IBRA, Oman — In
the arid vastness of this corner of the Arabian Peninsula, out where
goats and the occasional camel roam, rocks form the backdrop practically
every way you look.
But the stark outcrops and craggy ridges are
more than just scenery. Some of these rocks are hard at work, naturally
reacting with carbon dioxide from the atmosphere and turning it into
stone.
Veins of white carbonate minerals run through
slabs of dark rock like fat marbling a steak. Carbonate surrounds
pebbles and cobbles, turning ordinary gravel into natural mosaics.
Even pooled spring water that has bubbled up through the rocks reacts with CO2 to produce an ice-like crust of carbonate that, if broken, re-forms within days.
Scientists say that if this natural process, called carbon
mineralization, could be harnessed, accelerated and applied
inexpensively on a huge scale — admittedly some very big “ifs” — it
could help fight climate change. Rocks could remove some of the billions
of tons of heat-trapping carbon dioxide that humans have pumped into
the air since the beginning of the Industrial Age.
And by turning that CO2 into stone, the
rocks in Oman — or in a number of other places around the world that
have similar geological formations — would ensure that the gas stayed
out of the atmosphere forever.
“Solid carbonate minerals aren’t going anyplace,” said Peter B. Kelemen,
a geologist at Columbia University’s Lamont-Doherty Earth Observatory
who has been studying the rocks here for more than two decades.
Capturing and storing carbon dioxide is drawing
increased interest. The Intergovernmental Panel on Climate Change says
that deploying such technology is essential to efforts to rein in global
warming. But the idea has barely caught on: There are fewer than 20
large-scale projects in operation around the world, and they remove CO2 from the burning of fossil fuels at power plants or from other industrial processes and store it as gas underground.
What Dr. Kelemen and others have in mind is
removing carbon dioxide that is already in the air, to halt or reverse
the gradual increase in atmospheric CO2 concentration.
Direct-air capture, as it is known, is sometimes described as a form of
geoengineering — deliberate manipulation of the climate — although that
term is more often reserved for the idea of reducing warming by
reflecting more sunlight away from the earth.
Although many researchers dismiss direct-air
capture as logistically or economically impractical, especially given
the billions of tons of gas that would have to be removed to have an
impact, some say it may have to be considered if other efforts to
counter global warming are ineffective.
A few researchers and companies have built machines that can pull CO2 out of the air, in relatively small quantities, but adapting and enhancing natural capture processes using rocks is less developed.
“This one still feels like the most nascent piece of the conversation,” said Noah Deich, executive director of the Center for Carbon Removal, a research organization in Berkeley, Calif. “You see these sparks, but I don’t see anything catching fire yet.”
Dr. Kelemen is one of a relative handful of
researchers around the world who are studying the idea. At a geothermal
power plant in Iceland, after several years of experimentation,
an energy company is currently injecting modest amounts of carbon
dioxide into volcanic rock, where it becomes mineralized. Dutch
researchers have suggested spreading a kind of crushed rock along coastlines to capture CO2. And scientists in Canada and South Africa are studying ways to use mine wastes, called tailings, to do the same thing.
“It’s clear that we’re going to have to remove carbon dioxide from the atmosphere,” said Roger Aines,
who leads the development of carbon management technologies at Lawrence
Livermore National Laboratory in California and has worked with Dr.
Kelemen and others. “And we’re going to have to do it on a gigaton
scale.”
If billions of tons of CO2 are to be
turned to stone, there are few places in the world more suitable than
Oman, a sultanate with a population of 4 million and an economy based on
oil and, increasingly, tourism.
The carbon-capturing formations here, consisting
largely of a rock called peridotite, are in a slice of oceanic crust and
the mantle layer below it that was thrust up on land by tectonic forces
nearly 100 million years ago. Erosion has resulted in a patchy zone
about 200 miles long, up to 25 miles wide and several miles thick in the
northern part of the country, including here in the outskirts of Ibra, a
dusty inland city of 50,000. Even the bustling capital, Muscat, on the
Gulf of Oman, has a pocket of peridotite practically overlooking Sultan
Qaboos bin Said’s palace.
Peridotite normally is miles below the earth’s
surface. When the rocks are exposed to air or water as they are here,
Dr. Kelemen said, they are like a giant battery with a lot of chemical
potential. “They’re really, really far from equilibrium with the
atmosphere and surface water,” he said.
