These pathways or very similar ones them figure in a very large number of analyses and policy debates, as they correspond to three extremely important socio-political storylines. The authors assesses the three pathways in the light of Working Group I’s recently released contribution to the Intergovernmental Panel on Climate Change Fifth Assessment Report, which provides three specific global carbon dioxide (CO2) budgets, and associates them with specific risks of a global surface temperature increase of more than 2°C by the end of this century, relative to the 1850–1900 average.

The pathways’ key features and the findings of our analysis can be summarized thus:

  • The 1.5°C marker pathway is defined as the most challenging mitigation pathway that can still be defended as being techno-economically achievable. Emissions peak in 2014 and then decline (in all-gas terms) by as much as 7.1% per year. Cumulative emissions for 2000–2100 are 1,720 Gt CO2e). A comparison with the IPCC carbon budgets does not make it possible to estimate this pathway’s likelihood of keeping warming below 1.5°C, but it show that it has a significantly greater than 66% probability of staying below 2°C.
  • The 2°C marker pathway is fashioned after well-known and often-cited emissions pathways that are typically presented as having a “likely” chance of keeping warming below 2°C. Emissions peak in 2014 and then decline (in all-gas terms) by as much as 3.4% per year. Cumulative emissions for 2000–2100 are 2,380 Gt CO2e. A comparison with the IPCC budgets suggests that in fact, they have a less that 50% chance of holding warming below 2°C.
  • The G8 pathway, a marker of the high-level political consensus in developed countries, is based on a G8 declaration in 2008. Emissions peak in 2021, decline (in all-gas terms) by a maximum of 4.5% per year, and have a cumulative budget of 2,860 Gt CO2e. The authors find that its chance of keeping below 2°C is far less than 33%.

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