ASLR: very astute but you are neglecting aerosol forcing, If we completely halt aerosol emissions in 2027 then I am likely to agree that 2028 will reach +2.7C but under current conditions, with moderate reductions in aerosol emissions, continued significant anthropogenic carbon emissions and rapid sea ice-albedo feedback, I still expect (conservatively) to move above +2C by 2036.
jai,
You may well be correct; however, the linked reference indicates that the remaining carbon budget from 2015 may be as low as 590 GtCO2; and as CO₂-e emissions are around 50GtCO2 (which exceeds RCP 8.5 50%CL), it is easy to see that assuming ECS is 3C we could readily exceed the 2C limit by around 2030, or if ECS is 4C then we could exceed 2.7C by around 2032 to 2035, if we continue on our current BAU pathway for another 16 to 19 years.
Joeri Rogelj, Michiel Schaeffer, Pierre Friedlingstein, Nathan P. Gillett, Detlef P. van Vuuren, Keywan Riahi, Myles Allen & Reto Knutti (2016) "Differences between carbon budget estimates unravelled", Nature Climate Change, Volume: 6, Pages: 245–252, doi:10.1038/nclimate2868
http://www.nature.com/nclimate/journal/v6/n3/full/nclimate2868.htmlHowever, I note that the estimate of exceeding 2.7C by 2032 to 2035, does consider lag-time after the carbon budget has been exceeded, but does not consider the risk of accelerating Arctic Amplification due the potential early seasonal loss of Arctic Sea Ice Extent, nor the fact that the GWP of methane is higher than the authors of the reference assume; so even considering aerosol impacts, it may be possible that GMSTA could reach 2.7C around 2028.
Best,
ASLR
Edit: According to NOAA the CO₂ -equiv at the end of 2015 was 485 ppm; however, if one assumes that the GWP100 for methane is 35 instead of 25 (as assumed by NOAA), then NOAA's calculated value for the CO2-eq for 2015 would be
518ppm.