The factor 4 problem

Various scenarios concerning climate change and CO2 emissions have been produced, especially by the IPCC.

In the reference trend scenario shown in Figure 3.3, the emissions of carbon as CO2 related to energy consumption change from 7 to 14 Gt of carbon per year between the current period and 2050, i.e. from 26.6 to 52 Gt of CO2 (trend A). Such a trend would produce a level of CO2 in the atmosphere which is totally unacceptable in terms of impact on the climate.

We must therefore consider an alternative scenario on CO2 emission trends which would be compatible with the aim of limiting the average temperature increase to 2 °C compared with the pre-industrial situation.

Scenarios of this type have been drawn up in the European Union in the context of the Greenhouse Gas Reduction Pathways (GRP) study.

According to the two objective scenarios, the concentration levels of the six greenhouse gases would stabilise respectively at 550 ppm and 650 ppm

Gt C/year

Gt C/year

Stabilization of emissions C

Reference scenario

Division by 2 of emissions

2010 2020 2030 2040 2050 2060


Figure 3.3 Trend of CO2 emissions in tonnes of carbon per year

(450 ppm and 550 ppm for CO2 alone). In the first case, the temperature increase estimated on the basis of the IPCC results would be 1.6 °C and in the second case 2.5 °C [34].

If the CO2 emissions were stabilised now (trend B), the CO2 content in 2050 would be about 500 ppm. In practice, the possibility of stopping the rise in emissions immediately would be unrealistic, and they are likely to continue rising until 2020, even if we manage to slow down the rate of increase. Emissions will then have to decrease to reach a level close to half the current level by 2050, in order to respect the limit of an average temperature rise of 2 °C (trend C - scenario 450 ppm for CO2 alone).

If the long-term objective is to express the level of CO2 emissions as a figure per inhabitant in the world, which would seem fair, the emissions from the industrialised countries must be reduced by a factor of about 4 by 2050, in order to allow the emissions from the Third World countries and the developing countries to increase. France has assigned itself this objective [35].

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