related carbon dioxide emissions. Policies related to the carbon dioxide intensity of energy supply typically focus on promotion of low-carbon or zero-carbon sources of energy.

Conveniently, the percentage rate of change in carbon dioxide emission levels over time approximates the sum of the percentage rate of change across the four Kaya components. Table 18 shows the average rate of change of total carbon dioxide emissions and each individual Kaya component from 2008 to 2035 in the IEO2011 Reference case. The most significant driver of positive growth in energy-related carbon dioxide emissions is economic output per capita. The average annual growth rate of output per capita for non-OECD countries (3.6 percent from 2008 to 2035) in particular dominates all other Kaya components in the 27-year projection. For OECD countries, on the other hand, the 1.7-percent average annual increase in output per capita is nearly offset by the 1.5-percent annual decline in energy intensity.

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Except for Japan and Russia—where population is expected to decline from 2008 to 2035—population growth is also an important determinant of emissions increases. However, the population effect is less pronounced than the effect of output per capita (Figure 117). For non-OECD countries, increases in output per capita coupled with population growth overwhelm the improvements in energy intensity and carbon intensity. Although the same was true for the OECD countries over the period from 1990 to 2008, the projection horizon shows OECD growth in output per capita and population balanced by improvements in energy intensity and carbon intensity (Figure 118).

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Over the 2008-2035 projection period, energy intensity of economic output declines in all the IEO2011 regions. The trend is particularly pronounced in the non-OECD countries, where energy intensity of output decreases on average by 2.2 percent per year, compared with 1.5 percent per year in the OECD countries. Worldwide, the most significant decline in energy intensity of output is projected for China, at 2.6 percent per year. However, that decline is offset by the projected increase in China's output per capita, which grows by an average of 5.4 percent per year over the same period.

Carbon intensity of energy supply is also projected to decline in all the IEO2011 regions from 2008 to 2035, but to a lesser extent. The most significant declines in carbon intensity of energy supply are projected for Japan and OECD Europe, with annual decreases averaging 0.6 percent per year for both regions. Decreases in the consumption of liquids and coal (the most carbon-intensive fuels) in both regions, combined with increases in consumption of renewable energy, nuclear power, and natural gas, reduce the carbon intensity of the energy supply. For the OECD region as a whole, the average rate of decline in carbon intensity of energy supply over the 2008-2035 period is 0.4 percent per year, exceeding the non-OECD average of 0.2 percent per year. Still, the projected decrease in non-OECD carbon intensity would mark a departure from the 1990-2008 historical trend, when non-OECD carbon intensity remained nearly constant.