Here, we use “moderate” to describe emission cuts of about half between now and 2100. This amount of emission reduction roughly corresponds to what would be needed to achieve the goal established at Paris in 2015 of limiting average global warming to 2°C.
When you choose a city, the first line drawn shows which city currently has summer temperatures that your city could reach by 2100, assuming carbon emissions remain unchecked. Technically, this corresponds to a future emission scenario called RCP 8.5, which has generally mirrored our emissions over the past 10 years. Under RCP 8.5, land surface temperatures are projected to increase an average of 4.8°C (+/- 0.9°C) by 2100 over baseline (1985-2005) temperatures.
When you click “what about with moderate emission cuts,” the new line shows which city currently has summer temperatures that your city could reach by 2100 assuming the moderate emissions cuts described above. This corresponds to RCP 4.5, under which land surface temperatures are projected to increase an average of 2.4°C (+/- 0.6°C) by 2100 over baseline (1986-2005) temperatures.
Monthly average maximum daily temperature (Tmax) was taken as the median of a suite of 31 global climate models from the Coupled Model Intercomparison Project 5 (CMIP5).
Average summertime Tmax was calculated by averaging monthly Tmax for June, July and August in the northern hemisphere, and December, January and February in the southern hemisphere. This was done for two periods: 1996-2015 under RCP 8.5 (current), and 2080-2099 under RCP 4.5 and 8.5.
To match origin cities to destination cities we found a list of all cities whose current summer Tmax was within 0.5°C of the origin’s projected summer Tmax, then chose the geographically closest destination city. This was repeated for RCP 4.5 and RCP 8.5.