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Stabilization of Greeabouse Gases

4.5 All relevant greenhouse gases need to be considered in addressing stabilisation of greenhouse gas concentrations. First carbon dioxide is considered which, because of its importance and complicated behaviour, needs more detailed consideration than the other greenhouse gases.

Carbon dioxide

4.6 Carbon dioxide is removed from the atmosphere by a number of processes that operate on different timescales. It has a relatively long residence time in the climate system - of the order of a century or more. If net global anthropogenic emissions'' (i.e. anthropogenic sources minus anthropogenic sinks) were maintained at current levels (about 7 GtClyr including emissions from fossil fuel combustion, cement production and land-use change), they would lead to a nearly constant rate of increase in atmospheric concentrations for at least two centuries, reaching about 500 ppmv (approaching twice the pre-industrial concentration of 280 ppmv) by the end of the 21st century. Carbon cycle models show that immediate stabilisation of the concentration of carbon dioxide at its present level could only be achieved through an immediate reduction in its emissions of 50-70% and further reductions thereafter.

4.7

Carbon cycle models bave been used to estimate profiles of carbon dioxide emissions for stabilization at various carbon dioxide concentration levels. Such profiles have been generated for an illustrative set of levels: 450, 550, 650, 750 and 1000 ppmv. Among the many possible pathways to reach stabilization, two are illustrated in Figure 1 for each of the stabilization levels of 450, 550, 650 and 750 ppmv, and one for 1000 ppmv. The steeper the increase in the emissions (hence concentration) in these scenarios, the more quickly is the climate projected to change.

4.8

Any eventual stabilised concentration is governed more by the accumulated anthropogenic carbon dioxide emissions from now until the time of stabilisation, than by the way those emissions change over the period. This means that, for a given stabilised concentration value, higher emissions in early decades require lower emissions later on. Cumulative emissions from 1991 to 2100 corresponding to these stabilization levels are shown in Table 1, together with the cumulative emissions of carbon dioxide for all of the IPCC IS92 emission scenarios (see Figure 2 below and Table SPM-1 in the Summary for Policymakers of IPCC Working Group I for details of these scenarios).

4.9 Figure 1 and Table I are presented to clarify some of the constraints that would be imposed on future carbon dioxide emissions, if stabilization at the concentration levels illustrated were to be achieved. These examples do not represent any form of recommendation about how such stabilization levels might be achieved or the level of stabilization which might be chosen.

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For the remainder of Section 4. net global anthropogenic emissions“ (i.e. anthropogenic sources minus anthropogenic sinks) will be abbreviated to emissions".

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Figure 1 (a) Carbon dioxide concentration profiles leading to stabilisation a 450, SSO, 650 and 750 ppray following the part ways defined in IPCC (1994) (solid curves) and for pathways that allow emissions to follow IS92a until at least the year 2000 (dashed curves). A single profile that stabilises ar a carbon dioxide concentration of 1000 ppmv and follows (S922 emissions undil at least the year 2000 has also been defined. Stabilisation at concentrations of 450, 650 and 1000 ppmv would lead to equilibrium temperature increases relative to 1990" due to carbon dioxide alone (i.e. not including effects of other GHGs and aerosols) of about 1°C (range: 0.5 to 1.5°C); 2°C (range: 1.3 to 4*C) and 3.5 °C (range: 2 60 7°C) respectively. A doubling of the pre-industrial carbon dioxide concentration of 280 ppmv would lead to a concentration of S60 ppmv and doubling of the current concentration of 358 ppmv would lead to a concentration of about

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T20 ppmv.

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These numbers do not take into account the increase in temperature (0.1 to 0.7 °C) which would

Table 1 Total anthropogenic carbon dioxide emissions accumulated from 1991 to 2100 inclusive (GC) for the IS92 scenarios (see Table SPM-1 in the Summary for Policymakers of IPCC Working Group II) and for stabilisation at various levels of carbon dioxide concentration following the two sets of pathways shown in Figure 1 (a). The accumulated emissions leading to stabilisation of carbon dioxide concentration were calculated using a mid-range carbon cycle model. Results from other models could be up to approximately 15% higher or lower than those presented here.

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For comparison, emissions during the period 1860 to 1994 amounted to about 360 GIC, of which about
240 GC were due to fossil fuel use and 120 GIC due to deforestation and land-use change.
As in IPCC (1994) - see figure 1 (a) (solid curves).
Profiles that allow emissions to follow 1992a until at least the year 2000 - see figure I (a) (dashed curves).
Concentrations will not stabilise by 2100.

