Too Much Gas

PROBLEM: Can carbon sequestration reduce global levels
of carbon dioxide?

Long before global warming was a serious part of the political debate, ORNL researchers were logging greenhouse gases, tracing the carbon cycle, and tracking the effects of climate change. Oak Ridge researchers also were seemingly ahead of their time in developing and testing strategies to stabilize levels of carbon dioxide (CO₂) in the atmosphere. Today, equipped with comprehensive climate data and increasingly sophisticated computational models, climate scientists are able to predict, with increasing accuracy, trends in climate change and gauge the potential effectiveness of remediation efforts with unprecedented levels of certainty and detail.

Among these remediation strategies is carbon sequestration, a method of capturing and storing CO₂, the most common greenhouse gas. CO₂ is introduced into the atmosphere by activities, such as burning fossil fuels and various agricultural practices. Unlike some other atmospheric gases, CO₂ absorbs energy from sunlight, rather than reflecting it back into space. The process warms the CO₂, which in turn heats the surrounding atmosphere. Reduced to its most simple form, the earth's rising average temperature is linked to the growing volume of CO₂ produced by human activity.

According to the United Nations' Intergovernmental Panel on Climate Change, human activities now add 10 billion tons of CO₂ to the atmosphere every year. Projections that include a growing population, increased energy usage, and improved living standards, suggest that CO₂ emissions could rise to 20 billion tons annually by 2050. According to Gary Jacobs, head of ORNL's Environmental Sciences Division, the planet may be exceeding that trajectory already. "If our current trajectory continues through 2100," he says, "we will be looking at plausible severe warming on the order of about 6° C. An average increase of that magnitude could result in some regions, particularly in the northern latitudes, warming by as much as 10 to 15° C."

Climate simulations indicate that such levels of warming would have extreme consequences, including more frequent and severe droughts, floods, and storms. A significantly warmer climate would also have a negative impact in some regions, such as northeast India, on water resources, creating the possibility of famine and large-scale population migrations.

The goal of Jacobs and his staff of climate researchers is to develop strategies to help avoid such extreme scenarios by stabilizing the level of CO₂ in the atmosphere in the relatively near future. "If we had a magic wand and could stop all emissions today," says Jacobs, "we would still see warming on the order of a degree because so much CO₂ is already built into the climate cycle. The challenge of climate science is to figure out where we can put away, or sequester, several billion tons of CO₂ every year." Global climate models indicate that sequestration on the billions-of-tons scale would be enough to begin to stabilize atmospheric levels of CO₂ and at least slow the pace of global warming.

Jacobs anticipates that sequestration efforts will be ramped up in conjunction with increased use of technologies, such as nuclear, solar and wind, that do not produce CO₂ emissions. "The hope is that as these new energy technologies come into play, their increased efficiency will generate more power with less CO₂ to worry about," he says.

ORNL's carbon sequestration research is concentrated in three main areas: ocean sequestration, carbon capture and storage, and biosequestration.

ORNL's ocean sequestration research is focused in two categories. One approach is to fertilize the ocean's natural carbon cycle by seeding the water with iron to stimulate plankton growth, thereby pulling more CO₂ out of the atmosphere. When the plankton die, they sink to the bottom and are buried by sediment, along with the CO₂ they consumed. The other ocean-based approach involves injecting captured CO₂ deep into the sea where, because of the extreme pressure and cold temperatures, the gas would settle into dense pools and remain indefinitely.

Carbon capture and storage targets the source of the problem by concentrating the CO₂ emitted from coal- or gas-fired power plants and creating a supercritical fluid that can be stored below ground in geological formations, such as saline aquifers and abandoned oil wells. "Conceptually, we could store hundreds of billions of tons of CO₂ in these structures," Jacobs says.

However, despite the promise of ocean-based and geologic sequestration technologies, neither seems poised to be widely implemented in the near future. "Both ocean and geologic storage require a cost-effective means of separating and capturing the CO₂," Jacobs says. Large-scale demonstration projects are not scheduled for several years, so full implementation would likely be considerably later. "To get to point where we can annually sequester a couple of billion tons of CO₂ 'soon' would require major acceleration of R&D, demonstration projects, and long-term policy decisions that would incentivize utilities and others to make large-scale infrastructure investments," he says. "In addition, ocean sequestration would require international discussions on the possible impacts of the technology."

Still, Jacobs remains undeterred. "The biosequestration option—enhancing the natural uptake and storage of CO₂ in vegetation, plants and soil—is technically less challenging and provides near-term options for removing 1 to 2 billion tons of CO₂ per year. On a global scale, we lose every year roughly 1.5 billion tons of carbon back into the air through land use change, be it through deforestation, agricultural practices, development, or just poor land management." Fortunately, natural systems take up about 3.5 billion tons of CO₂, so they provide a net sequestering of 2 billion tons per year. Jacobs indicates that the primary challenge of biosequestration is figuring out how we can persuade this natural system to increase its appetite for CO₂. "Could we go from two tons to five?" he asks. "Can we get to 10? I certainly think we can get to five—possibly 10, but it would take significant management of global land resources at unprecedented levels."

Several options exist to achieve higher rates of biosequestration through land management. One is promoting low-till or no-till agriculture, because tilling the soil deeply releases the CO₂ that has been absorbed from the air by the plants and deposited in the soil by their roots. "That's not science as much as just good land management practice," explains Jacobs. "Globally, more carbon resides in the soil than in the vegetation."

"In Milan, Tennessee," he continues, "there is a switchgrass farm where we are studying the uptake of CO₂ in the soil. The soil in the fields accumulates a lot of carbon, so understanding how it is accumulated and how long it stays in the soil is important." For example, Jacobs and his team have found that if the switchgrass is harvested too early, or if the farmer tries to squeeze two plantings into a single season, then relatively little CO₂ and nitrogen are deposited into the soil. "It's better environmentally to let the switchgrass go through a frost and die—and then cut it so all of the CO₂ has been deposited below ground."

"We can also put a lot of carbon in trees," Jacobs says. "An opportunity arises if we receive an offset by using the trees to generate power or for fuel. So not only do we pull carbon out of the environment, we also replace other emissions from fossil fuel sources."

Jacobs notes that 30-50 percent of the land mass on earth is currently managed for agriculture, homes, and commercial activity, so thinking about biosequestration on a global scale is not unreasonable. "Where environmental science can really make a contribution is in understanding where the carbon can be stored on a biological or molecular scale," he says. "However, as land use changes—as we go from grasslands to trees or from deserts to grasses—the reflectivity of the land changes, and that, in turn, changes climate. If we alter present patterns on a large enough scale, we can change the hydrologic cycle locally, which can potentially modify climate trends on a global scale."

"So, in environmental sciences, we do fundamental studies on how to store carbon," Jacobs says. "Then our computational scientists and their improved earth system models provide us with the opportunity to analyze how these sequestration options stabilize the climate, how quickly that happens, and what new climate state we will inherit.

Jacobs and his colleagues have the most unique perspective, using biology at the molecular scale in the effort to tackle one of the biggest problems facing humankind.

Reference Material  

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