For instructors discussing the prospects for “The Road to Paris” at COP21 to help us build a bridge to a safer climatic future, a new study in the journal Nature would be a good student reading. The study draws upon the Intended National Determined Contributions of the more than 150 countries that have made such pledges to date,embodying 90% of the globe’s emissions. The study’s authors seek to assess both the prospects for limiting temperature changes to 2C from pre-industrial levels, as well as how much such pledges reduce the risk of the highest potential increases in temperatures. The authors emphasize that because temperature changes ultimately depend on cumulative emissions, it’s critical to assess the likely long-term paths of emissions commitments beyond the INDCs, which extend to only 2025 or 2030. This was calculated through the use of a global integrated assessment model. Also, the uncertainties associated with the global carbon cycle and climate system responses necessitates probabilistic assessments. The study utilizes two scenarios, a Paris-Continued minimum (2% annual rate) scenario assuming that countries proceed to reduce emissions at the same rate as required to achieve their INDCs between 2020-2030, and a Paris-Increased ambition scenario, assuming a 5% annual reduction beyond 2030.

The study’s conclusions include the following:

• The Paris-Continued scenario reduces the probability of temperatures increasing more than 4C in 2100 by 75% compared to the Reference-Low policy scenario, and by 80% from a Reference-No policy scenario;
  • The chance of exceeding 4C is virtually eliminated if mitigation efforts are increased beyond 2030, such as in the Paris-Increased ambition scenario
• There is an 8% probability of limiting temperature increases to 2C from pre-industrial levels In the Paris-Continued ambition; this increases to about 30% under the Paris-Increased scenario.
  • Scenarios to increase the probability of limiting temperatures to 2C to between 50-66% are plausible, but assume rapid emissions reductions after 2030, and many also include negative global emissions in the second half of the century, effectuated through the deployment of Bio-energy Carbon Capture and Sequestration (BECCS).
• To limit warming to any prescribed level in the future will necessitate ultimately reducing carbon dioxide emissions to zero. If this doesn’t transpire quickly beyond 2100, the prospects of both extreme temperature changes and exceeding the 2C threshold are substantially increased.