Facts4Paris: Temperatures in 2100 could be 3.5°C, 2.7° or less than 2°C warmer, depending on what happens after 2030.

Temperatures in 2100 depend largely on emissions after 2030—that is on what happens after the INDCs lapse:

• If we assume "no direct connection between efforts before and after 2030", we may be set for a 3.5°C temperature increase by 2100, according to the Climate Interactive group.
• If we assume "a similar level of effort will be undertaken by countries post-2030 as applies in the period 2020-2030", we may see a 2.7°C warming, says the Climate Action Tracker (and a report by the International Energy Agency).
• If, however, we include all scenarios from the IPCC Fifth Assessment Report, we still have some chance of maintaining temperatures below 2°C, according to the UNFCCC Aggregation Report. This would require a 3.3% annual emissions reduction rate from 2030 to 2050 (full range is 2.7% to 3.9%) with reductions continuing thereafter towards zero emissions in 2100.

There is another alternative; it is to change the trajectory before 2030 by reviewing and strengthening the INDCs in line with a 2°C target. This would lower the required emissions reduction rate to just 1.6% per year—a much less daunting task.

Notes:

1. Reduction rates here are measured against fixed 2010 emission levels, not against current emission levels from year to year; we think that this approach makes more sense (and the UNFCCC Aggregation Report agrees).
2. For a full explanation of the different assumptions used by the Climate Interactive and the Climate Action Tracker, read this comparative assessment.
3. Oh...and we'd like to mention that the climate model used by the IEA and the Climate Action Tracker to calculate the 2.7°C temperature increase is MAGICC, the model that we look after. We're glad that they have made good use of it, and you can too. It's available freely for use here: live.magicc.org.