Siim Sepp/Alamy

Estimates suggest that olivine could be used to sequester a significant proportion of carbon emissions.

Last week, a group of geoengineers met in Hamburg to discuss what on the face of it sounds like a very attractive idea: to soak up anthropogenic carbon emissions using only rocks and water. In particular, they want to help to mitigate climate change by crushing rocks and dropping them into the sea or spreading them on land. The meeting was hailed a success, but the idea is still far from fruition.

• Originality takes second place for young scientists with extreme workloads
• Why flawed diamonds are advancing quantum computing
• How plant-killing fungi protect forest diversity

The ‘weathering’, or breaking down, of rocks is a hugely important but very slow part of the carbon cycle. Natural weathering locks up atmospheric carbon dioxide by means of chemical reactions between common silicate minerals and air. For example, when magnesium-rich olivine, a rock of particular interest to geoengineers, is brought together with CO₂ and water under natural conditions, the resulting reaction forms magnesium carbonate and silicic acid, thereby removing and storing carbon.

But some scientists think that this natural process could be exploited to offset at least some of the carbon emitted by human activities. Rather than waiting for rocks to be slowly weathered away, olivine could be mined on an industrial scale, ground up, and spread over land or in the sea, speeding up these chemical reactions and sucking vast quantities of CO₂ out of the atmosphere. But this presents practical problems: according to one estimate, you would need to spread 5 gigatonnes of olivine on beaches annually to offset 30% of global CO₂ emissions (assuming 1990 levels of emissions; S. J. T. Hangx & C. J. Spiers Int. J. Greenhouse Gas Contr. 3, 757–767; 2009).

At the informal meeting, about 20 enhanced-weathering experts discussed recent research in the area and tried to summarize and coordinate future work, for example by agreeing to standardize experiments. Until now, there has been no organized research agenda for the fledgling field, says meeting convener Jens Hartmann, who works on geological cycles and carbon sequestration at the University of Hamburg in Germany. “It was very positive; we know we are now a community,” he says.

Hartmann points out that humans have been exploiting rock weathering for decades — for example, by spreading minerals such as olivine, pyroxenes and serpentines as fertilizers. “The question is, can we optimize it and can we do it in areas we are not doing it?” he says.

As with its use as a fertilizer, olivine would have to be finely crushed to maximize its exposure to carbon. Olaf Schuiling, a geochemist at Utrecht University in the Netherlands and a passionate advocate of enhanced weathering, proposes spreading coarse olivine grains on beaches that experience heavy seas. “There the grains are tumbling around in the surf and the waves, they collide, they abrade each other, and produce very rapidly a lot of tiny olivine slivers that weather quickly,” he says.

However, there is little evidence for the practical rates of weathering that could be expected if large amounts of olivine or other rocks were mined and spread on fields or dumped into the sea. This, in turn, means it is not clear how much would be needed to significantly mitigate carbon emissions, how long it would take to work or whether it would be cost and energy efficient.

In theory, one kilogram of olivine sequesters about one kilogram of CO₂, but the rate at which this happens can be slow. And the actual efficiency of sequestration will be much lower than 100%, because of the energy used — and emissions released — in grinding and transporting the rock. In some cases, this could emit more carbon than would be sequestered.

Francesc Montserrat, a marine benthic ecologist at the Royal Netherlands Institute for Sea Research in Yerseke, is trying to pin down the figures. He is using small tanks to measure the weathering of olivine in various conditions — including the impact of worms that live in and eat the sandy sediment. Montserrat’s experiments will test the idea that when these worms eat tiny grains of olivine they also help to break down the crust that can form on olivine’s surface, which slows down the weathering effect.

“You need to have some hard numbers to go to the authorities to say whether it will be safe enough to try it out,” he says. “We have good and very promising results, but there are still a lot of unknowns.”

Even advocates of this method of geo­engineering admit that large-scale enhanced weathering is not without risk. Olivine can contain toxic heavy metals such as nickel that could accumulate in the environment. Grinding rocks would produce dust, which might harm human health. And putting olivine into the sea could change the pH of the water, helping to combat ocean acidification driven by climate change but also potentially harming marine organisms by altering their environment.

Phil Renforth studies carbon sequestration and minerals at the University of Oxford, UK, and attended the Hamburg meeting. He says that there is a pressing need to conduct more work on enhanced weathering given that carbon emissions are likely to continue to rise, and because of the current focus on dealing with emissions by capturing them from power stations and storing them underground.

