With every gallon of gas it burns, your car exhausts roughly 20 pounds of carbon dioxide (CO2) [source: United States Dept. of Energy]. With 531 million cars worldwide in 2002, that can add up to a lot of CO2 released into the atmosphere pretty quickly [source: World Watch]. This is not to mention the carbon resulting from all of the coal-fire power plants, cleared forests, cow manure from farms and other sources. In total, we humans emit somewhere around 6 billion metric tons of CO2 each year [source: U.S. Dept. of Energy].
It's not that the Earth can't handle a little carbon dioxide. Just because people can't breathe pure CO2 doesn't mean it's bad. Plants love the stuff, using carbon dioxide as fuel for photosynthesis and emitting precious oxygen as waste. Photosynthesis is one part of the carbon cycle, one of Earth's biogeochemical processes.
Through this process, the extant carbon on the planet is shuffled from one place to another. Soil, oceans and the atmosphere all store carbon temporarily. Along the way, living organisms ingest CO2, effectively making them storehouses as well.
But what happens when the surplus gets to be too much? We don't really know what will happen if carbon stores eventually become flooded, as it appears they will with the accelerated rate at which we're releasing carbon dioxide.
Keep in mind, your car isn't generating the carbon dioxide released into the atmosphere. It was stored in the oil drilled out of the ground, and in the gasoline refined from it. But by burning it for energy, humans release it.
So if we're releasing too much CO2 into the atmosphere, can't we just capture it and stash it somewhere? Yes. Read about some plans to do just that on the next page.
Carbon Dioxide Capture and Storage
Carbon dioxide is important to the ecosystem, and not everyone's concerned about our premature introduction of carbon dioxide into the atmosphere. Some climate skeptics don't believe that global warming is a result of burning fossil fuels. But as studies of the effects on atmospheric carbon dioxide introduction from anthropogenic (human) sources build, more members of the scientific community are looking at ways of alleviating the stress that we appear to be putting on the carbon cycle.
To offset our CO2 emissions, some suggest capturing gaseous carbon dioxide before it can escape into the atmosphere. The ideal locations for carbon capture and storage (CCS) systems are at the very power plants that emit tons of carbon dioxide every day.
There are three different types of carbon capture: pre-combustion, post-combustion and oxyfuel combustion. The pre-combustion method requires separating the carbon dioxide from original energy sources, so it's not present when the fuel is burned. Post-combustion systems capture CO2 after it is burned off as waste but before it leaves a power plant's flue. Oxyfuel combustion adds nearly pure oxygen to captured CO2, and when burned together, easily separates the carbon dioxide out, making it easier to capture as waste.
While carbon capture and storage systems also require fossil fuel for energy, releasing more CO2, the United Nations' Intergovernmental Panel on Climate Change estimates that a power plant outfitted with a self-contained CCS system could successfully reduce net CO2 emissions by 85 to 95 percent [source: IPCC].
Once we've captured the carbon dioxide we emit, where do we put it? One suggested location is at the bottom of the ocean. The concept of deep ocean CO2 storage was first proposed by Dr. Michael Pilson. The theory is simple at its core: Liquefy the gaseous CO2, and introduce it to the ocean floor. The atmospheric pressure and low temperatures found in the deep ocean environment should keep the liquid CO2negatively buoyant, meaning it will sink rather than float. A predictable reaction between liquid CO2 and water under high pressure and low temperature causes carbon dioxide to turn into an icy compound called clathrate hydrate. In this form, the CO2 should maintain its integrity, preventing it from being absorbed by the seawater.
Toward the end of the 20th century, experiments were conducted to see if deep ocean carbon storage was feasible. Researchers at the Monterey Bay Aquarium Research Institute introduced liquid carbon dioxide to a beaker on the ocean floor at 3,600 meters (around 12,000 feet). The CO2 grew in volume and broke apart into globs, which were swept away by the current [source: CNN]. The plan needed revision: Releasing large amounts of carbon dioxide into the ocean can upset the ocean's ecosystem.
In 2008, a group of researchers from across the United States had an idea: It was fine for liquefied carbon dioxide to mimic a lava lamp, so long as it was stored safely and confined to areas with little or no marine life. Could they store the CO2 in giant bags? Read about this promising concept on the next page.
Carbon Dioxide Storage on the Abyssal Plain
The abyssal plain is a vast expanse of ocean that stretches from bottom of the continental slopes to the trench zones, where deep canyons like the Marianas Trench cut through the ocean floor. The abyssal plain begins at a depth of 4,000 meters (around 2.5 miles below sea level). There is little or no light to allow photosynthesis to take place, which means there's no native plant life. The food supply is made up of dead and decaying plant and animal matter that's sunk to the ocean floor. The animal population along the abyssal plain is sparse, and the area is large and generally flat.