4.10 Given cumulative emissions, and IPCC IS92a population and economic scenarios for 19902100, global annual average carbon dioxide emissions can be derived for the stabilization scenarios on a per capita or per unit of economic activity basis. If the atmospheric concentration is to remain below 550 ppmv, the future global annual average emissions cannot, during the next century, exceed the current global average and would have to be much lower before and beyond the end of the next century. Global annual average emissions could be higher for stabilization levels of 750 to 1000 ppmv. Nevertheless, even to achieve these latter stabilization levels, the global annual average emissions would need to be less than 50% above current levels on a per capita basis or less than balf of current levels per unit of economic activity'2.

4.11" The global average annual per capita emissions of carbon dioxide due to the combustion of fossil fuels is at present about 1.1 tonnes (as carbon). In addition, a net of about 0.2 tonnes per capita are emitted from deforestation and land-use change. The average annual fossil fuel per capita emission in developed and transitional economy countries is about 2.8 tonnes and ranges from 1.5 to 5.5 tonnes. The figure for the developing countries is 0.5 tonnes ranging from 0.1 tonnes to, in some few cases, above 2.0 tonnes (all figures are for 1990).

4.1214 Using World Bank estimates of GDP (gross domestic product) at market exchange rates, the current global annual average emission of energy-related carbon dioxide is about 0.3 tonnes per thousand 1990 US dollars output. In addition, global net emissions from land use changes are about 0.05 tonnes per thousand US dollars of output. The current average annual energy-related emissions per thousand 1990 US dollars output, evaluated at market exchange rates, is about 0.27 tonnes in developed and transitional economy countries and about 0.41 tonnes in developing countries. Using World Bank estimates of GDP at purchasing power parity exchange rates, the average annual energy-related emissions per thousand 1990 US dollars output is about 0.26 tonnes in developed and transitional economy countries and about 0.16 tonnes in developing countries."

Methanc

4.13 Atmospheric methane concentrations adjust to changes in anthropogenic emissions over a period of 9 to 15 years. If the annual methane emissions were immediately reduced by about 30 Tg CH. (about 8% of current anthropogenic emissions) methane concentrations would remain at today's levels. If methane emissions were to remain constant at their current levels, its concentration (1720 ppbv in 1994) would rise to about 1820 ppbv over the next 40 years.

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China registered its disagreement on the use of carbon dioxide emissions derived on the basis of a per unit of economic activity.

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The Panel agreed that this paragraph shall not prejudge the current negociations under the
UNFCCC.

The Panel agreed that this paragraph shall not prejudge the current negotiations under the
UNFCCC.

IS

These calculations of emissions per unit of economic activity do not include emissions from land

use changes or adjustments to reflect the informal economy.

Nitrous oxide

4.14 Nitrous oxide has a long lifetime (about 120 years) In order for the concentration to be Stabilized near current levels (312 ppbv in 1994), anthropogenic sources would need to be reduced immediately by more than 50%. If emissions of nitrous oxide were held constant at current levels, its concentration would rise to about 400 ppbv over several hundred years, which would increase its incremental radiative forcing by a factor of four over its current level.

Further points on stabilization

4.15 Stabilization of the concentrations of very long-lived gases, such as SF. or perfluorocarbons, can only be achieved effectively by stopping emissions. 4.16 The importance of the contribution of CO, to climate forcing, relative to that of the other greenhouse gases, increases with time in all of the IS92 emission scenarios (a to 1). For example, in the IS92a scenario, the CO, contribution increases from the present 60% to about 75% by the year 2100. During the same period, methane and nitrous oxide forcings increase in absolute terms by a factor that ranges between two and three. 4.17 The combined effect of all greenhouse gases in producing radiative forcing is often expressed in terms of the equivalent concentration of carbon dioxide which would produce the same forcing. Because of the effects of the other greenhouse gases, stabilisation at some level of equivalent carbon dioxide concentration implies maintaining carbon dioxide concentration at a lower level.

4.18 The stabilization of greenhouse gas concentrations does not imply that there will be no further climate change. After stabilization is achieved, global mean surface temperature would continue to rise for some centuries and sea level for many centuries.

5.

TECHNOLOGY AND POLICY OPTIONS FOR MITIGATION

5.1 The IPCC Second Assessment Report (1995) examines a wide range of approaches to reduce emissions and enhance sinks of greenhouse gases. This section provides technical information on options that could be used to reduce anthropogenic emissions and enhance sinks. of the principal greenhouse gases with a view to stabilizing their atmospheric concentrations; however, this analysis does not attempt to quantify potential macroeconomic consequences that may be associated with mitigation

5.2 Significant reductions in net greenhouse gas emissions are technically possible and can be economically feasible. These reductions can be achieved by utilizing an extensive array of technologies and policy measures that accelerate technology development, diffusion, and transfer in all sectors, including the energy, industry, transportation, residential commercial and agricultural/forestry sectors.

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