Mitigation implications of midcentury targets that preserve long-term climate policy options

Abstract

Midcentury targets have been proposed as a guide to climate change policy that can link long-term goals to shorter-term actions. However no explicit mitigation analyses have been carried out of the relationship between midcentury conditions and longer-term outcomes. Here we use an integrated assessment modeling framework with a detailed representation of the energy sector to examine the dependence of climate change outcomes in 2100 on emissions levels, atmospheric concentrations, and technology characteristics in 2050. We find that midcentury conditions are crucial determinants of longer-term climate outcomes, and we identify feasibility thresholds describing conditions that must be met by midcentury to keep particular long-term options open. For example, to preserve the technical feasibility of a 50% likelihood of keeping global average temperature at < 2 degrees C above preindustrial in 2100, global emissions must be reduced by about 20% below 2000 levels by 2050. Results are sensitive to several assumptions, including the nature of future socio-economic development. In a scenario with high demand for energy and land, being below 2 degrees C with 50% likelihood requires a 50% reduction in emissions below 2000 levels by 2050, which is only barely feasible with known technologies in that scenario. Results suggest that a greater focus on midcentury targets could facilitate the development of policies that preserve potentially desirable long-term options.

Fig. 1.

Emissions and CO₂-eq. concentration pathways to 2050 and resulting attainability ranges for the long-term (2100), assuming the B2 (Left) or A2r (Right) reference scenarios. Emissions include CO₂ from fossil fuel burning and land-use change. Colored shaded areas depict the feasible range of outcomes over the second half of the century associated with each 2050 emissions target.

Fig. 2.

Relationship between midcentury conditions and total energy system costs for a range of long-term CO₂-eq concentration targets, assuming the B2 (Left) or A2r (Right) reference scenarios. Upper panels show midcentury conditions for CO₂ emissions, Lower panels for share of zero-carbon sources in primary energy. Colored lines depict the relationship for each long-term CO₂-eq. concentration target, with the horizontal bars indicating the corresponding cost-optimal midcentury condition. The u-shape of the curves reflects rising costs if either too little or too much mitigation is undertaken by 2050, relative to the optimum. Costs are higher in the A2r scenario due to higher energy demand and slower rates of technological change relative to B2. The nearly vertical rise in costs indicate the location of the feasibility frontier; emissions higher than this level in 2050 make the long-term goal infeasible.

Fig. 3.

Feasibility frontier for the B2 (Red) and A2r (Blue) baseline scenarios compared to ranges of midcentury emissions associated with mitigation scenarios in the literature assessed in IPCC AR4 (Gray, 21). In our analysis, the B2 feasibility frontier does not apply to the IPCC stabilization category IV, because this target is attainable in the B2 baseline scenario without any mitigation by 2050.

Fig. 4.

Relationship between midcentury CO₂ emissions and the probability of being below a temperature change of 3 °C (Left) or 2 °C (Right) in 2100. Red area marks division between feasible (to the left) and infeasible (to the right) combinations of 2050 emissions levels and likelihoods of being below temperature thresholds in 2100. Green area marks ranges of least-cost midcentury conditions for different likelihoods of being below thresholds. Upper panels show the relationship for the B2 mitigation scenarios based on a single climate sensitivity PDF (26). Lower panels illustrate the baseline uncertainty, comparing results for the B2 (Shaded) and the A2r scenarios (dashed lines). Note different scales on the axes. Lower emissions levels by 2050 increase the likelihood of keeping temperature change < 2° C or < 3° C in 2100. For a given emissions level in 2050, the likelihood of a long-term temperature goal being feasible is generally lower in the A2r scenarios due to the greater energy demand to be met (Lower).