Given the ongoing combination of anthropogenic emissions+increased natural emissions+decreasing sink capacity, we need to be making policy from the atmospheric concentrations. Otherwise, we may be celebrating reductions in anthropogenic emissions while the CO2e accelerates away.
I believe that in the curated modern world that we live in (with traditionally high factors of safety against major negative impacts), public opinion is easily manipulated by those currently in power who benefit the most from sustaining a BAU pathway for a long as feasible. Such manipulations include both intellectual 'cheats' and 'othering', for instance:
1. Hansen as for a longtime advised that a sustainable atmospheric CO2 concentration must be limited to 350ppm (with limited emissions of other GHGs including the short-lived GHGs). However, policymakers cheat on this guidance by overshooting this limit and then implying that they will employ magical negative emissions technology to stay well below the 2C limit on GMSTA. Overshooting, the 350ppm limit not only strengthens natural positive feedback, and weakens carbon sinks, but it also risks triggering Hansen's ice-climate feedback that is ignored by AR5.
2. Claiming progress on CO2 emissions by promoting the use of shale gas to replace coal is a cheat because methane has a GWP100 of about 35 times that of CO2; so that if one includes ozone as a GHG then CO2e is above 530ppm.
3. Recent studies show that the ocean heat content is about 11% higher than assumed by AR5, and that the GMSTA referenced to 1750 is already (mid-2017) about 1.15C (per Hansen et al 2017). This implies that the climate model projections that policymakers use are biased to favor low projections for GMSTA.
4. The current (AR5) climate model projections are also biased on the low side regarding the risk of ice sheet mass loss in Antarctica.
5. Current (AR5) climate model projection do not adequately consider climate attractors like ENSO trends with global warming.
6. Regarding 'othering', current tax codes subsidize the use of fossil fuels, the Trump Administration is reversing US climate policy to the disadvantage of others, and the advanced nations are transferring climate risks to developing nations.
I could go on, but my basic point is that the public is confused into believing that we can safely continue BAU behavior for some years to come; while in truth we are well beyond reasonable limits already.
See also:
Hansen et al (2017), "Young people’s burden: requirement of negative CO₂ emissions", Earth Syst. Dynam., 8, 577–616,
https://doi.org/10.5194/esd-8-577-2017https://www.earth-syst-dynam.net/8/577/2017/esd-8-577-2017.pdf