That is in line with a 3.5 degrees warming scenario over the course of this century – still excluding albedo and carbon feedbacks. These numbers come from a new pledge calculation performed by Climate Interactive and MIT Sloan School of Management.
Why is the 860 ppm scenario overly pessimistic? It extrapolates countries’ Intended Nationally Determined Contributions (INDCs) which are for the Paris climate summit mostly focussed on the year 2030 over the rest of the century. Shown below are scenarios with increased levels of ambition beyond 2025/2030. (The 50% chance 2 degree pathway requires urgent increase of emission reductions even before 2025.)
Why is the 860 ppm scenario overly optimistic? It firstly assumes that countries develop effective national climate policies and actually reach their INDC emission goals. This has never before happened on a global scale. Therefore please also always take note of ‘business as usual’ – the scenario we are actually on, which could lead to 1250 ppm CO2eq. Also both the GHG concentration scenario and the linked warming (climate sensitivity) exclude feedbacks, many of which are likely to become significant amplifiers beyond 1.5/2 degrees warming.
According to a different group, Climate Action Tracker, the 2 degrees emissions gap for 2030 INDCs amounts to some 19Gt CO2eq – as shown in this graph.
© Rolf Schuttenhelm | www.bitsofscience.org