"We started out asking if it would be possible to predict the aerosol distribution pattern that would come closest to minimizing climate change," George Ban-Weiss of the Carnegie Institution for Science, US, told environmentalresearchweb. "To do this we used an approach often used in engineering called optimization. Since some people have begun to talk about engineering the climate as a possible last-resort fix for climate change, we wanted to see if results from a global climate model were predictable enough for this engineering approach to work."

Ban-Weiss and colleague Ken Caldeira found that the optimization approach did indeed work. But, as ever, there's a snag. Creating temperature conditions most like today's calls for a higher concentration of sulphate aerosols over the poles than at the equator. Minimizing disruption to the water cycle, on the other hand, requires a much more uniform sulphate distribution.

"Temperatures and the hydrological cycle cannot be minimized simultaneously, a consequence of the fact that the hydrological cycle is more sensitive to changes in sunlight than are surface temperatures," explained Ban-Weiss. "One implication is that when attempting to minimize changes in surface air temperature, the surface of Earth becomes overly dry in many latitude bands in the model. If the goal is to minimize changes in the hydrological cycle, many latitude bands have residual warming."

The team carried out five simulations using a global climate model; each employed a different distribution of aerosol by latitude, with a total aerosol loading of 10 megatonnes.

"The results from these so-called 'basis function' simulations were used as 'training' information for our optimization model," said Ban-Weiss. "We found that predictions of zonal average surface temperatures and water runoff from the optimization model were very similar to that from the global climate model."

Ban-Weiss and Caldeira are keen to stress that their study only uses one climate model and does not include all processes that are important in reality, such as socio-political consequences, changes in stratospheric ozone, ocean circulation alterations, aerosol transport and microphysics.

"Our results are illustrative and do not provide a sound basis for making policy decisions," said Ban-Weiss. "We also point out that geoengineering options can never reverse all of the consequences of greenhouse-gas emissions. For example, it doesn't reverse ocean acidification. And it obviously has associated risk. So geoengineering is not an alternative to greenhouse-gas emissions reductions."

Next the researchers would like to look at the effects of geoengineering at smaller scales than latitude bands, for example attempting to minimize climate change at all grid cells in the model.

"This could be important because there could be longitudinal shifts in climate (in the east-west direction) that do not show up in our latitude averages," said Ban-Weiss. "In addition we could use other metrics of climate change besides surface temperature and runoff – some examples could include minimizing changes in polar sea ice or monsoonal cycles."

According to Ban-Weiss, the linearity of the climate response to various aerosol forcings also suggests a way of addressing paleo-climate studies. "If we can understand the response to changing solar insolation, carbon dioxide levels, vegetation type, continental position, ice sheet volume, and so on – each individually – and then estimate the climate consequences of a combination of those effects through linear super-position, it means that we do not need to get the entire story to have at least a partial understanding of paleo-climate dynamics," he said.

The researchers reported their work in ERL.