It is currently strongly debated to what extent geo-engineering options should be considered to limit climate change. There are arguments both supporting and opposing its use to address the threat of rising global mean temperature due to greenhouse gas emissions and its impact on the environment. A European project has been set up to weigh up arguments for and against geo-engineering concepts.
Some of the heat generated from short-wave solar radiation, which warms the Earth during the day, struggles to escape back into space when it is emitted as long-wave thermal radiation from the Earth's surface. A large fraction of this thermal radiation is absorbed by greenhouse gases, such as carbon dioxide or methane, in the atmosphere which re-emit some of the heat back to Earth a process known as the 'greenhouse effect'. As a consequence, an increase in the greenhouse gas content in the air due to fossil fuel burning and land-use change leads to a rise in global mean surface temperature.
The Implicc project, funded by the EU's FP7 programme, was set up to assess the feasibility and implications of technologies that could be employed to alter the radiative budget of the Earth the balance between incoming energy from the sun and the outgoing thermal radiation in order to cool the earth down. Scientists have suggested a number of technical options to combat climate change, generally referred to as geo-engineering. The Implicc project looked in particular at the possible undesirable side effects this approach may have on crucial parts of the Earth system and humankind.
"The goal of Implicc was to increase the level of knowledge about the feasibility and implications of existing suggestions to engineer the climate through solar radiation management (SRM)," says project coordinator Hauke Schmidt from the Max Planck Institute for Meteorology in Hamburg. "In this respect, we were particularly interested in what climate effects would result from SRM."
The Implicc consortium, which included five higher educational and research institutions in France, Germany and Norway, performed studies on the effectiveness, side effects and economic implications of three geo-engineering schemes: space-borne reflectors; sulphur injections into the stratosphere; and the engineering of low-level marine clouds through sea-salt injections.
The idea behind the "brightening of marine clouds" is to increase, through sea-salt seeding, the number of cloud condensation nuclei (CCN) that would lead to more but smaller cloud droplets, which are more efficient at reflecting solar radiation.
SRM drawbacks uncovered
"Our research showed that the effectiveness of sea-salt seeding may be lower than expected because the injected sea salt provides a large surface area not only for water vapour but also for gaseous sulphuric acid to condense on," says Dr Schmidt. "The latter could otherwise lead to the formation of new CCN."
The research carried out on sulphur injections, which are thought to recreate a cooling effect similar to that caused by the expulsion of volcanic sulfur into the atmosphere, also proved inconclusive. "There is still a lot of uncertainty concerning the amount of sulfate needed to create a certain climate effect. However, our study provides hints that it may be less effective than estimated from the study of volcanic eruptions," Dr Schmidt acknowledges.
Implicc's research into the climate effects of SRM showed that the approach would lead to a global reduction in rainfall and evaporation, the coordinator cautions. "The different models used in our project showed robustly that precipitation and evaporation decreases would affect large land masses in the mid-latitudes of the northern hemisphere, such as Canada and the US, central and northern Europe and Asia.
"What we can safely say is that solar radiation management would produce a climate different from historic precedents with globally less precipitation," says Dr Schmidt.
The researchers discovered that the number of low clouds would decrease through SRM, meaning that its intended effect cooling the surface would be counteracted to some extent and more SRM would be necessary than initially expected.
"We have concluded from the Implicc results that the potentially strong climate responses of SRM suggest that climate engineering cannot be seen as a substitute for a policy pathway of mitigating climate change through the reduction of greenhouse gas emissions," notes Schmidt.