due to Arctic geoengineering, but there is still a residual precipitation increase (up to 7%) in most monsoon regions associated with the residual CO 2 induced warming in the tropics. The ITCZ shift due to our Global geoengineering simulation, where aerosols (20 Mt) are prescribed uniformly around the globe, is much smaller and the precipitation changes in most monsoon regions are within ±2% as the residual CO 2 -induced warming in the tropics is also much less than in Arctic and Polar geoengineering. Further, global geoengineering nearly offsets the Arctic warming. Based on our results we infer that Arctic geoengineering leads to ITCZ shift and leaves residual CO 2 induced warming in the tropics resulting in substantial precipitation decreases (increases) in the Northern (Southern) hemisphere monsoon regions.

Abstract. We perform a source attribution for tropospheric and ground-level ozone using a novel technique that accounts separately for the contributions of the two chemically distinct emitted precursors (reactive carbon and oxides of nitrogen) to the chemical production of ozone in the troposphere. By tagging anthropogenic emissions of these precursors according to the geographical region from which they are emitted, we determine source–receptor relationships for ground-level ozone. Our methodology reproduces earlier results obtained via other techniques for ozone source attribution, and it also delivers additional information about the modelled processes responsible for the intercontinental transport of ozone, which is especially strong during the spring months. The current generation of chemical transport models used to support international negotiations aimed at reducing the intercontinental transport of ozone shows especially strong inter-model differences in simulated springtime ozone. Current models also simulate a large range of different responses of surface ozone to methane, which is one of the major precursors of ground-level ozone. Using our novel source attribution technique, we show that emissions of NOx (oxides of nitrogen) from international shipping over the high seas play a disproportionately strong role in our model system regarding the hemispheric-scale response of surface ozone to changes in methane, as well as to the springtime maximum in intercontinental transport of ozone and its precursors. We recommend a renewed focus on the improvement of the representation of the chemistry of ship NOx emissions in current-generation models. We demonstrate the utility of ozone source attribution as a powerful model diagnostic tool and recommend that similar source attribution techniques become a standard part of future model intercomparison studies.

In this study, we examine the statistics of precipitation extreme events in a model simulation of solar radiation management (SRM) geoengineering. We consider both intensity and frequency-based extreme indices for precipitation. The analysis is performed over both large-scale domains as well as regional scales (22 Giorgi land regions). We find that precipitation extremes are substantially reduced in geoengineering simulation: the magnitude of change is much smaller than those that occur in a simulation with elevated atmospheric CO 2 alone. In the geoengineered climate, though the global mean of the intensity of extreme precipitation events is slightly less than in control climate, substantial changes remain on regional scales. We do not find significant changes in the frequency of precipitation extremes in geoengineering simulation compared to control simulation on global and regional scales. We infer that SRM schemes are likely to reduce precipitation extremes and the associated impacts on a global scale. However, we note that a comprehensive assessment of moral, social, ethical, legal, technological, economic, political and governance issues is required for using SRM methods to counter the impacts of climate change.

In this study, we examine the statistics of temperature extremes in a model simulation of solar radiation management (SRM) geoengineering. We consider both intensity and frequency-based extreme indices for temperature. The analysis is performed over both large-scale domains as well as regional scales (22 Giorgi land regions). We find that temperature extremes are substantially reduced in geoengineering simulation: the magnitude of change is much smaller than that occur in a simulation with elevated atmospheric CO 2 alone. Large increase (~10-20 K) in the lower tails (0.1 percentile) of T min and T max in the northern hemisphere extra-tropics that are simulated under doubling of CO 2 are reduced in geoengineering simulation, but significant increase (~4-7 K) persist over high-latitude land regions. Frequency of temperature extremes is largely offset over land regions in geoengineered climate. We infer that SRM schemes are likely to reduce temperature extremes and the associated impacts on a global scale. However, we note that a comprehensive assessment of moral, social, ethical, legal, technological, economic, political and governance issues is required for using SRM methods to counter the impacts of climate change.