On account of the long atmospheric lifetime of CO2, the level of accumulated CO2 emissions already in the atmosphere plays an important role in determining the average temperature at the Earth’s surface for decades to come.
The “carbon budget,” or “emissions budget,” represents the threshold of CO2 emissions accumulated since the pre-industrialized period that must not be exceeded between now and 2100 in order to respect a given target temperature. Of the different models used by the IPCC, most estimate that the carbon budget allowing us to respect the 2°C limit is 2,900 billion tonnes of CO2. In 2011, emissions had already used up around two thirds of the carbon budget.(26)
These models estimate that the cumulative CO2 emissions remaining if the budget is to be respected for the period from 2012 to 2100 must be limited to between 630 billion and 1,180 billion tonnes of CO2.(27) Given the current rate of reductions of GHG emissions based on existing policies, the carbon budget could be exhausted by around 2034.(28)
It is, however, possible to respect the 2°C limit even while temporarily exceeding the carbon budget in the short run. However, this excess must subsequently be compensated for (sometime around 2065) with a negative global carbon balance. Such a scenario is achievable if anthropogenic GHG emissions are at a certain point more than compensated for by the absorption of carbon associated with reforestation and by the capture and storage of CO2.
Table 1-2 illustrates the evolution of net emissions through to the end of the 21st century that is required by the carbon budget in order to have a greater than 66% probability of respecting the 2°C limit.
Another method used by the IPCC to illustrate the same goal emphasizes achieving an atmospheric concentration target of 430 to 480 parts per million of CO2 equivalent by 2100. The different scenarios in which there are no extra efforts on the part of governments to reduce GHG emissions arrive at an atmospheric concentration of 450 parts per million of CO2 equivalent by 2030, and at concentrations varying from 750 to 1,300 parts per million of CO2 equivalent by 2100.(29)
Stabilizing the amount of warming at 2°C implies a substantial reduction in anthropogenic GHG emissions between now and 2050. At that time, in addition to significant energy efficiency gains, we will have to get from three to four times more of our energy from renewable sources, from nuclear power, and from biofuels, or from fossil fuels paired with carbon capture and storage. Between 2040 and 2070, the energy sector’s emissions will have to be reduced by 90% compared to the 2010 level.(30)
Table 1-3 illustrates the GHG reductions required in the 21st century in order to respect the 2°C goal.
Net emissions required to respect the 2°C limit with a greater than 66% probability, gigatonnes of CO2
Source: This is a median based on 19 different scenarios. United Nations Environment Programme, The Emissions Gap Report 2014: A UNEP Synthesis Report, November 2014, p. 15.
Maximum annual global emissions and changes compared to emissions in 1990 and 2010 in order to respect the 2°C limit with a greater than 66% probability, gigatonnes of CO2 equivalent
Source: This is a median based on 18 different scenarios. Authors’ calculations. United Nations Environment Programme, The Emissions Gap Report 2014: A UNEP Synthesis Report, November 2014, pp. xvi and 16.
26. United Nations Environment Programme, The Emissions Gap Report 2014: A UNEP Synthesis Report, November 2014, p. 2.
28. Price Waterhouse Cooper, IPCC carbon budget to 2100 will be used by 2034 according to PwC analysis, Press release, November 14, 2013.
29. Ottmar Edenhofer et al., op. cit., footnote 22, p. 8.
30. Ibid., pp. 12 and 18.