Geologists have mapped 6,000 miles of large rock formations in the United States that could be used to store some of the excess carbon dioxide building up in Earth’s atmosphere. The carbon dioxide released by fossil fuel burning has been continually accumulating in the atmosphere since the start of the Industrial Revolution. While some of the greenhouse gas is taken up by plants and absorbed by the ocean, a significant amount is still hanging out in the air, trapping some of the heat that Earth's surface would otherwise radiate to space and thereby warming the globe. Scientists and engineers have proposed several ways to artificially trap and store some of this excess carbon dioxide in underground aquifers and other large rock formations. Now scientists at Columbia University’s Earth Institute and the U.S. Geological Survey have surveyed the United States and found 6,000 square miles (15,500 square kilometers) of so-called ultramafic rocks at or near the surface that could be ideal for storing the excess gas. The locations of the rocks are detailed in a USGS report. Originating deep in the earth, these rocks contain minerals that react naturally with carbon dioxide to form solid minerals, a process called mineral carbonation that could make for an ideal storage mechanism. Other so-called carbon sequestration schemes have focused on storing carbon dioxide in liquid or gas form, but these proposals have met with concerns about leaks. The major drawback to natural mineral carbonation is its slow pace: normally, it takes thousands of years for rocks to react with sizable quantities of carbon dioxide. But scientists are experimenting with ways to speed the reaction up by dissolving carbon dioxide in water and injecting it into the rock, as well as capturing heat generated by the reaction to accelerate the process. "It offers a way to permanently get rid of carbon dioxide emissions,” said Juerg Matter, a scientist at Columbia’s Lamont-Doherty Earth Observatory, where a range of projects to tackle the issue is underway. The United States' ultramafic rocks could be enough to stash more than 500 years of U.S. carbon dioxide production, said the report's lead author, Sam Krevor, a graduate student working through the Earth Institute's Lenfest Center for Sustainable Energy. Most of the locations are conveniently clustered in strips along the east and west coasts — some near major cities including New York, Baltimore and San Francisco. "We're trying to show that anyone within a reasonable distance of these rock formations could use this process to sequester as much carbon dioxide as possible," Krevor said. Klaus Lackner, who helped originate the idea of mineral sequestration in the 1990s, hopes a global mapping effort can be undertaken to find more such storage areas. "It's a really big step forward," he said. Another rock, common volcanic basalt, also reacts with carbon dioxide, and efforts are underway to map this rock type in detail as well.

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