Christiana Figueres and six co-authors and about sixty five co-signatories published a paper in Nature (28/6/2017) entitled “Three years to safeguard our climate”. http://www.nature.com/news/three-years-to-safeguard-our-climate-1.22201 It emphasizes the need to have emissions begin to decline after 2020, in particular the article says “According to an April report (prepared by Carbon Tracker in London, the Climate Action Tracker consortium, the Potsdam Institute for Climate Impact Research in Germany and Yale University in New Haven, Connecticut), should emissions continue to rise beyond 2020, or even remain level, the temperature goals set in Paris become almost unattainable”. (Italics added). They argue that to achieve the Paris objective of a 2C rise, emissions must fall from 2020 on.
But Global Warming (GW) depends on the energy imbalance, or “forcing”, which in turn depends not on annual emissions but the level of atmospheric CO2 (ACO2) and other greenhouse gasses. Obviously it is helpful to have annual emissions decline because they add to the level of ACO2 in subsequent years. But it is the level of greenhouse gases, and the blocking of outgoing energy (in the form of infra-red (heat) energy), that causes temperature to rise.
To see this consider what would happen if we were able to stop all fossil CO2 emissions today. (Obviously we cannot do this, but it provides a useful thought experiment.) Current temperature is about 1.1C above pre-industrial https://library.wmo.int/opac/doc_num.php?explnum_id=3414 (P.5). SOx emissions and particulate matter shade the earth, reducing incoming sunlight, they provide about 0.5C of cooling, so a couple of weeks after closing all coal plants global temperature would probably rise to 1.6C above pre-industrial. The graph on the next page shows average temperature (above to 20th century average) has risen from 0.7C to 0.9C from 2014 to 2017 or 0.2C in three years or 0.07C per year. Using these estimates, with zero fossil CO2 emissions, we would reach 2C above pre-industrial by 2023. And 3C above pre-industrial fifteen years later, or by 2038. And this is if we stopped using fossil fuels today.
The above paragraph implies that we are going to blow through the 2C limit in short order. (If we kept coal plants open for the benefit of their short run shading (“geo-engineering”) effect we could stay under 2C for another say 7 years until about 2030. I am not advocating keeping coal plants open, just doing the math.)