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Paris Agreement targets and overshoot scenarios
Katsumasa Tanaka
NIES, Japon and IPSL
Jeudi 22/11/2018, 11h00-12h00
Bat. 714, P. 1129, LSCE Orme des Merisiers

I will present two studies related to the Paris Agreement (Tanaka and O’Neill, 2018, Nature Climate Change; Tanaka, Boucher, Ciais, Yamagata, et al. in prep).

  1. The Paris Agreement stipulates that global warming be stabilized at well below 2 °C above pre-industrial levels, with aims to further constrain this warming to 1.5 °C. However, it also calls for reducing net anthropogenic greenhouse gas (GHG) emissions to zero during the second half of this century. Here, we use a reduced-form integrated assessment model to examine the consistency between temperature- and emission-based targets. We find that net zero GHG emissions are not necessarily required to remain below 1.5 °C or 2 °C, assuming either target can be achieved without overshoot. With overshoot, however, the emissions goal is consistent with the temperature targets, and substantial negative emissions are associated with reducing warming after it peaks. Temperature targets are put at risk by late achievement of emissions goals and the use of some GHG emission metrics. Refinement of Paris Agreement emissions goals should include a focus on net zero CO2—not GHG—emissions, achieved early in the second half of the century.
  2. Given the levels of emission reductions pledged by countries till today, it is increasingly relevant to consider overshooting the temperature targets at least temporarily. However, it is not well understood yet whether it is possible to decrease the temperature after overshoot. Central to our concern is climate-carbon feedbacks, which may release more CO2 from soil and possibly plant biomass to the atmosphere under higher warming, thereby amplifying overshoot. However, the strength of such feedbacks is not well-known. Such positive but uncertain feedbacks could have important consequences on emission pathways that might have to be taken over next generations to meet the Paris goals. This study explores how 2°C and 1.5°C overshoot pathways can be influenced by climate-carbon feedbacks and analyzes the climatic and economic consequences. We compute emissions scenarios based on a cost-effectiveness principle by using a simple Integrated Assessment Model. In the uncertainty analysis, we consider “learning”: that is, possible changes in our understanding of the strength of carbon cycle feedbacks, given new observations in the future. An expected key finding is that, if feedbacks turn out to be stronger than assumed today, it would incur substantial abatement costs to keep up with the Paris goals. Furthermore, our results will point to a key policy message: do not bet on temperature overshoot and negative emissions to achieve the Paris targets, supporting early mitigation actions toward a stabilization without overshoot.
Contact : Masa Kageyama
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