But appearances--or invisibility, as the case may be--can deceive. Despite its tininess and commonness, the relationship of CO2 to global warming has made it one of the most scientifically and politically important substances in history.
The problem is this: An excess of CO2 in the atmosphere is trapping too much of the sun's heat. That heat, scientists say, is changing Earth's climate, which affects such things as the world's food and water supplies, economies and, thanks to the melting of the polar ice caps, the hundreds of millions of people living on the planet's coastlines.
Most of the excess CO2 comes from the burning of fossil fuels. The United Nations' International Panel on Climate Change reports that in just the past four decades, CO2 emissions have increased 80 percent. In 2006, the most recent year for which data was available, humans emitted just over 29 billion metric tons of CO2. The U.S. alone accounted for 6 billion metric tons. To put that into perspective, 1 billion tons equals the weight of 2,740 Empire State Buildings.
Some people see these statistics as evidence of environmental irresponsibility, but the scientists and engineers looking for solutions to the carbon problem see them as a challenge. If humans made the carbon problem, then humans can solve it.
Unfortunately, there is no silver-bullet solution. Scientists say fixing the carbon problem will require conservation, advances in renewable energies and lifestyle changes for people around the world. An array of solutions will have to work together to cut the carbon problem down to size.
One of the most promising solutions proposed so far is believed to be geologic carbon sequestration, the process of storing excess CO2 in underground rocks. Scientists say this technology, elements of which have been used for decades in the oil industry, could safely store hundreds of billions of tons of CO2, practically forever. However, sequestration's opponents argue that too little is known about the long-term effects of storing massive volumes of CO2 underground. On top of that, the ownership of and liability for underground storage space is still unclear. Gov. Brian Schweitzer and the Montana Legislature have just begun sorting through those legal issues. Meanwhile, scientists continue work on the technology behind sequestration.
What is clear about sequestration is that it will not be cheap, and so far it remains mostly untested on the massive scale that will be necessary to significantly reduce the world's net CO2 emissions.
But scientists and engineers at Montana State University and its partners are researching ways to make sequestration possible. Since 2003, almost two dozen researchers in departments across MSU have been building on the university's experience with agriculture-related sequestration and expanding into geologic sequestration. Using advanced geology, biochemistry, computer imaging and fieldwork, these scientists hope to make sequestration a reality in the 21st century and buy other scientists the time they need to build humanity's carbon-neutral future.
Subsurface geology: Knowing the hole
MSU researcher Dave Bowen leads geologic research for the Big Sky Carbon Sequestration Partnership, a research group based at MSU. The partnership includes three national laboratories, a dozen universities, industry partners, tribal nations and international research groups. All are working together on a trio of projects to determine the viability of sequestration in the northwestern U.S. It is one of seven regional partnerships across the country funded by the U.S. Department of Energy.
Since it formed in 2003, the partnership has received three major awards from the Department of Energy, totaling more than $85 million. Of that, almost $19 million has stayed at MSU.
Bowen's specialty is stratigraphy, the science of making three-dimensional maps of the underground. These maps, he said, are like a pile of high-resolution photos, each one representing a period frozen in time.
"You can look at it and understand what the landscape was and how it evolved over time," he said.
What can you learn about carbon sequestration from maps of the subsurface? For one, you can learn how fluids move when they're underground, a concept that's vital to keeping CO2 securely stowed, Bowen said.
Geologic carbon sequestration starts at a "point source," a power plant or other source of CO2. There, CO2 is filtered out of the plant's waste gases and then compressed to around 1,000 pounds per square inch of pressure, turning the gas into a liquid-like state. This "supercritical" CO2 then is transported to a sequestration site and pumped underground, where it behaves like any other liquid.
Bowen's 3-D maps help him understand how porous the subsurface is, which allows him to predict how CO2 will move through the ground after injected. The maps also help Bowen determine how much CO2 will fit into below-ground storage sites in the Big Sky Partnership's region.
In fact, estimating the region's CO2 capacity was the first stage of the partnership's work. Its estimates show that existing oil and gas fields in the region--ideal sequestration sites--could hold as much as 1 billion metric tons of CO2, and the region's basalt formations could hold even more--33 to 134 billion metric tons.
Capacity is not really an issue, Bowen said. The region likely has more than enough subsurface space to hold all of its own CO2 emissions--and then some.
