Three types of technologies have been proposed as solutions to the challenge of global climate change. The first is clean energy technology (for example, wind, solar, or biofuels), which can produce electricity or power cars without burning fossil fuels. The second is conservation: everything from turning off unused lights to designing more energy efficient buildings and appliances can reduce the carbon footprint of homeowners and corporations. A third option is to permanently remove greenhouse gases from the atmosphere by sequestration, or locking them up into a stable reservoir. This is a familiar concept; a few years ago it was trendy for rock stars to pay for the planting of trees to offset the carbon emissions from their tours. However, environmental groups quickly raised an obvious concern: trees rot when they die, releasing all of the carbon they removed from the atmosphere!
That’s why it’s exciting that geologists Peter Keleman and Jurg Matter of Columbia University have proposed an innovative method to remove carbon dioxide (CO2) from the atmosphere. They suggest using a rock called peridotite, which naturally reacts with CO2, to act as a global filter, removing some of our emissions from the atmosphere. This could form a permanent reservoir for CO2 and add to a growing toolkit of solutions for global climate change.
The level of CO2 in the earth’s atmosphere has increased by 36% when compared to pre-industrial levels (about 300 years ago). This CO2 mostly comes from burning fossil fuels for energy, although other factors such as changing land use patterns have contributed to the increase. Rising levels of CO2 are a concern because the gas is transparent to shorter wavelength visible light from the sun, and allows this light to reach Earth’s surface, but CO2 traps the longer wavelength heat energy that the earth radiates out to the atmosphere. Thus, even a small increase in CO2 results in more heat being trapped on Earth. Observations of global temperature and other climate factors leave no scientific doubt that humans are causing global climate change.
This raises the question: what should be done about it? In spite of public concern and political will in some parts of the world for major measures to combat climate change, global CO2 emissions are projected to increase from 28.1 trillion kilograms (kg) annually in 2005 to 42.3 trillion kg per year in 2030. Most of this increase is predicted to come from the developing world, where reducing poverty will remain a higher political priority than cutting emissions. The need for technologies that either reduce emissions or remove atmospheric CO2 in a cost-effective manner will therefore only increase.
Old Rocks and New Ideas
Keleman and Matter developed their idea for carbon sequestration based on their research in the Sultanate of Oman, a country on the southeast tip of the Arabian Peninsula. They went to Oman to investigate one of the largest surface formations of a rock called peridotite, which is composed of minerals that naturally react with CO2, removing it from the atmosphere. They set out to learn how much CO2 this huge peridotite outcrop, about 200 by 50 kilometers (km) in area, naturally removes from the atmosphere.
Peridotite is relatively uncommon on the surface of the earth; it is generally found below the earth’s crust in a layer called the mantle. In a few places – such as Oman, Papua New Guinea, and Greece – collisions of the earth’s tectonic plates have brought large chunks of mantle peridotite close to the surface. When they are exposed to water, the minerals that form peridotite start to react with gases dissolved in the water, undergoing a process known as chemical weathering. In one common weathering reaction, the minerals react with CO2 to become carbonate minerals. The CO2 for this reaction generally comes from the atmosphere. Limestone and marble are common examples of rocks made of carbonate minerals. Carbonates are stable, so CO2 that reacts with peridotite will not re-enter the atmosphere.
In the November 11, 2008 issue of the Proceedings of the National Academy of Sciences, Keleman and Matter reported that carbon sequestration through the reaction of peridotite with CO2 is happening quickly in Oman. They estimated the current natural rate of carbon sequestration at approximately 40 million kg of atmospheric CO2 per year. This is only a tiny fraction of the 28.1 trillion kg of CO2 emitted by human activities in 2005, but just as you can increase the amount of oil you get out of the earth by drilling, Keleman and Matter reasoned that perhaps we can increase the rate of this reaction with some tricks used by the oil industry.
Before we talk about oil, let’s consider how to increase the rate of a reaction. One way to increase the rate of a reaction is to increase the surface area of a solid reactant by breaking it down. Just as fine-grained table salt takes less time to dissolve in water than coarse rock salt, smaller chunks of peridotite will react with CO2 faster. Another way to increase the reaction rate is to change its temperature, for example more salt dissolves in water at higher temperatures. Similarly, heating the environment of the peridotite increases the rate of weathering reactions.
Keleman and Matter propose that these techniques combined could be useful to increase the rate of carbon sequestration in peridotite, but how should it be done on a massive scale? As oil extraction became more sophisticated, geologists and engineers developed a method called hydraulic fracture, in which water is pumped down a well to “crack” rock that holds oil and create spaces for the oil to seep towards the well. Using this technique solves the first problem of making the size of the chunks of peridotite smaller, without having to dig them all up and grind them mechanically, which is too costly.
The challenge of increasing the temperature of the reaction is slightly more complicated. Fortunately, the reaction of peridotite minerals to carbonates is exothermic, meaning it releases heat! Keleman and Matter propose a two-step process to speed up the reactions that remove CO2 from the atmosphere. They propose hydraulic fracturing of the peridotite rock and then pumping concentrated CO2 and hot water down the borehole. The hot water and high concentration of CO2 would speed up the natural weathering reaction. The reaction would release heat and increase the temperature further, eliminating the need to pre-heat the water. At that point the reaction would be fast and could be controlled by one factor: the amount of water pumped into the borehole. The authors argue that this technique, in Oman alone, could consume more than 1 trillion kg of CO2 per year, a substantial fraction of the world’s total emissions!
Exciting Ideas and Challenges
You have probably heard the adage that genius is one percent inspiration, ninety-nine percent perspiration. This exciting new idea is no different. While peridotite represents a promising natural “dump” to hold our excess carbon, there are technical, economic, and political hurdles that remain. Although this idea is very exciting, field trials have not yet been conducted. Furthermore, the current proposal for carbon sequestration calls for concentrating the carbon to inject it below the surface, which is an energy-intensive procedure. Because of the high startup costs in drilling, obtaining enough funding for field tests may be difficult, although oil companies with the expertise and resources to conduct trials might be convinced to help.
The ongoing recession may lead to stimulus spending on research and development for renewable energy, but projects such as this one could benefit as well. On the other hand, falling oil prices and tighter budgets could cut private demand for renewable energy, while this project would probably always need the help of public funding. Politically, convincing governments to get behind centralized carbon sequestration projects might be easier than convincing them to make economically painful decisions to cap emissions, but that remains to be seen. There is still no “magic bullet” to stop climate change, but at least there are plenty of ideas.
–Marshall P. Thomas, Harvard Medical School
For More Information:
The EPA has good background information about climate change science, policy, and the economics of climate change:
< http://www.epa.gov/climatechange/ >
The Department of Energy also has extensive information about climate change and research into clean energy and carbon sequestration technologies:
< http://www.energy.gov/environment/climatechange.htm >
Keleman, P and Matter, J. In situ carbonation of peridotite for CO2 storage. PNAS. 105:17295-17300. (2008)