The rocks are so extensive, Dr. Kelemen said, that
if it was somehow possible to fully use them they could store hundreds
of years of CO2 emissions. More realistically, he said, Oman could store at least a billion tons of CO2 annually. (Current yearly worldwide emissions are close to 40 billion tons.)
While the formations here are special, they are
not unique. Similar though smaller ones are found in Northern
California, Papua New Guinea and Albania, among other places.
Dr. Kelemen first came to Oman in the 1990s, as
the thrust-up rocks were one of the best sites in the world to study
what was then his area of research, the formation and structure of the
earth’s crust. He’d noticed the carbonate veins but thought they must be
millions of years old.
“There was a feeling that carbon mineralization was really slow and not worth thinking about,” he said.
But in 2007, he had some of the carbonate
dated. Almost all of it was less than 50,000 years old, suggesting that
the mineralization process was actually much faster.
“So then I said, O.K., this is pretty cool,” Dr. Kelemen said.
Since then, in addition to continuing his crust research, he has spent
much time studying the prospects for harnessing the mineralization
process — among other things, learning about the water chemistry, which
changes as it flows through the rocks, and measuring the actual uptake
of CO2 from the air in certain spots.
For much of this decade he has also led a
multinational effort to drill boreholes in the rock, a $4 million
project that is only partly related to carbon capture. In March the
drilling was nearing completion, with scientists and technicians sending
instruments down the holes, which are up to 1,300 feet deep, to better
characterize the rock layers.
The rocks here may be capable of capturing a
lot of carbon dioxide, but the challenge is doing it much faster than
nature, in huge amounts and at low enough cost to make it more than a
pipe dream. Dr. Kelemen and his colleagues, including Juerg Matter, a researcher from the University of Southampton in England who was involved in the Icelandic project, have some ideas.
One possibility, Dr. Kelemen said, would be to drill pairs of wells and pump water with dissolved CO2
into one of them. As the water traveled through the rock formation
carbonate would form; when it reached the other well the water, now
depleted of CO2, would be pumped out. The process could be repeated over and over.
There is a lot that is unknown about such an
approach, however. For one thing, while pumping water deep into the
earth, where temperatures and pressures are higher, could make the
process of mineralization go tens of thousands of times faster, so much
carbonate might form that the water flow would stop. “You might clog
everything up, and it would all come to a screeching halt,” Dr. Kelemen
said.
Experiments and eventually pilot projects are needed to better
understand and optimize this process and others, Dr. Kelemen said, but
so far Omani officials have been reluctant to grant the necessary
permits. The researchers may need to go elsewhere, like California,
where the rocks are less accessible but the state government has set
ambitious targets for reducing emissions and is open to new ways to meet
them.
Dr. Kelemen and Dr. Aines have had preliminary
discussions with California officials about the possibility of
experimenting there. “We would certainly be a willing and eager partner
to help them with it,” said David Bunn, director of the State Department
of Conservation.
Perhaps the simplest way to use rocks to
capture carbon dioxide would be to quarry large amounts of them, grind
them into fine particles and spread them out to expose them to the air.
The material could be turned over from time to time to expose fresh
surfaces, or perhaps air with a higher CO2 concentration could be pumped into it to speed up the process.
But a quarrying and grinding operation of the
scale required would be hugely expensive, scar the landscape and produce
enormous CO2 emissions of its own. So a few researchers are
asking, Why not use rocks that have already been quarried and ground up
for other purposes?
Such rocks are found in large amounts at mines
around the world, as waste tailings. Platinum, nickel and diamonds, in
particular, are mined from rock that has a lot of carbon-mineralization
potential.
Gregory Dipple,
a researcher at the University of British Columbia who has been
studying mine tailings for more than a decade, said early on he found
evidence that waste rock was forming carbonate without any human
intervention. “It was clear it was taking CO2 from the air,” he said.
Dr. Dipple is now working with several mining
companies and studying ways to improve upon the natural process. The
goal would be to capture at least enough CO2 to fully offset a mine’s carbon emissions, which typically come from trucks and on-site power generation.
Evelyn Mervine, who has worked with Dr. Dipple and Dr. Kelemen and now works for De Beers, the world’s largest diamond company, is studying a similar approach and hopes by next year to conduct trials at one or more of the company’s mines.
“We don’t think from a scientific perspective
it would be that difficult or expensive — we can be carbon-neutral,” she
said. “And in the mining industry that is extraordinary.”
“Relative to the global problem, it’s really
just a drop in the bucket,” Dr. Mervine said. “But it sets a really good
precedent.”
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