The real issue is the volume of CO2 that would be injected into the ground. Oil and gas companies have been injecting CO2 underground for decades to enhance production, and those companies even have stored their products underground temporarily. But underground storage never has been tried with the volumes--billions of tons--sequestration will require to be useful. That makes understanding the subsurface even more important, he said.
Pore space: Small spaces for a big problem
One of the misconceptions about sequestration is that there are massive empty spaces underground where supercritical CO2 will be stored, similar to an underground storage tank.
"There's not some big cave under there," said Lee Spangler, director of the Big Sky Partnership and of MSU's Energy Research Institute. "We're storing CO2 in the pore space, and there are a lot of mechanisms that will keep it underground."
First among those mechanisms, Spangler explained, is the underground environment itself. Sites are chosen so that an impermeable, nonporous layer of rock is above the storage level. This "cap rock" physically holds the CO2 underground.
Upon injection, the CO2 immediately finds its way into the rock's pores, the tiny, porous veins that form naturally in such minerals as basalt and sandstone. If kept at depth long enough, the CO2 eventually will dissolve into the deep groundwater or "brine," an undrinkable mix of dissolved minerals. That CO2-laden brine is heavier than the water around it, and so it sinks beneath the normal brine, Spangler said.
Eventually, the CO2 reacts with the minerals in the brine and turns into a solid form, usually limestone.
How long does it all take? The CO2 is trapped beneath the cap rock immediately. Decades and centuries will see the CO2 dissolve and sink. It will take further centuries or millennia to see it permanently trapped in limestone, Spangler said.
Planning for safety
All that means that sequestration doesn't end after the last ton of CO2 has been injected. The next step is monitoring the site, and many scientists at MSU and elsewhere are working on what Spangler calls "just-in-case" technologies, instruments to scan sequestration sites and watch for any escaping CO2.
Spangler emphasized that properly chosen sequestration sites don't leak, but even if they did, a CO2 leak would pose almost no danger to humans or the environment. Leaks would, however, be costly to the companies that spent millions of dollars to put the CO2 underground in the first place. Letting CO2 escape would be like allowing an investment in equipment and technology just float away.
Last summer, MSU hosted a month-long series of experiments to test more than 20 different technologies for detecting leaks. The experiments drew researchers from around the world and tested devices ranging from sensors to measure the stress on plants caused by excess CO2 to advanced laser detection systems.
The experiments took place just west of MSU, where the university has built a one-of-a-kind test site. There, a 240-foot pipe buried in the ground releases small amounts of CO2 to simulate leaks. The site provides researchers with conditions not yet available in the real world, Spangler said.
"The problem is that when surface detection has been deployed at sequestration sites, those sites have been very well chosen and don't leak, so there's nothing to detect," he said. "This site allows us to test detection technologies to make certain they will work."
Biofilm barriers: Gumming up the works
"The whole business of carbon sequestration needs to have ways to stop leakage before it occurs," said Al Cunningham, a civil engineering professor and founding member of MSU's Center for Biofilm Engineering, who is working on another just-in-case technology.
Cunningham works with biofilms, colonies of bacteria living together in a carbohydrate slime. Biofilms are extremely common and form on most wet surfaces.
It's the slime--known as "extracellular polymer"--that holds one of the keys to sealing the tiny fractures in the rock that sequestered CO2 might use to escape back to the surface. The polymer is a non-living, slimy, glue-like substance that "basically plugs up the free pore space," Cunningham said.
All that would be needed would be to inject bacteria into the sequestration site and feed them. The bacteria would then do their normal thing, in the process producing enough polymer to form a "biofilm barrier" that seals the site.
Biofilm barriers are not a new idea. For 15 years, scientists at the biofilm center have been working on ways to use biofilms to prevent the spread of groundwater contamination and even to degrade those contaminants.
But it's not all about just sliming the rocks.
"We've discovered a certain kind of bacteria that will precipitate calcium carbonate, given the right conditions," he said.
That means that, under the right conditions, the barrier bacteria will turn a small amount of sequestered CO2 into limestone, forming new rock to more effectively seal the site.
Cunningham said this "biomineralization" process takes only a few days and can use common, easy-to-grow bacteria. The hope is to someday find bacteria already living beneath sequestration sites that can do the job, which would save having to pump them underground.
