Climate engineering using atmospheric aerosols could lead to significant cooling to help offset climate change, according to a new study – but it would still mean greenhouse gas emissions would need to be curbed.
From the University of Eastern Finland
It is possible to significantly slow down and even temporarily stop the progression of global warming by increasing the atmospheric aerosol concentration, shows a new study from the University of Eastern Finland. However, climate engineering does not remove the need to reduce greenhouse gas emissions.
The study used global climate models to analyse the ability of atmospheric aerosols to cool down the climate, as well as the consequences of their use. The study focused on methods of climate engineering, which intentionally and artificially increase the atmospheric aerosol concentration in order to cool down the climate.
Furthermore, the cooling effects of current atmospheric aerosol emissions were analysed. The study found that aerosol particles injected into the stratosphere proved extremely efficient in cooling down the climate. The method mimics massive volcanic eruptions which release aerosol particles into the stratosphere that reflect solar radiation back into space, thus cooling down the climate even up to years. Atmospheric aerosols injected into the troposphere, on the other hand, can effectively impact the climate through cloud formation. Atmospheric aerosols increase the number of cloud droplets in clouds and make them whiter, which means that they can more effectively reflect solar radiation back into space.
The study also showed that current traffic and industry induced aerosol emissions cool down the climate. However, their cooling effect on the global temperature is significantly smaller than the warming effect of current greenhouse gas emissions. Nevertheless, it would be possible to harness, for example, global airline traffic and ship traffic for the purposes of atmospheric temperature regulation by increasing the sulphuric concentrations of fuels. This would make it possible to significantly increase stratospheric aerosol concentrations and cloud reflectivity in open sea. However, sulphuric concentrations of fuels would have to be increased beyond the levels defined in international agreements. In addition, the cooling effect would mainly be targeted at the northern hemisphere, which is responsible for a far greater share of global traffic than the southern hemisphere.
Atmospheric aerosols not enough, greenhouse gas restrictions vital
The study also shows that not even the most promising methods of climate engineering can cool down the climate, unless the growth of greenhouse gas emissions can be brought under control. This is indicated by a study that analysed the climate effects of a volcanic eruption at a time when aerosol concentrations in the stratosphere were increased for climate engineering purposes. The cooling effect of the volcanic eruption was significantly smaller than it would have been under normal circumstances. The sulphur dioxide released in the volcanic eruption combined with the sulphur dioxide injected into the stratosphere for climate engineering purposes leads to relatively larger particle sizes in comparison to a volcanic eruption in current conditions. The ability of large particles to reflect solar radiation is weaker and their life cycle in the atmosphere shorter than those of smaller particles.
In practice, the consequences would be similar in a situation where the stratospheric aerosol concentration is increased for climate engineering purposes. If greenhouse gas emissions continue to grow, reversing the resulting global warming by climate engineering would require the injection of increasingly large amounts of aerosols into the atmosphere. The consequence would be increasingly large relative particle sizes with a smaller cooling effect, thus weakening the relative effect of climate engineering. This means that climate engineering is not able, not even in theory, to reverse global warming caused by growing greenhouse emissions, if they continue to increase at the current rate also in the future. Moreover, climate engineering can’t fully reverse all consequences of increased atmospheric carbon dioxide concentrations, such as changes in rainfall. Climate change should be mitigated by reducing greenhouse gases, while climate engineering — even at its best — could provide only temporary relief in situations calling for extreme measures.
Both explosive volcanic eruptions, which emit sulfur dioxide into the stratosphere, and stratospheric geoengineering via sulfur injections can potentially cool the climate by increasing the amount of scattering particles in the atmosphere. Here we employ a global aerosol-climate model and an Earth system model to study the radiative and climate changes occurring after an erupting volcano during solar radiation management (SRM). According to our simulations the radiative impacts of the eruption and SRM are not additive and the radiative effects and climate changes occurring after the eruption depend strongly on whether SRM is continued or suspended after the eruption. In the former case, the peak burden of the additional stratospheric sulfate as well as changes in global mean precipitation are fairly similar regardless of whether the eruption takes place in a SRM or non-SRM world. However, the maximum increase in the global mean radiative forcing caused by the eruption is approximately 21 % lower compared to a case when the eruption occurs in an unperturbed atmosphere. In addition, the recovery of the stratospheric sulfur burden and radiative forcing is significantly faster after the eruption, because the eruption during the SRM leads to a smaller number and larger sulfate particles compared to the eruption in a non-SRM world. On the other hand, if SRM is suspended immediately after the eruption, the peak increase in global forcing caused by the eruption is about 32 % lower compared to a corresponding eruption into a clean background atmosphere. In this simulation, only about one-third of the global ensemble-mean cooling occurs after the eruption, compared to that occurring after an eruption under unperturbed atmospheric conditions. Furthermore, the global cooling signal is seen only for the 12 months after the eruption in the former scenario compared to over 40 months in the latter. In terms of global precipitation rate, we obtain a 36 % smaller decrease in the first year after the eruption and again a clearly faster recovery in the concurrent eruption and SRM scenario, which is suspended after the eruption. We also found that an explosive eruption could lead to significantly different regional climate responses depending on whether it takes place during geoengineering or into an unperturbed background atmosphere. Our results imply that observations from previous large eruptions, such as Mount Pinatubo in 1991, are not directly applicable when estimating the potential consequences of a volcanic eruption during stratospheric geoengineering.
Laakso, A., Kokkola, H., Partanen, A.-I., Niemeier, U., Timmreck, C., Lehtinen, K. E. J., Hakkarainen, H., and Korhonen, H.; Radiative and climate impacts of a large volcanic eruption during stratospheric sulfur geoengineering; Atmospheric Chemistry and Physics; 16, 305-323, doi:10.5194/acp-16-305-2016, 2016.
University of Eastern Finland news release via EurekAlert!