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ONE MORNING EACH WEEK a scientist takes a stroll on the barren upper slopes of Hawaii's Mauna Loa volcano, with a basketball-sized glass sphere in hand. At some point the researcher faces the wind, takes a deep breath and strides forward while twisting open a stopcock in the sphere.

With a whoosh lasting no more than a few seconds, five litres of the most pristine air on the planet replaces the vacuum inside the thick-walled orb.

Every couple of weeks, a more thoroughly wrapped-up researcher at the South Pole conducts the same ritual. At these remote sites and dozens of others around the world, instruments also sniff the air, adding measurements of atmospheric chemistry to a dataset that stretches back for more than 50 years.

This nearly continuous record results from one of the most comprehensive and longest-running Earth science experiments in history, says Ralph F. Keeling, a climate scientist at Scripps Institution of Oceanography in La Jolla, California. He carries on the effort his father, Charles Keeling, began as a graduate student back in the 1950s.

Several trends emerge from the data, says Keeling. First, in the northern hemisphere, the atmospheric concentration of carbon dioxide (CO₂) rises and falls by about seven parts per million (ppm) over the course of a year (around a current average of approximately 380 ppm).

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 seven ppm saw-tooth variation in CO₂ is superimposed on an average concentration that's rising steadily. Today's average is more than 380 ppm, compared with 315 ppm 50 years ago, and 280 ppm prior to the Industrial Revolution. And it's still rising by about 2 ppm each year, mainly due to combustion of fossil fuels.

Largely because CO₂ traps heat, Earth's average temperature has climbed by about 0.74° C over the past century, a trend that scientists expect will accelerate. In the next 20 years, the average global temperature is projected to rise at least another 0.4° C.

Squelching additional temperature increases depends on limiting - if not eliminating - the rise in CO₂ levels. And as Keeling says, "It's clear that if we want to stabilise CO₂ concentrations in the atmosphere, we need to stop the rise in fossil fuel emissions."

But halting the increase in atmospheric CO₂ doesn't necessarily mean doing away with fossil fuels. Many experts think that capturing CO₂ emissions, rather than simply reducing them, could ultimately provide climate relief.

Possible solutions range from boosting natural forms of carbon capture and storage - fertilising the oceans to enhance algal blooms, say, or somehow augmenting the soil's ability to hold organic matter - to schemes for snatching CO₂ from smoke stacks and disposing of it deep underground or in seafloor sediments.

Success in sequestering carbon depends on meeting two major 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.

Organisms that dominate the base of the world's food chains soak up quite a bit of CO₂ - currently about two per cent 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 metabolise their food and breathe out CO₂, and decomposition of dead plants and animals also releases large amounts of greenhouse gas.

Over the long haul, though, ecosystems can sequester significant amounts of carbon. For instance, 20 to 30 per cent of the carbon in the world's soil is locked in the northern hemisphere's peat lands, wetlands that accumulate plant matter and soil, with most of that accumulating since the end of the last Ice Age about 11,000 years ago.

Recent data suggest that North American ecosystems sequester, on average, 505 million tonnes 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.

But most of that sequestered carbon - about 301 million tonnes - is locked away in forests or in the wood products harvested from them, notes Anthony W. King, an ecosystem scientist at Oak Ridge National Laboratory in Tennessee, USA. He and his colleagues reported their analysis of these carbon sinks in November 2007 in an assessment issued by a consortium of U.S. government agencies. "New, vigorously growing forests are where most carbon sequestration takes place," he says.

Some researchers, including Ning Zeng, an atmospheric scientist at the University of Maryland in College Park, seek to harness the prodigious carbon-storing power of forests. Right now, forest floors worldwide are lined with coarse wood - everything from twigs and limbs shed during growth to entire fallen trees - containing about 65 billion tonnes of carbon, says Zeng.

Left undisturbed, that material would return its carbon to the atmosphere via decomposition or wildfire. Bury that wood in an oxygen-poor environment, however, and the carbon could be locked away for centuries.

Furthermore, Zeng notes, each year the world's forests produce enough coarse wood to lock away about 10 billion tonnes of carbon. Burying just half of that would significantly counteract the 6.9 billion tonnes 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 US$14 per tonne - the environmental toll could be substantial.

Coarse wood collected from the average square kilometre of forest could contain 500 tonnes of carbon, Zeng reported in December 2007 in San Francisco at a meeting of the American Geophysical Union. That volume of wood would fill a trench 10 m wide, 10 m deep and 25 m long. To sequester five billion tonnes of carbon a year, 10 million such trenches would need to be dug and filled. That's one every three seconds.

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.

Adding iron to the surface waters in some seas could help reduce CO₂ build-up in the atmosphere and forestall climate change, some scientists suggest. In the late 1980s, American 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."

Or maybe not. Recent studies in the North Atlantic and North Pacific confirm that natural algal blooms can indeed sequester CO₂, but in many cases the phenomenon may be only temporary, with little if any carbon making its way into deep water or seafloor sediments.

In late 2004 and early 2005, a similar study near the Crozet Islands southeast of South Africa further demonstrated that natural algal blooms result in only modest carbon sequestration.

Peter Statham, a marine biogeochemist at the National Oceanography Centre in Southampton, England, and his colleagues installed sediment traps at a depth of 2,000 m at several spots near the islands. South of the islands, particles drifting down through a one square-metre area carried a total of only 0.087 g of carbon each year; north of the islands, where ocean currents carry dissolved iron and other eroded minerals, the carbon flux to deep water was almost five times higher.

