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). {WGIII 1.3, SPM}
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. 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.
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}