Introduction and Overview

The concentration of greenhouse gases, including CO2 and methane (CH4), in the atmosphere has been steadily rising since the beginning of the Industrial Revolution (Etheridge et al., 1996, 2002; NRC, 2010). Anthropogenic CO2 emissions from the combustion of fossil fuels have been the main contributor to rising CO2-concentration levels in the atmosphere, followed by CO2 emissions from land use, land use change, and forestry (LULUCF).

Chapter 5 analyzes the anthropogenic greenhouse gas (GHG)-emission trends until the present and the main drivers that explain those trends. This chapter serves as a reference for assessing, in following chapters, the potential future emissions paths, and mitigation measures.

For a systematic assessment of the main drivers of GHG-emission trends, this and subsequent chapters employ a decomposition analysis based on the IPAT and Kaya identities (see Box 5.1).

Chapter 5 first considers the immediate drivers, or factors in the decomposition, of total GHG emissions. For energy, the factors are population, gross domestic product (GDP) (production) and gross national expenditure (GNE) (expenditures) per capita, energy intensity of production and expenditures, and GHG-emissions intensity of energy. For other sectors, the last two factors are combined into GHG-emissions intensity of production or expenditures. Secondly, it considers the underlying drivers defined as the processes, mechanisms, and characteristics of society that influence emissions through the factors, such as fossil fuels endowment and availability, consumption patterns, structural and technological changes, and behavioural choices.

Underlying drivers are subject to policies and measures that can be applied to, and act upon them. Changes in these underlying drivers, in turn, induce changes in the immediate drivers and, eventually, in the GHG-emissions trends.

The effect of immediate drivers on GHG emissions can be quantified through a straight decomposition analysis; the effect of underlying drivers on immediate drivers, however, is not straightforward and, for that reason, difficult to quantify in terms of their ultimate effects on GHG emissions. In addition, sometimes immediate drivers may affect underlying drivers in a reverse direction. Policies and measures in turn affect these interactions. Figure 5.1 reflects the interconnections among GHG emissions, immediate drivers, underlying drivers, and policies and measures as well as the interactions across these three groups through the dotted lines.