Artificially altering earth’s climate – or geoengineering – might have found a new fan this weekend in the shape of Charles Clover, writing in the Sunday Times. While “blotting out the sun” might be somewhat of an overstatement of the process, Clover seems increasingly convinced that “global tinkering” may someday be necessary.

His article refers to a recent paper looking at how much it might cost to release tiny particles, or aerosols, into the atmosphere. Once there, the particles reflect sunlight back out to space, preventing it from warming up the earth’s surface. The process, called solar radiation management, mimics the global cooling effect after giant volcanic eruptions.

The study suggests that using modified aeroplanes or airships, sulphuric acid could be released in the atmosphere at an affordable cost. But sulphuric acid isn’t the only type of particle that has this effect. Other research shows that particles of titanium dioxide would be better at reflecting light, so engineers would need to use less of it to artificially create a cooling effect.

Low cost delivery

The new study, published in Environmental Research letters, compares the cost of artificially cooling the planet under a selection of both realistic and theoretical techniques.

To counter the man-made warming projected for the next 50 years, the authors calculate it would be necessary to release about 4 million tonnes of sulphuric acid into the atmosphere each year. And the most efficient way to do this would be by tapping into large-scale air circulations.

Air rises at the tropics, and from there circulates around the world in great currents. This makes it the ideal spot for releasing particulate matter. But how high up the materials are released is also important – higher usually means better dispersal, but the cost of technology goes up with altitude too. Based on modelling, the authors suggest that between 18-25 kilometres up is probably the best compromise between cost and efficiency.

Knowing roughly where the sulphur dioxide should be dispersed, the scientists were able to make some crude assumptions to compare the relative costs of getting it from the ground to the atmosphere:

A cost comparison of different systems to deliver aerosols to the atmosphere. Source: adapted from McCellan et al. 2012

For the sake of simplicity, our table shows the cheapest ways to transport and release material about 20km up, but the authors gave a number of other options.

Designing new aeroplanes suited to flying at higher altitudes looks to be the cheapest option, at a cost of $0.7-$1.5 billion per year. A modified airship could also deliver one million tonnes of sulphur to the same height in the atmosphere for around $1 billion per year, but the authors suggest the costs here are less certain as the technology is still in its infancy.

The same is true of many of the suggested techniques. Delivering aerosols, either in gas or a semi-liquid (slurry) form, through pipes suspended from floating platforms – both of these are theoretical systems, at or beyond the limits of today’s technologies. Firing sulphur into the atmosphere using rockets and guns reaches higher altitudes, but would be much more expensive than the alternatives.

Of course, these costs are just to deliver one million tonnes of sulphur, but in practice larger volumes would be needed. The authors suggest that delivering enough aerosol to counter projected warming would cost between $2-$8 billion each year – which is comparatively less, they say, than the cost of mitigation or ‘climate damages’.

But these costs might also change if a substance other than sulphur was used. Indeed, other research suggests that it may not be the best aerosol for the job.

Perfect particulates

Following a volcanic eruption, it’s the particles of sulphuric acid which cause cooling, but they aren’t the only type of particle to have this effect. A study in Nature Climate Change suggests that minerals like titanium dioxide might instead be better at reflecting light back out to space – which means a smaller amount of material needs to be used.

A comparison of density and refractive index for a range of aerosols. Source: Pope et al. 2012

The researchers suggest that titanium dioxide offers the best balance of reflectivity and weight. Although titanium dioxide is heavier than sulphur, much less of it would be needed to reflect light back out to space. Considering the weight matters especially when the aerosols are going to be delivered intermittently – with aeroplanes and airships for example.

To exemplify this, the authors demonstrate how much sulphuric acid was released in the 1991 Mount Pinatubo eruption – around 30 million tonnes. They calculate that just 10 million tonnes of titanium dioxide would have been needed to recreate the cooling effect – just one third of the mass.

In reality, minerals like titanium dioxide need to be carried in a gas or liquid, rather than released as a solid – which means transporting a bigger mass and volume of material than just the mineral itself. But using titanium dioxide would still mean considerable fuel savings compared to sulphuric acid, which likely means considerable cost savings.

Scientists remain cautious

While neither of these papers advocate geoengineering, or attempt to factor in the risks it poses, they do fulfil a research need as geoengineering is increasingly being considered in climate policies.  Studies like these suggest that geoengineering is in theory feasible and affordable, but there are a number of reasons scientists are concerned about implementing it in practice.

Scientists are also concerned that research on economic feasibility might move the focus away from the risks of geoengineering. Responding to the cost analysis, Dr Matt Watson, who leads the SPICE project on geoengineering,  warned:

“…we must not get drawn into discussion where economics becomes the key driver. Impact on humanity and ecosystems must be, and continue to be, of primary consideration.”

Unintended impacts are perhaps one of the biggest fears around geoengineering. The release of particulates could affect the regional climate including cloud formation and rainfall patterns, with knock-on effects for the growth of plants and crops. Particulates could also interact with the ozone layer in a harmful way.

Moreover, while geoengineering could prove useful, it doesn’t address the greenhouse gas emissions rise driving climate change, or its other impacts like ocean acidification. While temperatures could be controlled, the climate would still be different.

Professor of Atmospheric Physics at Imperial College London, Joanna Haigh explained:

“There is no evidence that [geoengineering] would enable the climate to stabilise in a state similar to that which it would occupy naturally at lower greenhouse gas concentrations.”

With all of these negative factors, geoengineering is unlikely to be implemented on a large scale any time soon. The threat of harmful side effects currently outweighs the desire for action. But since global greenhouse gas emissions are still rising, research like this is becoming increasingly relevant.