Sammendrag
Scenario studies have shown that in order to limit global warming to 2°C above pre-industrial levels, negative CO2 emissions are required. Currently, no safe and well-established technologies exist for achieving such negative emissions. Hence, although carbon dioxide removal may appear less risky and controversial than Radiation Management (RM) techniques, the latter type of climate engineering (CE) techniques cannot be ruled out as a future policy option. The EXPECT project, funded by the Norwegian Research Council, explores the potential and risks of RM through Earth System Model Simulations. We here describe results from a study that simulates a 21st century transition from an RCP8.5 to a RCP4.5 scenario through Radiation Management. The study uses the Norwegian Earth System Model (NorESM) to compare the results from the following three RM techniques: a) Stratospheric Aerosol Injections (SAI); b) Marine Sky Brightening (MSB); c) Cirrus Cloud Thinning (CCT). All three simulations start from the year 2020 and run until 2100. Whereas both SAI and MSB successfully simulate the desired negative radiative forcing throughout the 21st century, the CCT simulations have a +0.5 W m-2 residual forcing (on top of RCP4.5) at the end of the century. Although all three techniques obtain approximately the same global temperature evolution, precipitation responses are very different. In particular, the CCT simulation has even more globally averaged precipitation at year 2100 than RCP8.5, whereas both SAI and MSB simulate less precipitation than RCP4.5. In addition, there are significant differences in geographical patterns of precipitation. Natural variability in the Earth System also exhibits sensitivity to the choice of RM technique: Both the Atlantic Meridional Overturning Circulation and the Pacific Decadal Oscillation respond differently to the choice of SAI, MSB or CCT. We will present a careful analysis, as well as a physical interpretation of the above results.
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