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Implications and risks of engineering solar radiation to limit climate change

Final Report Summary - IMPLICC (Implications and risks of engineering solar radiation to limit climate change)

Executive Summary:
Within the IMPLICC project, five partner institutes from France, Germany and Norway have studied the effectiveness, side effects, risks and economic implications of climate engineering through different solar radiation management techniques suggested to limit climate change. The main tools used in these studies were state-of-the-art numerical Earth system models (in some cases augmented by specific treatments of atmospheric aerosols and chemistry) and an economic model. One central question was what climate would result from the application of three different CE techniques: the reduction of solar irradiance (through space mirrors); the enhancement of the reflection of solar radiation through stratospheric sulfate aerosols; and the manipulation of marine clouds through injection of sea salt. One novel aspect of IMPLICC in the context of climate engineering research was the implementation of a model intercomparison study in order to identify robust climate response patterns.
In an idealized experiment with large greenhouse gas forcing balanced globally by the reduction of solar irradiance it was shown that it may be possible to compensate the increase of global mean temperature. However, the increase in global total precipitation that is expected in scenarios with enhanced greenhouse gas concentrations would be overcompensated by solar radiation management: a geoengineered climate would have less precipitation than a natural climate of the same global mean temperature. The model intercomparison showed that precipitation decreases – under the chosen scenarios - would particularly affect large land masses in the mid-latitudes of the Northern hemisphere, i.e. Canada and the US, central and northern Europe and Asia. The simulation of a scenario with a much smaller degree of geoengineering, where just the increase of climate forcing through a moderate greenhouse gas emission scenario after the year 2020 would be compensated, showed, not surprisingly, a much smaller climate impact. Because of the weakness of the forcing, the regional patterns of the simulated responses are also less robust than under strong forcing. It was, however, clearly shown that an abrupt termination of climate engineering efforts would lead to very rapid climate change. Numerical results of these simulations are made available to the international scientific community for further exploitation that is currently ongoing for example within the international “Geoengineering Model Intercomparison Project” (GeoMIP).
The estimation of economic implications of climate change and climate engineering on long timescales has obvious limitations. However, our simulations suggest that climate engineering under a moderate emission scenario may not be economically advantageous. This could be different under high-emission scenarios, but also it is then unclear if the economic importance of side-effects would become significant.
IMPLICC has also made progress on microphysical processes involved in the aerosol-based radiation management methods, which help determine their effectiveness. It has become clear that the effectiveness of the methods depends strongly on the implementation, e.g. on the size of emitted sea salt particles. However, uncertainties concerning the amount of aerosol necessary to reach a certain climate effect remain.
It has become clear during the course of the project that some of the remaining uncertainties concerning implications of climate engineering are caused by limited understanding of climate processes in general, which are not necessarily specific to climate engineering. The manipulation of marine clouds, for example, is based on aerosol-cloud interaction processes which are one of the major open questions of climate research, independent of the origin of aerosols. Injecting sulfur into the stratosphere would not only have radiative but also dynamical effects. Dynamical stratospheretroposphere coupling would need to be better understood in order to fully appreciate the effects of such climate engineering.
Finally, it needs to be noted that the climate response is only one aspect that has to be considered when the implementation of climate engineering techniques is discussed. Other potential side effects specific to some methods, as well as political, ethical, legal and further economic implications also need to be taken into account.

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