|The Global Greenhouse Regime. Who Pays? (UNU, 1993, 382 p.)|
|Part I Measuring responsibility|
|3 Assessing emissions: five approaches compared|
Cumulative CO2, energy only
As it takes many decades for CO2 to be removed from the atmosphere, the increase in concentration of CO2 from pre-industrial levels is largely due to CO2 emitted in past decades. In this respect, historical CO2 emissions are much more relevant to the level of committed atmospheric warming than are current emissions. Emissions from past energy use, however, make up a smaller portion of total CO2 release than today's fossil fuel emissions because CO2 from land clearing may have been roughly comparable with fossil fuel related CO2 until the middle of this century (see Figure 3.1). Emissions from fossil fuel combustion since the start of the industrial revolution are estimated to be 175 to 215 gigatonnes (GT) of carbon (C), representing between about 55 and 70 per cent of total anthropogenic CO2 release (IPCC 1990). Contributions to warming, however, are considerably lower because CO2 is but one of the gases contributing to the heating effect. Considering both the change in fossil fuel emissions over time (Keeling 1973; Marland et al. 1990) and the estimated contribution of CO2 to total warming (IPCC 1990), it is calculated that cumulative CO2 emissions from energy may contribute about 40 per cent of the warming effect of the trace gases now in the atmosphere.
Figure 3.1 Historic CO2 emissions compared
Cumulative CO2, energy and biota
About 60 per cent of the warming effect of anthropogenic greenhouse gas emissions in the atmosphere is thought to be from CO2 or cumulative CO2 emissions from energy and biota (IPCC 1990).
CO2, energy only (current)
Although current CO2 emissions from energy is the least comprehensive of the five categories considered here, CO2 from energy alone is the major warming source from current emissions. Emissions from energy consumption contribute about 65 per cent of the expected warming effect of the trace gases now being emitted, if halocarbons are excluded from the total. This CO2 share of warming reflects the use of the IPCC's GWP for a 100 year period, published in 1992 (IPCC 1992). The estimated warming is only 60 per cent of the total if the IPCC's 1990 GWPs are employed and no doubt will change as the IPCC revises in the future.
Partial CH4 and CO2
This category, as defined above, covers about 80 per cent of the warming effect of current greenhouse gas emissions excluding halocarbons. Halocarbons have been omitted from the current emissions total because they are already being phased out under the Montreal Protocol.
The comprehensive approach to emission measurement theoretically represents 100 per cent of current greenhouse gas emissions.
The influence of time horizon
The relative comprehensiveness of the energy and modified comprehensive approaches varies considerably depending on how much of the heating effect of the different gases is taken into account. In Figure 3.2, current emissions are compared using three CO2 equivalence indices including the potential heating effect of the gases over a 20, 100, and 500 year time horizon. In the 20 year time horizon, CO2 from energy contributes only about 45 per cent of the total heating effect because the index based on the shorter time horizon does not capture the ultimate heating effect of CO2, which continues many decades beyond the atmospheric residence time of CH4, the next most important greenhouse gas. Accordingly, the proportion of the total heating contribution due to CO2 from energy is much higher about 70 per cent - over the longer time horizon.
Figure 3.2 Contributions to total emissions by source