Panel (a) shows ensemble mean global mean near-surface air temperature in historical and RCP simulations from CanESM2
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Figure 2. Panel (a) shows ensemble mean global mean near-surface air temperature in historical and RCP simulations from CanESM2. Pale lines labelled 'GHG' show global mean temperature in simulations with well-mixed GHG changes only. The pink line labelled 'RCP 4.5—2000 aerosols' shows simulations with fixed 2000 aerosol and aerosol precursor emissions and all other forcings following RCP 4.5. Panel (b) shows an estimate of the aerosol contribution to temperature change since 2000 derived by subtracting the temperature anomaly relative to 1995–2005 in the simulations with GHG changes only (dark coloured lines) or 2000 aerosols simulations (light pink line) from the anomaly in the RCP simulations. Panel (c) shows the rate of temperature change in each of the simulations, calculated from a difference in mean temperature between the following and preceding 11-year periods for each year. The warming rate in 2000 is higher than in any previous year of the historical simulations. Panel (d) shows an estimate of the ratio of the aerosol-induced temperature changes to the total temperature change relative to the year 2000 for each RCP. Dark lines show ratios derived from the difference between the RCP simulations and the GHG-only simulations, and the light pink line shows the ratio derived from the difference between the RCP 4.5 and RCP 4.5—2000 aerosols simulations. All temperatures were smoothed with an 11-year running mean prior to calculating this ratio. Shaded bands show estimates of the 5–95% uncertainty ranges in the ensemble mean ratios, derived by taking the standard deviation of the ratio from each of the five independent pairs of simulations with and without aerosol changes, dividing by the square root of five (the ensemble size), and multiplying by a Student-t statistic with a cutoff value of 0.05 and four degrees of freedom (one less than the ensemble size). All values shown are five-member ensemble means.
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.