Scientists work to put the greenhouse gas in its place
each week, a scientist takes a stroll on the barren upper slopes
Once every couple of weeks, a parka-clad researcher at
the South Pole conducts the same ritual. At these remote sites and dozens of
others, instruments also sniff the air, adding measurements of atmospheric
chemistry to a dataset that stretches back more than 50 years. The nearly
continuous record results from one of the longest-running, most comprehensive
earth science experiments in history, says Ralph F. Keeling, a climate
scientist at Scripps Institution of Oceanography in
Several trends pop out of the data, says Ralph Keeling. First, in the Northern Hemisphere the atmospheric concentration of carbon dioxide rises and falls about 7 parts per million over the course of the year. The concentration typically reaches a peak each May, then starts to drop as the hemisphere’s flush of new plant growth converts the gas into sprouts, vegetation and wood. In October, the decomposition of newly fallen leaves again boosts CO2 levels. Populations of algae at the base of the ocean’s food chain follow the same trend, waxing each spring and waning each autumn.
A second trend is that each year’s 7-ppm, saw-tooth variation in CO2 is superimposed on an average concentration that is steadily rising. Today’s average is more than 380 ppm, compared with 315 ppm 50 years ago. And it’s still rising, about 2 ppm each year, mainly from burning fossil fuels.
Largely because CO2 traps heat, Earth’s average temperature has climbed about 0.74 degrees Celsius over the past century (SN: 2/10/07, p. 83), a trend that scientists expect will accelerate. In the next 20 years, the average global temperature is projected to rise another 0.4 degrees C.
Squelching additional temperature increases depends on limiting, if not eliminating, the rise in CO2 levels, many scientists say. And, Keeling says, “It’s clear that if we want to stabilize CO2 concentrations in the atmosphere, we need to stop the rise in fossil fuel emissions.”
But halting the increase in amounts of CO2 in the air doesn’t necessarily mean doing away with fossil fuels. Many experts suggest that capturing CO2 emissions, rather than only reducing them, could ultimately provide climate relief.
Possible solutions range from boosting natural forms of carbon capture and storage, or sequestration — fertilizing the oceans to enhance algal blooms, say, or somehow augmenting the soil’s ability to hold organic matter — to schemes for snatching CO2 from smokestacks and disposing of it deep underground or in seafloor sediments.
Success in sequestering carbon comes down to meeting two challenges: How to remove CO2 from the air (or prevent it from getting there in the first place) and what to do with it once it has been collected.
Doing it naturally
Organisms that dominate the base of the world’s food chains soak up quite a bit of CO2 — currently about 2 percent of the atmosphere’s stockpile each growing season. That gas, plus sunlight and other nutrients, is converted into carbon-rich sugars and biological tissues that nourish humans and all other animals. Unfortunately, most of that carbon makes its way back to the atmosphere rather quickly: Animals metabolize their food, breathing out CO2. Decomposition of dead plants and animals likewise produces the greenhouse gas.
Over the long haul, though, ecosystems can sequester significant amounts of carbon. About 30 percent of the carbon in the world’s soil is locked in peat lands of the Northern Hemisphere, for instance, with most of that accumulating since the end of the last ice age about 10,000 years ago (SN: 2/10/01, p. 95).
Recent data suggest that North American ecosystems
sequester, on average, 505 million metric tons of carbon each year. Some
accumulates as organic material in soil, wetlands or the carbon-rich sediments
deposited in the continent’s rivers and lakes. More is stored in woody plants
that have invaded grasslands or trees that have taken over shrublands. Most of
the sequestered carbon, about 301 million tons, is locked away in North
American forests or in the wood products harvested from them, notes Anthony W.
King, an ecosystem scientist at Oak Ridge National Laboratory in
“New, vigorously growing forests are where most carbon sequestration takes place,” King says.
researchers, including Ning Zeng, an atmospheric scientist at the
Furthermore, Zeng notes, each year the world’s forests naturally produce enough coarse wood to lock away about 10 billion tons of carbon. Burying just half of that amount would significantly counteract the estimated 6.9 billion tons of carbon released into the atmosphere each year via fossil fuel emissions.
