IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Synthesis Report

2.1 Emissions of long-lived GHGs

The radiative forcing of the climate system is dominated by the long-lived GHGs, and this section considers those whose emissions are covered by the UNFCCC.

Global GHG emissions due to human activities have grown since pre-industrial times, with an increase of 70% between 1970 and 2004 (Figure 2.1).[5] {WGIII 1.3, SPM}

Global anthropogenic GHG emissions

Figure 2.1

Figure 2.1. (a) Global annual emissions of anthropogenic GHGs from 1970 to 2004.[5] (b) Share of different anthropogenic GHGs in total emissions in 2004 in terms of CO2-eq. (c) Share of different sectors in total anthropogenic GHG emissions in 2004 in terms of CO2-eq. (Forestry includes deforestation.) {WGIII Figures TS.1a, TS.1b, TS.2b}

Carbon dioxide (CO2) is the most important anthropogenic GHG. Its annual emissions have grown between 1970 and 2004 by about 80%, from 21 to 38 gigatonnes (Gt), and represented 77% of total anthropogenic GHG emissions in 2004 (Figure 2.1). The rate of growth of CO2-eq emissions was much higher during the recent 10-year period of 1995-2004 (0.92 GtCO2-eq per year) than during the previous period of 1970-1994 (0.43 GtCO2-eq per year). {WGIII 1.3, TS.1, SPM}

Carbon dioxide-equivalent (CO2-eq) emissions and concentrations

GHGs differ in their warming influence (radiative forcing) on the global climate system due to their different radiative properties and lifetimes in the atmosphere. These warming influences may be expressed through a common metric based on the radiative forcing of CO2.

  • CO2-equivalent emission is the amount of CO2 emission that would cause the same time-integrated radiative forcing, over a given time horizon, as an emitted amount of a long-lived GHG or a mixture of GHGs. The equivalent CO2 emission is obtained by multiplying the emission of a GHG by its Global Warming Potential (GWP) for the given time horizon.[6] For a mix of GHGs it is obtained by summing the equivalent CO2 emissions of each gas. Equivalent CO2 emission is a standard and useful metric for comparing emissions of different GHGs but does not imply the same climate change responses (see WGI 2.10).
  • CO2-equivalent concentration is the concentration of CO2 that would cause the same amount of radiative forcing as a given mixture of CO2 and other forcing components.[7]

The largest growth in GHG emissions between 1970 and 2004 has come from energy supply, transport and industry, while residential and commercial buildings, forestry (including deforestation) and agriculture sectors have been growing at a lower rate. The sectoral sources of GHGs in 2004 are considered in Figure 2.1c. {WGIII 1.3, SPM}

The effect on global emissions of the decrease in global energy intensity (-33%) during 1970 to 2004 has been smaller than the combined effect of global income growth (77%) and global population growth (69%); both drivers of increasing energy-related CO2 emissions. The long-term trend of declining CO2 emissions per unit of energy supplied reversed after 2000. {WGIII 1.3, Figure SPM.2, SPM}

Differences in per capita income, per capita emissions and energy intensity among countries remain significant. In 2004, UNFCCC Annex I countries held a 20% share in world population, produced 57% of the world’s Gross Domestic Product based on Purchasing Power Parity (GDPPPP) and accounted for 46% of global GHG emissions (Figure 2.2). {WGIII 1.3, SPM}

Regional distribution of GHG emissions by population and by GDPPPP

Figure 2.2Figure 2.2

Figure 2.2. (a) Distribution of regional per capita GHG emissions according to the population of different country groupings in 2004 (see appendix for definitions of country groupings). (b) Distribution of regional GHG emissions per US$ of GDPPPP over the GDP of different country groupings in 2004. The percentages in the bars in both panels indicate a region’s share in global GHG emissions. {WGIII Figures SPM.3a, SPM.3b}

  1. ^  Includes only carbon dioxide (CO2 ), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphurhexafluoride (SF6), whose emissions are covered by the UNFCCC. These GHGs are weighted by their 100-year Global Warming Potentials (GWPs), using values consistent with reporting under the UNFCCC.
  2. ^  This report uses 100-year GWPs and numerical values consistent with reporting under the UNFCCC.
  3. ^  Such values may consider only GHGs, or a combination of GHGs and aerosols.