On one level, it`s a bit Geekery. We know very well what temperatures are doing today, and we certainly have a very detailed picture through the 20th century. That should be enough. The most comprehensive and recent assessment of mitigation trajectories consistent with the Paris Agreement is the latest IPCC special report of 1.5°C (SR1.5). The Policy Makers` Summary (MPS) 3 defined mitigation trajectories compatible with 1.5°C as trails without overflow or with limited exceedances. These pathways limit average global warming throughout the period 21. 1.5 °C without exceeding this level (“no-overshoot”) or let warming fall below 1.5 °C by the end of the century (about 1.3 °C of warming at 2100) after a brief and limited exceedance of the average peak warming below 1.6 °C towards the 2060s (“low-overshoot”). Conference of the Parties (COP21) in December 2015, 195 nations adopted the Paris Agreement2. The first instrument of its kind, the pioneering agreement, aims to strengthen the global response to the threat of climate change by “keeping the rise in global average temperature well below 2°C above pre-industrial levels and striving to limit the temperature increase to 1.5°C above pre-industrial levels”. I was present at the plenary session where the contribution of Group I to the IPCC AR5 was adopted line by line.

There was certainly the agreement of all government delegates on the proposal to use 1850-1900 to represent pre-industrial levels and, as a result, the term pre-industrial was replaced by the more precise/direct 1850-1900, where the latter had been used. .