Cunningham said the biofilm center could be ready for large-scale tests in a couple of years.
So could this process be used to turn large volumes of CO2 into limestone? Cunningham didn't rule out the possibility, but he pointed out that even if it were possible to use biomineralization on a massive scale, it would not solve the root problem: too much CO2 being emitted into the atmosphere.
"The idea behind sequestration is to buy civilization time to convert to energy sources that don't release carbon," he said.
The partnership's plans
The Big Sky Partnership's sequestration work proceeds in phases. The first phase estimated the storage capacity of the partnership's region, which includes Montana, Wyoming, Idaho and parts of Oregon, Washington and the Dakotas.
The second phase is an actual drill site, located west of Walla Walla, Wash., in the town of Wallula, where the partnership will pump 3,000 metric tons of CO2 into a massive basalt formation that underlies the Columbia River Plateau. Drilling on Phase II began in January.
Researchers working on Phase II hope to learn whether basalt formations like the one near Wallula will make good commercial sequestration sites. Lab tests show that CO2 turns into solid minerals rapidly in basalts. Researchers want to see if this works underground as well.
The third phase began in November, when the Department of Energy awarded the partnership $67 million to fund an eight-year project that calls for injecting a million tons of CO2 per year for three years into a sandstone formation 11,000 feet below the surface near Big Piney, Wyo. The other five years of the project will be spent studying the site and the results of the injection. Phase III work will begin sometime in the next year or two.
Scientists on Phase III want to learn how the sandstone formation responds to being injected with a commercial-sized volume of CO2, which will be piped from a nearby natural gas processing plant. Importantly, they also want to evaluate the process, from transporting to storing, to learn more about how it could be adapted for commercial use.
Economics: Follow the money
Few people in Montana know more about the potential cost of sequestration than John Antle, an agricultural economics professor at MSU and the partnership's lead economist.
"Sequestration is very expensive," he said, "because no one has tried very hard yet to figure out how to do it cheaply."
Putting carbon into the ground is only part of sequestration. That part is doable, Antle said. What's uncertain, he said, is whether companies will put up the money to make it happen.
Right now, without taxes on carbon emissions or government regulations to cap emissions, the only real motivation for businesses to look at sequestration is to create good will or to speculate on sequestration's future.
"Neither of those is enough incentive," said Antle, who also studies the potential for sequestering carbon in forest growth and agricultural soils. "The government will need to pass legislation that makes carbon emissions costly so there's an incentive to reduce them."
While Antle doesn't have a lot of data to plug into his economic models yet, he does think that sequestration is affordable on a national scale. Not cheap, but affordable.
"It's certainly not going to shut down the economy as some people would have you believe," he said. "That's just the naysayers trying to scare people off."
But establishing serious sequestration in the U.S. will be akin to building a new industry such as the railroads or telephone system, Antle said. And while the basic science makes building such systems possible, that's not enough.
"Basic science is great, but we also need science motivated to solve a problem," he said. "And we need to demonstrate that it's economically feasible."
Coal: The reason we need sequestration
Montana holds a quarter of the country's coal reserves and one-sixteenth of the world's supply. That reserve represents enormous revenue for the state, Spangler said. It also represents a lot of potential CO2 emissions.
The U.S. does want to develop renewable energy, he said. The problem is that until supplies of renewable energy become more consistent and plentiful, fossil fuels--including the estimated 200 years worth of coal Earth holds--will remain the backbone of the country's energy supply.
"If we're going to use all this coal, it would be best to use it in a climate-friendly fashion," he said. That means sequestering the CO2 generated by burning that coal.
Developing methods to deal with that CO2 is important, Spangler said, considering recent industrial growth around the world. China, in particular, stands out as a country that needs ways to manage its emissions.
China's construction of coal-burning plants has boomed, so much so that China overtook the U.S. as the world's top CO2 emitter in 2008. China's coal consumption has doubled since 2000, and if a way to clean up its emissions isn't found, experts say China's CO2 emissions will outweigh those of all other industrialized countries over the next 25 years.
So finding methods to sequester carbon is not just important to Montana and the U.S., Spangler said. It's vital to buying the time the world needs to develop technologies that don't emit carbon at all.
"It provides the short-term bridge for the long-term solution to be developed," he said.