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The potential climate impact of solar geoengineering is examined using climate model simulations by artificially reducing the incoming solar radiation at the top of the atmosphere. Climate scenario simulations indicate that a doubling of atmospheric carbon dioxide (2xCO2) induces a surface temperature rise which is amplified over the poles primarily during the respective winter. The warming also causes intensification and poleward shift of the global precipitation pattern. In our model, a 2.1% globally uniform solar reduction can largely compensate the global mean warming caused by a doubling of CO2. We find that solar shading is efficient to restore the temperature at the region where the background sunshine is strong, regionally at low-latitudes, seasonally during summer. Solar shading would lead to an overall weakening of the global hydrological cycle, resulting in a large-scale drought. A 3.6% solar reduction in the tropics can largely reduce the tropical and global warming as well. However, it reduces the precipitation at the central tropics, while increase the precipitation over the monsoon region. Comparatively, a 14% solar reduction over the poles can effectively prevent the polar summer temperature increase and sea-ice retreat. However, caused by the increased temperature gradient, polar solar shading increases the storm activity at high-latitudes, especially during summer when the solar reduction reaches its maximum amplitude. Our simulations show that solar shading could be an effective way to stabilize the polar cryosphere. Nevertheless, it has a strong impact on the hydrological cycle and provides a heterogenous regional climate signal.

Recent variability in West African monsoon rainfall has been shown to be influenced by multiple ocean-atmosphere modes, including the El Niño Southern Oscillation, Atlantic Multidecadal Oscillation and the Interdecadal Pacific Oscillation. How these modes will change in response to long term forcing is less well understood. Here we use four transient simulations driven by changes in orbital forcing and greenhouse gas concentrations over the past 6,000 years to examine the relationship between West African monsoon rainfall multiscale variability and changes in the modes associated with this variability. All four models show a near linear decline in monsoon rainfall over the past 6,000 years in response to the gradual weakening of the interhemispheric gradient in sea surface temperatures. The only modes that show a long-term trend are those associated with the strengthening of the El Niño Southern Oscillation from the mid-Holocene onwards. There are marked multi-centennial oscillations superimposed on the long-term trend in monsoon rainfall which are strongly associated with multi-centennial oscillations in the Atlantic Multidecadal Oscillation and in tropical Atlantic sea surface temperatures in all models. However, the influence of different modes on interannual to multi-decadal rainfall variability is not consistent across the models. This is driven in one case by the Atlantic Multidecadal Oscillation and changes in Mediterranean sea surface temperatures, by tropical sea surface temperature changes in another, and in the fourth model by a combination of the Atlantic Multidecadal Oscillation and the El Niño Southern Oscillation. Furthermore, multiple inter-basin teleconnections are associated with significant rainfall anomalies in each model.

Satellite-observed sea surface temperature (SST) provides an unprecedented opportunity to evaluate the ongoing global warming and has recently reached a milestone of 40 years temporal coverage. One of the major warming patterns captured by satellites is relatively strong subtropical ocean warming across all ocean basins. This pattern was widely interpreted as a manifestation of the Pacific Decadal Oscillation, a natural climate variability. However, we find that the observed warming pattern, characterized by strong warming over the subtropical oceans and mild warming or even cooling over the subpolar oceans, is dynamically consistent with where the surface water converges and diverges. By comparing observations with paleo-reconstructions and simulated short-term and long-term SST response to CO2 forcing, we propose that the observed warming pattern is likely in a transient early warming stage constrained by background ocean circulation. In the long term, ocean warming at high latitudes is expected to exceed the temporally dominating subtropical ocean warming. The committed, but lagged, high-latitude ocean warming has potential to reshape the ocean-atmosphere circulation and threaten the stability of marine-terminating ice sheets.