IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group III: Mitigation of Climate Change Intensities

The Kaya identity (Kaya, 1990) is a decomposition that expresses the level of energy related CO2 emissions as the product of four indicators: (1) carbon intensity (CO2 emissions per unit of total primary energy supply (TPES)), (2) energy intensity (TPES per unit of GDP), (3) gross domestic product per capita (GDP/cap) and (4) population. The global average growth rate of CO2 emissions between 1970 and 2004 of 1.9% per year is the result of the following annual growth rates: population 1.6%, GDP/cap[12] 1.8%, energy-intensity of –1.2% and carbon-intensity –0.2% (Figure 1.5).

Figure 1.5

Figure 1.5: Intensities of energy use and CO2 emissions, 1970–2004.

Data Source: IEA data

A decomposition analysis according to the refined Laspeyeres index method (Sun, 1998; Sun and Ang, 2000) is shown in Figure 1.6. Each of the three stacked bars refers to 10-year periods and indicates how the net change in CO2 emissions of that decade can be attributed to the four indicators of the Kaya identity. These contributions – to tonnes of CO2 emissions – can be positive or negative, and their sum equals the net emission change (shown for each decade by the black line).

Figure 1.6

Figure 1.6: Decomposition of global energy-related CO2 emission changes at the global scale for three historical and three future decades.

Sources: IEA data World Energy Outlook 2006 (IEA, 2006a)

GDP/capita and population growth were the main drivers of the increase in global emissions during the last three decades of the 20th century. However, consistently declining energy intensities indicate structural changes in the global energy system. The role of carbon intensity in offsetting emission growth has been declining over the last two decades. The reduction in carbon intensity of energy supply was the strongest between 1980 and 1990 due to the delayed effect of the oil price shocks of the 1970s, and it approached zero towards the year 2000 and reversed after 2000 At the global scale, declining carbon and energy intensities have been unable to offset income effects and population growth and, consequently, carbon emissions have risen. Under the reference scenario of the International Energy Agency (IEA, 2006a) these trends are expected to remain valid until 2030; in particular, energy is not expected to be further decarbonized under this baseline scenario.

Of the major countries and groups of countries – North America, Western Europe, Japan, China, India, Brazil, Transition Economies – only the Transition Economies (refers to 1993–2003 only) and, to a lesser extent, the group of the EU15 have reduced their CO2 emissions in absolute terms.

The decline of the carbon content of energy (CO2/TPES) was the highest in Western Europe, but the effect led only to a slight reduction of CO2 in absolute terms. Together with Western Europe and the Transition Countries, USA/Canada, Japan and – to a much lesser extent – Brazil have also reduced their carbon intensity.

Declining energy intensities observed in China and India have been partially offset by increasing carbon intensities (CO2/TPES) in these countries. It appears that rising carbon intensities accompany the early stages of the industrialization process, which is closely linked to accelerated electricity generation mainly based on fossil fuels (primarily coal). In addition, the emerging but rapidly growing transport sector is fuelled by oil, which further contributes to increasing carbon intensities. Stepped-up fossil fuel use, GDP/capita growth and, to a lesser extent, population growth have resulted in the dramatic increase in carbon emissions in India and China.

The Transition Economies of Eastern Europe and the former Soviet Union suffered declining per capita incomes during the 1990s as a result of their contracting economies and, concurrently, total GHG emissions were greatly reduced. However, the continued low level of energy efficiency in using coal, oil and gas has allowed only moderate improvements in carbon and energy intensities. Despite the economic decline during the 1990s, this group of countries accounted for 12% of global CO2 emissions in 2003 (Marland et al., 2006).

The challenge – an absolute reduction of global GHG emissions – is daunting. It presupposes a reduction of energy and carbon intensities at a faster rate than income and population growth taken together. Admittedly, there are many possible combinations of the four Kaya identity components, but with the scope and legitimacy of population control subject to ongoing debate, the remaining two technology-oriented factors, energy and carbon intensities, have to bear the main burden.

  1. ^  Purchasing power parity (PPP) at 2000 prices and exchange rates.