It sounds like an environmentalist's dream: technology that can trap greenhouse gas before it harms the atmosphere. But the technique - carbon sequestration - is the darling of the Bush administration and Big Oil. So how green is it to store CO2 in oceans, forests, coal deposits, and rock formations? Here's a rundown of four methods.
SALINE AQUIFERS
Technology: CO2 captured from coal-fired power plants is injected into saline wells. The depth of these aquifers provides enough pressure to keep the CO2 in liquid form, and the reservoirs are permanently isolated under dense, impenetrable rock.
Potential storage: 500 billion tons in the US
Backed by: Electricity producers
Where: In New Haven, West Virginia, energy innovator Battelle is experimenting with storing CO2 produced by an American Electric Power plant.
Pros and cons: Reduces emissions but promotes fossil fuel power plants - leading to more emissions.
Green factor: Moderate
DEEP COAL SEAMS
Technology: CO2 is shot as deep as 4,500 feet into unminable coal beds, where it is quickly absorbed. Added bonus: The CO2 displaces methane, which can be burned as fuel.
Potential storage: The US sits on an estimated 4 trillion tons of coal, 90 percent of it unminable due to seam thickness and depth. Worldwide, such beds could sequester 150 billion tons of CO2.
Backed by: Coal industry and power producers
Where: New Mexico's San Juan Basin
Pros and cons: Encourages the use of cleaner-burning natural gas, but also increases the production of coal, one of the most polluting fossil fuels.
Green factor: Moderate
OFFSHORE SEABED AQUIFERS
Technology: CO2 is injected into natural undersea saline aquifers thousands of feet below the surface. An impervious bed of rock seals the CO2 inside.
Potential storage: Unknown, but could exceed all land-based storage methods
Backed by: Oil companies with offshore rigs
Where: Norway. Statoil deposits about 1 million tons of recovered CO2 annually into the Utsira Sand, a saline formation below the North Sea floor.
Pros and cons: A convenient way to store CO2 emissions from offshore drilling, but nobody knows the impact on sea life if the CO2 escapes.
Green factor: Low
FORESTS
Technology: Plant a tree. New forests increase the absorption of C02 through photosynthesis.
Potential storage: Over 40 years, an acre of additional forest removes 150 to 200 tons of CO2.
Backed by: Environmentalists and the energy industry, which gets good PR for enviro-friendliness
Where: The US has hundreds of forest-based projects; other major efforts are under way in Costa Rica and Indonesia.
Pros and cons: Preserves the environment, but some scientists worry that CO2 storage in forests isn't fail-safe. Trees die.
Green factor: High
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