A new study in the journal Nature Climate Change examines the level of permissible greenhouse emissions to limit temperature increases to less than 2C relative to pre-industrial levels, Joeri Rogelj, et al., Emission Pathways Consistent with a 2C Global Temperature Level 1-6 (Oct. 2011). The study extends path-dependent assessments such as UNEP’s The Emissions Gap Report. Among the take-aways from the study:

1. There is increasing evidence that we may need negative emissions “growth” to effectuate declines of temperature at a reasonable time scale; this is attributable to slow ocean mixing, which both delays warming associated with anthropogenic emissions and also limits amount of cooling for many decades to centuries. Technologies that might help effectuate negative emissions include carbon capture and sequestration (CCS) and bio-energy;
2. Pathways with a “likely” (defined as greater than 66%) chance of staying below 2C requires median 2020 emissions of 44 Gt CO2e (compared to 48 GT CO2e currently), and 20 Gt by 2050;
3. Atmospheric concentrations of CO2 in 2100 consistent with a 2C target are approximately 425ppm, or 465ppm CO2e;
4. For more scenarios in the study’s data set, stabilization of emissions in 2030 is more consistently with a “likely” chance to stagy below 3C instead of 2C.

This is an excellent reading in a module on long-term responses to climate change. The conclusion that 70% of the scenarios that provide emissions pathways that hold temperatures below 2C require negative emissions could stimulate some good discussion about the importance of CCS technologies in future climate change policy portfolios, or geoengineering technologies, e.g. air capture or ocean iron fertilization. This piece also would provide excellent data for in-class cliamte simulations.