A climate forcing tolerable window

Using TIAM and its climate module, the possible tolerable futures were explored in terms of GHG accumulation and temperature change, using three possible values for the wanted radiative forcing: 3.5, 4.0 and 4.5 W/m2 (with no possible overshooting during the time horizon).

This analysis considers a climate sensitivity (change in equilibrium atmospheric temperature induced by a doubling of CO2 concentration) of 2.9 oC. It also considers that all countries cooperate to the World reduction of emissions. In other words, the solutions represent first-best solutions.

Temperature increase

The surface average temperature (SAT) reaches 3.3 o C in 2087, in the Reference case, more than 2.5 o C when the maximal forcing is 4.5 W/m2, almost 2.3 o C for 4.0 W/m2 and finally slightly more than 2 o C for 3.5 W/m2 (Figure 1). Of course, longer term temperatures would continue to grow in the future, depending on the future emissions. This means that only the limit of 3.5 W/m2 would be able to satisfy the limit of 2 o C desired by the EU.

Figure 1: Temperature, GHG concentration and GHG emissions

GHG concentration and emission trajectory

The GHG concentrations and emissions (Figure 1) are computed considering the three gases CO2 , CH4 and N2O in CO2 equivalent, using the Global Warming Potentials of CH4 and N2O as provided by IPCC, 2007). Annual World emissions are multiplied by almost 4 between 2006 and 2087 in the Reference case. They are reduced in 2087 by a factor 4 to 5.3 in the constrained scenarios (factor 1.5 to 2.8 in 2050).

The GHG concentration reaches almost 980 ppm-equiv at the end of the time horizon in the Reference case, against 535 to 650 ppm in the constrained scenarios. It also appears that the emissions converge to close values in all scenarios at the end of the time horizon; while the trajectories are more different during the transition period. In other words, a less stringent forcing target makes possible a delay in the reduction of the emissions.

Finally, China dominates the future World emissions (up to almost 50% of the global emissions in the Reference) as well as the future reductions (also up to almost 50% of the World reductions). However, the contribution by China to the World reductions appears to be higher in the mid-term horizon (close to 50%) than at the end (slight decrease up to 43% of the World reductions). The contribution by India is far less high, with up to 11% of the World emissions and 16% of the World reductions.

Let's remember that these scenarios consider a perfect cooperation of all countries. Any agreement involving a reduced number of countries will of course require stronger relative reductions by these countries in order to satisfy a same climate target. Hence the reduction factor 2 discussed in Europe by 2050.

Cumulative emissions

The cumulative emissions (Figure 2) over the period 2005-2050 are particularly important given their role in the bargaining progress between OECD and emergent countries: the cumulative emissions can be considered as an emission budget to be allocated amongst the different countries in order to satisfy the wanted limit related to the radiative forcing. The negotiation about burden sharing between OECD and emerging nations could therefore be reduced schematically to a split of this total budget between the two groups of countries.

Figure 2: Cumulative emissions

The obtained GHG budget over 2005-2050 is 457 GtC (Figure 2) in order to comply with a maximal radiative forcing of 3.5 W/m2 (no overshooting) until the end of the century.

Efficient technology decisions in the power sector

At the World level and on a long term perspective (Figure 3), the climate strategies rely on the penetration of low-emissions technologies, being power plants with carbon capture and sequestration and renewable, substituting conventional oil/gas/coal power plants. Such substitution occurs as soon as 2015 under the 3.5 W/m2 target, while it is delayed until 2030 under the 4.5 W/m2.

Focusing on the 3.5 W/m2 limit, China and India (Figure 4) follow the general dynamic as theWorld, dominated by the penetration of power plants with carbon capture and sequestration before 2050 (renewables also penetrate, especially in China, but in the second part of the century not shown here).

Figure 3: Electricity production at the World level

Let's keep in mind that: a full cooperation between all countries is assumed here, meaning also a full availability of the new technologies in developing countries; and the question of who pays for the development of these technologies, and more generally, for the climate strategies, is not treated here. In other words, the scenarios presented in this section help answer the question what to do, without deciding about who pays.

For example, the Clean Development Mechanism of the Kyoto Protocol, or any other technology agreement between countries X and Y would mean that a country X pays for the mitigation actions implemented in country Y.

Figure 4: Electricity production in India and China (2005-2050)

Remark: On the 3.5 W/m2 scenario, it is worth noting that as seen in Figure 4, China and India both need to begin to see large scale penetration of CCS by 2015, which is implausible at best, but evidence of what needs to be done if 3.5 W/m2 would need to be met and this should not be downplayed.

Efficient technology decisions in the power sector

The substitution of fossil fuels by less-emitting fuels (gas, biofuels, electricity) is of course observed in the final energy sectors (Figure 5). The most affected sectors are: energy-intensive industry, where gas replaces coal especially in the energy uses; and transport, where alcohols and, to a lesser extent, hydrogen (produced from coal with carbon capture and sequestration) penetrate from 2040. In India and China, similar decisions occur (Tables 5.5 and 5.6).

Figure 5: World final energy consumption

Among the other efficient strategies, the demand for the energy services, which are elastic to their own prices in TIAM, are reduced in the climate scenarios, contributing to a reduction of the energy consumption. The most important reductions are observed in the industry sectors as well as the residential and commercial demands for energy services strongly dependant on electricity (electric appliances, lighting), representing potential changes of behaviours. The reductions of the latter in China and India are relatively higher than the World average. Indeed, the price elasticities for these demands are considered higher in developing countries than in industrialized countries. These reactions will be more deeply explored in the coupled framework TIAM-GEMINI-E3.

Table 1: Final energy consumption in China
Table 2: Final energy consumption in India

Cost analysis

The price reaches 50 to 255 $/t in 2050, and more than 250 to 430 $/t at the end of the time horizon, depending on the forcing target (Figure 5). The relative stabilization of the price in the 3.5 W/m2 case proves that the most difficult reduction effort concerns the mid-term period in this case. In the other cases the efforts grow regularly. This follows the form of the emission trajectory, where the mid-term reduction is much higher in the 3.5 W/m2 case compared to the others (Figure 5). The loss of surplus (the cost of the climate strategy) increases more than 4-5 fold when choosing the 3.5 W/m2 target versus the 4.5 W/m2 (Table 3). The equivalent Net Present Value of the cost represent 1.3% of the GDP1 in the case 3.5 W/m2.

The amount of investments needed for the 3.5 W/m2 scenario (Table 4) represent 56% of the total cost of the strategy at the World level, and respectively 34%, 48% and 65% of the total cost of the strategies implemented in China, India, and Western Europe. As noted before, the computation of regional costs does not mean that each country will have to pay for the strategy implemented in its territory. Additional investments needed in China represent 17% of the total World additional investments, against 12% for India and 11% for the Western Europe. The high future emissions of China are behind this high level of investment needed in the country to implement the mitigation potential.

Figure 6: CO2 price ($/t)
Table 3: Total cost
Table 4: Distribution of total cost





1. Even though costs are expressed as % of GDP, these costs do not represent GDP losses but rather loss of World surplus.