evidence: The combination of all radiative forcings generally matches longer-term changes in observed temperatures quite well. There is some year-to-year variability, primarily from El Niño events, that is not driven by changes in forcings. There are also periods from 1900-1920 and 1930-1950 where some larger disagreements are evident between projected and observed warming, both in this simple model and in more complex climate models. [...] Future radiative forcing from CO2 is expected to continue to increase if emissions rise. Aerosols, on the other hand, are projected to peak at today’s levels and decline significantly by 2100, driven in large part by concerns about air quality. This reduction in aerosols will enhance overall warming, bringing total warming from all radiative forcing closer to warming from greenhouse gases alone. The RCP scenarios assume no specific future volcanic eruptions, as the timing of these is unknowable, while solar output continues its 11-year cycle. [...] While natural forcings from solar and volcanoes do not seem to play much of a role in long-term warming, there is also natural variability associated with ocean cycles and variations in ocean heat uptake.