The purpose of this paper is to clarify the magnitude of the climate challenge we face globally and the role that the four largest greenhouse gas emitters – China, the U.S.A., the European Union and India – could potentially play, if they decided on a “deep collaboration”. As stated in IPCC’s 1.5°C report, the challenge is indeed to bring global emissions down to a level where they could be compensated for by anthropogenic carbon capture from the atmosphere. In this paper, we focus on the abatement of CO2 emissions as they represent two thirds of total GHG emissions3. By doing so, we recognise that confining our data to CO2 ignores other important gases (methane, nitrous oxides, fluorinated gases) and their emission dynamics. The paper proceeds along three stages. In section 2. “Where we stand, a global view”, we recall the dynamics of atmospheric concentrations for two major GHGs, CO2 and methane. In section 3. “Looking back”, we analyse in more detail the trends and bifurcations in the emissions for each of the four constituencies we are considering. Finally, in section 4. “Where we need to go”, we analyse for the same constituencies representative scenarios that will allow us to contrast current developments with more constrained trajectories meeting the Paris commitments and, further on, net zero ambitions.