While the price tag for this technique would be
relatively reasonable — photosynthesis
is free, and burying the wood would cost about $14 per ton — the environmental toll could be substantial. Coarse wood
collected from the average square kilometer of forest could contain about 500
tons of carbon, Zeng reported in December in
5 billion tons of carbon each year, logging crews would need to dig and fill 10 million such trenches, about one every three seconds.
“This is not an environmentally friendly method” of carbon sequestration, Zeng admits.
Life at sea
In certain parts of the oceans, especially along the western coasts of large continents, nutrient-rich waters fuel the growth of algae and other phytoplankton. Their growth pulls CO2 from the atmosphere. Many parts of the ocean, however, lack one or more vital nutrients, particularly dissolved iron, and are therefore nearly devoid of life (SN: 8/4/07, p. 77).
Adding iron to the surface waters in some seas could help reduce CO2 buildup in the atmosphere and forestall climate change, some scientists suggest. In the late 1980s, oceanographer John Martin, an early proponent of this idea, boasted: “Give me half a tanker of iron, and I’ll give you the next ice age.”
not. Recent studies in the
Peter Statham, a marine biogeochemist at the National Oceanography
Many uncertainties remain about how effective any artificial attempts to boost algal growth might be, says Statham. First of all, he notes, scientists aren’t sure which forms of iron are the ones that marine phytoplankton find most nutritious. And the long-term effects of adding the wrong type of iron — or maybe even the right one, he adds — could damage marine ecosystems for years. “There’s a huge gap in our understanding of these phenomena,” he says.
Finally, fertilizing the seas to sequester carbon, even with no bad side effects, may have little if any effect on climate. “Even in the most favorable circumstances, oceans would sequester only a small fraction of the carbon dioxide that humans are emitting,” Statham argues.
Down and away
Today, coal and petroleum each account for about 40 percent of global CO2 emissions. Of the two, however, coal poses by far the larger threat to future climate. For one thing, coal produces more CO2 per unit of energy than any other fossil fuel — about twice that generated by burning natural gas, for example. Also, coal is abundant and therefore relatively cheap: The amount of carbon found in the world’s coal reserves is about triple that locked away in petroleum and natural gas deposits.
Worldwide, coal-fired power plants each year generate
about 8 billion tons of CO2, an amount that contains about 2.2
billion tons of carbon. And, says Daniel Schrag, a geochemist at
All told, the coal-fired power plants built in the next 25 years will, during their projected 50- to 60-year lifetimes, generate about 660 billion tons of CO2, says George Peridas, an analyst with the Natural Resources Defense Council office in San Francisco. That’s about 25 percent more than all the CO2 that humans have produced by burning coal since 1751, a period that encompasses the entire Industrial Revolution.
coal-fired power plants are point sources of immense volumes of CO2,
they’re tempting targets for sequestration efforts, says Tom Feeley, an
environmental scientist at the National Energy Technology Laboratory in
In current power plants, CO2-absorbing materials would be placed in a stream of 200°C emissions, mostly nitrogen with between 3 and 15 percent CO2. The active materials could either absorb the gas, just as a sponge sops up water, or chemically bind to it.
Materials called metal-organic frameworks (SN: 1/7/06, p. 4) fall into the category of CO2 sponges. In their gaseous state, CO2 molecules fly about at great speeds and keep a considerable distance from each other, but inside the pores of some of these crystalline sieves, the molecules line up and cram close together, says Rahul Banerjee, a chemist at the University of California, Los Angeles.
Discovering the reactions that produce a substance that effectively captures CO2 takes time. So, Banerjee and his colleagues recently adopted a technique common in the pharmaceutical industry: They used a computer-controlled device to automatically dispense various combinations and concentrations of reactants into each of 96 tiny wells on a single plate — each well, in essence, its own 300-microliter beaker — which was then heated. The researchers then assessed the CO2-sopping ability of the resulting crystals.