At this depth, the temperature hovers around 2 degrees Celsius (35.6 degrees Fahrenheit) and the atmospheric pressure exerted by the force of gravity is 5,880 pounds per square inch (413.3 kg/sq. cm) [source: University of Hawaii]. That's a much different environment than what we're used to at sea level, where the atmospheric pressure is 14.7 pounds per square inch (1.03 kg/sq. cm) [source: Texas A&M University], and the average global temperature was 14.77 degrees Celsius (58.6 degrees Fahrenheit) in 2005 [source: Earth Policy Institute]. Featuring these conditions, the abyssal plain is an ideal location to store liquefied carbon dioxide.
Dr. David Keith proposed that the abyssal plain be the site for huge bags made of polymers, about 600 feet (183 meters) in diameter to serve as storage containers for liquid carbon dioxide. The CO2 would be delivered to the ocean via pipeline, like crude oil is delivered to refineries. Each bag could hold about two days' worth of the world's carbon dioxide emissions -- 160 million metric tons [source: Natural Sciences and Engineering Research Council]. One of the things that makes Keith's proposal so attractive is that the technology to make it happen already exists. We currently have CO2 pipeline delivery system technology, and both pre- and post-combustion capture systems are already extant.
Keith spoke about his idea in a lecture to the American Association for the Advancement of Science in February 2008. If his concept is put into practice, the giant containment bags should prevent damage to the oceanic ecosystem by preventing the release of large amounts of CO2 into the ocean. Keith says the carbon dioxide's negative buoyancy would keep the gas from surfacing [source: Natural Sciences and Engineering Research Council].
With the amount of storage space needed to contain the world's CO2 emissions, the abyssal plain may be only one locale where we keep our carbon dioxide. Depositories deep in the Earth's crust are another location being evaluated, which makes sense, since this is where most of the fossil fuel we refine came from in the first place.
Simply storing the CO2 might sound like sweeping a problem under a rug, but it's difficult to say what technology humanity will have at its disposal one or two centuries from now. It's possible we will have discovered some use for the compound we don't currently have. It could prove to be an energy source in the future. If peak oil theorists are correct, we will have largely discontinued our use of fossil fuels and the carbon cycle may be able to handle slow releases of CO2 from storage.
Interestingly, we may have also developed a way to recreate the situation that produced our fossil fuels in the first place. Using gravity and carbon, we could theoretically synthesize fossil fuels. By capturing the CO2 emitted and reusing it as an ingredient in this synthesis, we could create a closed system that could meet global energy needs without adversely affecting the carbon cycle. To successfully pull off such a system, we would need plenty of CO2 in the future. If Keith's bag system works, it will be there waiting.
For more information on Earth processes and other related topics, visit the next page.
CO2 Problem: Author's Note
I pitched this article several years ago after hearing about the concept of carbon capture and storage. The whole concept is beautiful: We keep burning fossil fuels as much as we like, but we capture the carbon dioxide before it can pollute the air and we store it. In this scenario, the storage is in large bags at the sea bottom, where temperatures are cool enough and the pressure great enough that captured CO2 gas will convert to a globby, semi-solid state that is easier to contain.
I also came across other ideas for where we can store carbon dioxide, like in empty aquifers, and a number of ways to capture it, like before, during or after combustion. But all of them gave me the same sense of excitement, that humans could not only use, but also capture and reuse their waste. The end goal of carbon capture and sequestration, I learned, is to form a closed circle, where spent CO2 is repressurized into useable carbon fuel again and again. Not only does it cut down on pollution, it provides energy security as well. Years later, I came across this example as a central theme of the new Anthropocene age of geology -- humans using ingenuity to both exploit and protect the planet, causing as little damage as possible along the way.
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CO2 Problem: Cheat Sheet
Stuff you need to know:
- Humans emit about 6 billion metric tons of carbon dioxide each year.
- A movement is afoot to begin carbon capture and sequestration programs to contain and store waste carbon dioxide from power plants and ultimately cars and other transport.
- One major proposal for storing captured CO2 is in large bags at the bottom of the ocean.
- Each of these massive bags could hold about 160 million metric tons of CO2, about two days' worth of global emissions.
- At these depths, CO2 will convert to a semi-solid state, making it easy to maintain in storage than in a gaseous state.
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