Before 2100, will there be a 5-year period with an average global temperature >3.6˚C warmer than the 1861-1880 baseline?

Without additional efforts to reduce GHG emissions beyond those in place today, global emissions growth is expected to persist, driven by growth in global population and economic activities. Global mean surface temperature increases in 2100 in baseline scenarios—those without additional mitigation—range from 3.7°C to 4.8°C above the average for 1850–1900 for a median climate response.

Given these estimates of the baseline scenarios of unmitigated emissions, studies exploring particular effort-sharing mitigation frameworks, have estimated substantial global financial flows associated with mitigation in scenarios to limit warming during the 21st century to less than 2°C. But there is also a non-negligible chance that unmitigated emissions will lead to global temperature increases much higher than the median estimated outcome. More generally, estimates of temperature increases resulting from greenhouse emissions have a “fat” right tail, meaning that there is a low, but non-negligible chance of very high temperature increases.

In particular, it has been argued that there is a decent chance that the unmitigated emissions might result in a >6.4ºC change in global mean surface temperature. Then, even with the systems to reduce temperatures by 2.8ºC (as might be required in baseline scenarios to achieve the 2ºC target), mean global temperature might be still be at least as high as 3.6ºC, despite substantial mitigation efforts.

The estimated humanitarian impacts of climate changes are likely highly nonlinear: marginal temperature increases are expected to cause more damage at already-increased temperatures (i.e. going from 3ºC to 4ºC is expected to be significantly worse than going from 1ºC to 2ºC). According to the IPCC's 2014 report,

The risks associated with temperatures at or above 4°C include severe and widespread impacts on unique and threatened systems, substantial species extinction, large risks to global and regional food security, consequential constraints on common human activities, increased likelihood of triggering tipping points (critical thresholds) and limited potential for adaptation in some cases.