In less than three months, the researchers generated 16 new zeolites, a type of metal-organic framework composed of aluminum silicates, Banerjee and his colleagues reported in the Feb. 15 Science. Three of the zeolites are highly porous, with each gram of the material having a large surface area — where CO2 molecules can attach — of between 1,000 and 2,000 square meters. A 1-liter sample of one of those supersponges, a substance dubbed ZIF-69, could hold up to 83 liters of CO2 under normal atmospheric pressure.
team of scientists has produced a CO2-absorbing substance — one
that binds the gas via a chemical reaction
painting an organic compound called aziridine on a wafer of silica. Unlike
previously developed aminosilica materials, the new substance has a high
storage capacity for CO2, says Christopher W. Jones, a chemical
engineer at Georgia Institute of Technology in
Capturing vast amounts of power plant emissions is just half the task. The next step is storage. Many scientists propose locking CO2 underground or in the deep ocean.
Under high pressure, as in ocean depths below 500 meters, CO2 is a dense liquid, not a gas, and doesn’t mix well with water. Therefore it’s possible to deep six CO2 on the ocean floor, but many researchers have concerns about how large pools of concentrated CO2 might affect ecosystems there (SN: 6/19/99, p. 392). The CO2 might slowly dissolve into the surrounding water, creating acidic conditions.
A new and relatively simple twist on the deep-ocean technique may address many such concerns. If liquid CO2 is blended with a mixture of seawater and pulverized limestone, the CO2 breaks up into globules that are 200 to 500 micrometers in diameter and coated with limestone powder, says Dan Golomb, a physical chemist at the University of Massachusetts, Lowell. The resulting emulsion has a consistency between that of milk and mayonnaise. Injected into the deep sea, the limestone-veneered droplets sink about 200 meters per day, lab tests suggest. As the droplets dissolve into the surrounding water or break up as they jostle about on the seafloor, the limestone’s carbonate dissolves too, buffering much of the resulting acidity, like a tiny Tums. Golomb and his colleagues described their carbon-dumping process last July in Environmental Science & Technology.
Immense volumes of subterranean strata are a tempting dumping ground, too. Some types of rock formations are naturally impervious to the flow of gases and liquids. In fact, some of these geological reservoirs have already proven themselves by sequestering naturally formed CO2 for millions of years. Oil companies have been mining that CO2, transporting it through pipelines and pumping it into the ground to enhance the recovery of petroleum from faltering oil fields for decades — an irony indeed to think that CO2 is being pumped into the ground so that petroleum, a raw material for even more CO2, can be extracted.
In many regions of the world, saline aquifers lie deep beneath the ground. Because that salty water isn’t suitable for drinking, some of those strata, especially those sandwiched between or capped by impervious rocks, could be used to store CO2. Scientists estimate that such reservoirs might hold hundreds of years’ worth of captured emissions.
of CO2 in ancient volcanic rocks may provide an even more secure
sequestration technique. A multimillion-dollar field test soon to be under way
tests suggest that liquid CO2 will chemically react with basalt to
produce various minerals, including calcium carbonate, in a matter of months,
says Pete McGrail, an environmental engineer at Pacific Northwest National
Later this year, McGrail and his colleagues will inject between 1,000 and 3,000 tons of liquid CO2 — enough, give or take, to fill an Olympic-sized swimming pool — into the porous rocks at a depth of about 1 kilometer. Then, researchers will assess the effectiveness of their sequestration by occasionally collecting fluid samples at the injection site. Analyses suggest that this volume of CO2 will react to form carbonate minerals within five years, says McGrail.
sequestration technique is deemed suitable, the region’s ancient basalts could
hold a volume of CO2 approaching that emitted by every coal-fired
power plant in the
And that time may be coming soon, says
“Carbon Dioxide Capture and Storage: Summary for Policymakers,” IPCC Special Report, 2005. www.ipcc.ch/pdf/special-reports/srccs/srccs_summaryforpolicymakers.pdf
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