Abstract
Global aerosol and ozone distributions and their associated radiative forcings were simulated between 1850 and 2100 following a recent historical emission dataset and under the representative concentration pathways (RCP) for the future. These simulations were used in an Earth System Model to account for the changes in both radiatively and chemically active compounds, when simulating the climate evolution. The past negative stratospheric ozone trends result in a negative climate forcing culminating at −0.15 W m−2 in the 1990s. In the meantime, the tropospheric ozone burden increase generates a positive climate forcing peaking at 0.41 W m−2. The future evolution of ozone strongly depends on the RCP scenario considered. In RCP4.5 and RCP6.0, the evolution of both stratospheric and tropospheric ozone generate relatively weak radiative forcing changes until 2060–2070 followed by a relative 30 % decrease in radiative forcing by 2100. In contrast, RCP8.5 and RCP2.6 model projections exhibit strongly different ozone radiative forcing trajectories. In the RCP2.6 scenario, both effects (stratospheric ozone, a negative forcing, and tropospheric ozone, a positive forcing) decline towards 1950s values while they both get stronger in the RCP8.5 scenario. Over the twentieth century, the evolution of the total aerosol burden is characterized by a strong increase after World War II until the middle of the 1980s followed by a stabilization during the last decade due to the strong decrease in sulfates in OECD countries since the 1970s. The cooling effects reach their maximal values in 1980, with −0.34 and −0.28 W m−2 respectively for direct and indirect total radiative forcings. According to the RCP scenarios, the aerosol content, after peaking around 2010, is projected to decline strongly and monotonically during the twenty-first century for the RCP8.5, 4.5 and 2.6 scenarios. While for RCP6.0 the decline occurs later, after peaking around 2050. As a consequence the relative importance of the total cooling effect of aerosols becomes weaker throughout the twenty-first century compared with the positive forcing of greenhouse gases. Nevertheless, both surface ozone and aerosol content show very different regional features depending on the future scenario considered. Hence, in 2050, surface ozone changes vary between −12 and +12 ppbv over Asia depending on the RCP projection, whereas the regional direct aerosol radiative forcing can locally exceed −3 W m−2.
Highlights
Reactive greenhouse gases and aerosols are key climate forcing agents, comparable to CO2 in terms of anthropogenic net radiative forcing intensity, but with contrasting regional effects
In order to use these fluxes in the Interaction between Chemistry and Aerosol (INCA) model, we reported the individual hydrocarbon fluxes on INCA species or surrogate species as described in Folberth et al (2006), we spatially interpolated the fluxes to the model resolution (3.75° 9 1.9°) and we applied a linear interpolation between decades
Each emission trajectory is compatible with a distinct climate pathway and includes gridded emissions for aerosol and ozone precursors in decadal increment
Summary
Reactive greenhouse gases (methane and ozone) and aerosols are key climate forcing agents, comparable to CO2 in terms of anthropogenic net radiative forcing intensity, but with contrasting regional effects. The fourth assessment report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) cites global average net radiative forcing due to anthropogenic changes in concentrations from preindustrial era of: ?0.48 W m-2 for methane, ?0.35 W m-2 for tropospheric ozone and -0.05 W m-2 for stratospheric ozone. The associated ranges around the values reaches 160 % for direct effect and can reach more than double the forcing itself for the indirect effects. These species are responsible for the dominant uncertainties in the radiative forcing evaluation (Forster et al 2007)
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