According to the Intergovernmental Panel on Climate Change (IPCC), human activities since pre-industrial times have increased the global average temperature by about 0.8 degrees Kelvin and this figure is projected to rise to somewhere between 1.8 and 4 degrees towards the end of this century. Such a substantial change in climate conditions is expected to have tremendous implications for humans and the biosphere in general, and urges for immediate mitigation actions. In the event that such remediation measures fail, it is discussed to intervene on the climate change with targeted geo-engineering techniques. One of these methods entails the removal of carbon dioxide (CO2) and greenhouse gases (GHGs) from the atmosphere. Alternatively, solar radiation management (SRM) techniques reduce the amount of sunlight absorbed by the Earth in an attempt to offset any effects posed by increased GHG concentrations. The EU-funded 'Implications and risks of engineering solar radiation to limit climate change' (IMPLICC) project proposed to study the efficiency, side-effects and economic implications of SRM techniques. Scientists concentrated on three main methods, namely space-borne reflectors placed between the Earth and the Sun, sulphur injections into the stratosphere and low-level marine clouds. Using Earth system models, IMPLICC partners set out to perform climate simulations of the impact of the different geoengineering techniques. Results indicated that the effectiveness of solar forcing is less than that of CO2 forcing, and in order to balance the global temperature increase, 10–20 % more engineering would be necessary than expected. Another major result is that SRM techniques would not be able to restore a historical climate because if the temperature increase would be fully balanced, the precipitation increase will be overcompensated. The principle underlying sulphur injections in the stratosphere lies in the creation of an aerosol layer that would essentially reflect the sunlight. To investigate the beneficial effect of this approach without affecting the ozone layer, researchers performed various simulation experiments that assessed the effectiveness of emissions at different altitudes and latitudes. With respect to the manipulation of cloud whitening as a means of balancing the effect of GHGs, partners discovered that in general the most sensitive areas for this technique were found over the tropical oceans. Overall, IMPLICC studies of geo-engineering techniques unveiled some uncertainty regarding the magnitude of the radiative effects brought about by these methods. It also suggested that comprehensive observations of related natural phenomena might be helpful in order to more accurately assess the potential of these methods.
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