One of the most plausible and well studied geoengineering proposals is the global dimming idea. Fling some sulphate particles into the stratosphere, reflect a greater proportion of solar energy back into space, and reduce the intensity of the solar energy reaching the Earth’s surface. Nature has already proved that the idea works – global temperatures dropped by around 0.5°C in the months following the eruption of Mount Pinatubo in 1991 – and it sounds simple and straightforward. However, new research shows that the mechanisms behind global dimming are much more complex than previously thought, and that such a project would have serious negative consequences for tropical climate.

Until now, most climate models have simulated global dimming geoengineering projects by ‘turning down’ the Sun. "It is a very common shortcut, but it isn’t realistic because it doesn’t include the effects of aerosols on the stratospheric energy balance," explained Angus Ferraro from the University of Reading, UK. To rectify this shortcut, Ferraro and his colleagues ran model simulations which included the impact that extra aerosols would have on the stratospheric energy balance, and compared them with simple solar dimming model runs.

When they ran their more complex simulation, they found that injecting reflective aerosols into the stratosphere would have a huge impact on tropical circulation and rainfall. They found that the extra aerosols required to counterbalance surface warming caused by a quadrupling of atmospheric carbon dioxide would reduce rainfall in the tropics by one third.

"If we think about the rainfall in Indonesia, for example, an average reduction of 30% would be like making the very driest years of today the very wettest years in a geoengineered climate," said Ferraro. When they carried out the same model run but simply ‘turned down the Sun’, the impact on tropical circulation and rainfall vanished. The findings are published in Environmental Research Letters (ERL).

Simple ‘turn down the Sun’ climate simulations don’t show this effect because they don’t look at the effect the aerosols themselves have on the stratosphere. In their more complex simulation the researchers show that the aerosols emit radiation, which travels down and gets absorbed into the troposphere. This warms the upper troposphere and makes the tropical atmosphere more stable. "When you have warmer air overlying colder air, you don't get upward motion because this kind of setup is stable," said Ferraro. As a result, tropical circulation weakens and the dampened upward motion reduces tropical rainfall.

This doesn’t necessarily preclude that such geoengineering shouldn’t be used, but the new model reveals that there are likely to be undesirable side effects, which would have to be taken into account. "In this case we show that caution is required when interpreting model results from solar dimming experiments as analogues for stratospheric aerosol geoengineering," wrote Ferraro and his colleagues.

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