“We’re putting all our eggs in one basket if we’re only looking at one method,” he says. There’s a real need to diversify the portfolio.”

Pete Bucktrout/BAS

A 2012 attempt to drill through a 3-kilometre-thick Antarctic ice sheet faced technical difficulties.

Christmas Day 2012 was a very bad day for glaciologist Martin Siegert and his team of Antarctic researchers. After weeks of equipment failures, Siegert was forced to halt an ambitious attempt to drill into a lake deep beneath the West Antarctic Ice Sheet. “The decision was difficult to make but easy enough to call,” he noted in his field diary. Exhausted and disappointed, the team packed up.

• Originality takes second place for young scientists with extreme workloads
• Why flawed diamonds are advancing quantum computing
• How plant-killing fungi protect forest diversity

But it has not given up. Over the past year, researchers, engineers and officials involved in the US$12-million drilling project, funded by the UK Natural Environment Research Council, have carried out and responded to several internal reviews into the reasons for its failure. And now, in a paper under review by the Annals of Glaciology, Siegert, who is based at the University of Bristol, UK, and his colleagues have summarized the problems that they suffered at Lake Ellsworth and laid out options for putting them right. They think that several years of engineering work will be required to develop improved technology for a more reliable drill, but they say that success is achievable.

“I am glad to see that they plan to publish their drilling efforts,” says John Priscu, a glaciologist at Montana State University in Bozeman who has worked on similar lake-drilling projects in western Antarctica. “It will quell rumours and provide a solid bit of groundwork on which they can move forward.”

Lake Ellsworth is one of several hundred lakes beneath Antarctica’s ice sheets (see ‘Hidden lakes’). Scientists suspect that the extended basins, isolated for possibly millions of years, support specially adapted forms of life. Organisms that may thrive in the extreme environment could even bear clues as to the biology of extraterrestrial life, such as any that might exist in a suspected ocean beneath the icy surface of Europa, one of Jupiter’s moons.

Expand

The Ellsworth project was in planning for more than ten years. The teardrop-shaped lake, about 15 kilometres long and up to 156 metres deep, and in a valley, was extensively charted with seismic methods and ice-penetrating radar by the team before the drilling attempt.

After arriving at the lake in early December 2012, Siegert’s team had hoped to cut through the 3-kilometre-thick ice sheet in a single 72-hour effort. A specially developed hot-water drilling technique, devised by engineers at the British Antarctic Survey, was designed to minimize air and water pollution.

According to the paper, problems started when the boiler that was intended to melt large quantities of snow to provide hot water for the drill failed to work properly because of short-circuiting in its control panel. More severe problems followed. The two parallel drills — one to drill the main borehole to reach the lake, and one to create a reservoir cavity to recirculate drilling water — ran too slowly. Other failures, including of components designed to ensure vertical drilling, exacerbated the problems.

“The drilling was essentially undertaken blindly,” says Siegert. Probably because one or both holes were not drilled vertically, the cavity failed to link with the main borehole. Water also leaked into the cavity drill and froze the hose in the drill hole. Attempts to remove the hose failed, so it had to be cut. At that point, and with not enough fuel left to reach the lake, Siegert gave up.

His report into the project suggests a number of steps to improve the drilling system: sensors need to be thoroughly tested for reliability under conditions comparable to Antarctica’s harsh environment, spare parts must be available on the site, and a field team must include electrical engineers to assist with on-site operations.

The method is not fundamentally flawed, however. Last year, Priscu and his team successfully used a hot-water drill to explore Lake Whillans, a small body of water on the edge of the Ross Ice Shelf in western Antarctica. The lake, which is not as deep as Ellsworth, does seem to harbour microbial life (see Nature http://doi.org/q3m; 2013).

There are other reasons for pursuing hot-water drill technology. In 2012, Russian scientists broke into Lake Vostok, by far the largest of Antarctica’s hidden lakes, using a kerosene-fuelled drill. But their samples are spoiled with drill fluid and the bacteria they contain are probably contaminant species.

Consideration of what went wrong at Ellsworth should result in a revised plan for a return mission, says Mahlon Kennicutt, chair of the Lake Ellsworth advisory committee. The team hopes to formally propose a second attempt in five years.

“Antarctic science is by its nature risky,” says Kennicutt. “However, the potential gains in knowledge outweigh the costs and the risks in most cases, and this is especially true for the exploration of subglacial aquatic environments.”