Panel (a) compares SO2 emissions prescribed over the historical period in the CMIP5 simulations (solid grey) with three inventory-based estimates of historical emissions [1, 10, 11]
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Figure 1. Panel (a) compares SO2 emissions prescribed over the historical period in the CMIP5 simulations (solid grey) with three inventory-based estimates of historical emissions [1, 10, 11]. Note that, unlike the other estimates shown, the Klimont et al  estimates exclude emissions from open burning from grasslands, savannahs, forests and deforestation, though this contribution is relatively small. SO2 emissions over the 21st century specified in the RCP 2.6, 4.5 and 8.5 scenarios  (solid coloured lines) are compared with those specified in the SRES A1, A2, B1 and B2 scenarios  (dashed coloured lines). The pink line shows the emissions in the simulation with fixed 2000 aerosol emissions. Panel (b) shows the simulated global atmospheric burden of SO4 in the historical simulations (grey), RCP simulations (colours), and RCP 4.5—2000 aerosols simulations (pink).
The representative concentration pathway (RCP) scenarios all assume stringent emissions controls on aerosols and their precursors, and hence include progressive decreases in aerosol and aerosol precursor emissions through the 21st century. Recent studies have suggested that the resultant decrease in aerosols could drive rapid near-term warming, which could dominate the effects of greenhouse gas (GHG) increases in the coming decades. In CanESM2 simulations, we find that under the RCP 2.6 scenario, which includes the fastest decrease in aerosol and aerosol precursor emissions, the contribution of aerosol reductions to warming between 2000 and 2040 is around 30%. Moreover, the rate of warming in the RCP 2.6 simulations declines gradually from its present-day value as GHG emissions decrease. Thus, while aerosol emission reductions contribute to gradual warming through the 21st century, we find no evidence that aerosol emission reductions drive particularly rapid near-term warming in this scenario. In the near-term, as in the long-term, GHG increases are the dominant driver of warming.