Return to different climate states by reducing sulphate aerosols under future CO₂ concentrations

It is generally believed that anthropogenic aerosols cool the atmosphere; therefore, they offset the global warming resulting from greenhouse gases to some extent. Reduction in sulphate, a primary anthropogenic aerosol, is necessary for mitigating air pollution, which causes atmospheric warming. Here, the changes in the surface air temperature under various anthropogenic emission amounts of sulphur dioxide (SO₂), which is a precursor of sulphate aerosol, are simulated under both present and doubled carbon dioxide (CO₂) concentrations with a climate model. No previous studies have conducted explicit experiments to estimate the temperature changes due to individual short-lived climate forcers (SLCFs) in different climate states with atmosphere–ocean coupled models. The simulation results clearly show that reducing SO₂ emissions at high CO₂ concentrations will significantly enhance atmospheric warming in comparison with that under the present CO₂ concentration. In the high latitudes of the Northern Hemisphere, the temperature change that will occur when fuel SO₂ emissions reach zero under a doubled CO₂ concentration will be approximately 1.0 °C, while this value will be approximately 0.5 °C under the present state. This considerable difference can affect the discussion of the 1.5 °C/2 °C target in the Paris Agreement.