Yet 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 - and maybe even the right type - could damage marine ecosystems for years. "There's a huge gap in our understanding of these phenomena," he says.

Finally, fertilising the seas to sequester carbon, even with no bad side effects, may have little if any effect on climate. "Even in the most favourable circumstances, oceans would sequester only a small fraction of the carbon dioxide that humans are emitting," Statham argues.

Today, coal and petroleum each account for about 40 per cent of global CO₂ emissions. Of the two, however, coal poses by far the larger threat to future climate. Coal produces more CO₂ per unit of energy than any other fossil fuel - burning natural gas only generates 60 per cent of the CO₂ emissions, 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 eight billion tonnes of CO₂, an amount that contains about 2.2 billion tonnes of carbon. And, says Daniel Schrag, a geochemist at Harvard University in Cambridge, Massachusetts, emissions are poised to get even worse: about 110 power plants fuelled by pulverised coal are now at various stages in the permitting process in the United States, and China reportedly cuts the ribbon on one such plant every week or so.

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 tonnes of CO₂, says George Peridas, an analyst with environmental advocacy group the Natural Resources Defence Council in San Francisco, California. That's about 25 per cent more than all the CO₂ that humans have produced by burning coal since 1751, a period that encompasses the entire Industrial Revolution.

Because 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 U.S. National Energy Technology Laboratory in Pittsburgh, Pennsylvania. He and his colleagues are studying ways to capture emissions, ranging from using CO₂-hungry materials to sop the gas from smokestacks to building new types of plants that burn coal altogether differently.

The goal is to develop techniques for large-scale field tests by 2012 that can capture at least 90 per cent of a power plant's CO₂ emissions but boost the price of its electricity by no more than 20 per cent.

In current power plants, CO₂-absorbing materials would be placed in a stream of 200°C emissions, mostly nitrogen with only 3 to 15 per cent CO₂. The active materials could either absorb CO₂, just as a sponge sops up water, or chemically bind to the gas.

Materials called 'metal-organic frameworks' 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 reactions that effectively capture 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 96 tiny wells on a single plate - each well, in essence, its own 300-microlitre beaker - which was then heated.

Researchers then assessed the CO₂-sopping ability of the resulting crystals. In less than three months, as Banerjee and his colleagues reported in the U.S. journal Science in February 2008, the researchers generated 16 new 'zeolites', a type of metal-organic framework composed of aluminum silicates. (A computer representation of a zeolite is shown above).

Three of the zeolites are highly porous, with each gram of the material having a large surface area (to which CO₂ molecules can attach themselves) of between 1,000 and 2,000 m2. A one-litre sample of one of those supersponges - a substance dubbed ZIF-69 - could hold up to 83 L of CO₂.

Another team of scientists has produced a CO₂-absorbing substance - one that binds the gas via a chemical reaction - by 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 the Georgia Institute of Technology in Atlanta.

The chemical reaction can be reversed by heating the CO₂-saturated material, enabling researchers to capture the gas and dispose of it. A series of lab tests indicates that the material, whose amine-rich coating is tightly bound to the silica substrate, retains its capacity to soak up CO₂ after nearly a dozen cycles, the researchers reported in the Journal of the American Chemical Society in March 2008.

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 found at ocean depths below 500 m - CO₂ is a dense liquid, not a gas, and doesn't mix well with water. Therefore it is possible to deposit CO₂ on the ocean floor. However, many researchers have concerns about how large pools of concentrated CO₂ might affect ecosystems there. 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 pulverised limestone, the CO₂ breaks up into globules 200 to 500 micrometres 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 somewhere between that of milk and mayonnaise. Injected into the deep sea, lab tests suggest the limestone-veneered droplets sink about 200 m per day. As they dissolve into the surrounding water or break up on the ocean floor, the limestone's carbonate dissolves too, buffering much of the resulting acidity, like a tiny antacid tablet.

Golomb and his colleagues described their carbon-dumping process in the U.S. journal Environmental Science & Technology in July 2007. Another method uses large quantities of a highly reactive form of limestone to lock away and dispose of dissolved CO₂ in surface waters (see "Cleaning up CO₂ with a twist of lime", Cosmos Online).

Immense volumes of subterranean strata are another tempting dumping ground. Some types of rock formations are naturally impervious to the flow of gases and liquids and have, in fact, 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 parts 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 such reservoirs could hold hundreds of years' worth of captured emissions.

Disposal of CO₂ in ancient volcanic rocks may provide an even more secure sequestration technique. Lab tests suggest that liquid CO₂ will chemically react with basalt to produce various minerals, including calcium carbonate, in a matter of months.

Therefore, according to Pete McGrail, an environmental engineer at Pacific Northwest National Laboratory in Richland, Washington State, concerns about CO₂ escaping from its underground prison are minimised. Thick layers of basalt, the result of widespread volcanic activity between six million and 17 million years ago, underlie the area surrounding McGrail's lab.

Although most think of basalt as impervious, many of these deposits are porous because they were frothy when they cooled or they cracked extensively when subsequent flows of lava heated them up or weighed them down.

Late in 2008, McGrail and his colleagues will inject between 1,000 and 3,000 tonnes of liquid CO₂ - enough, give or take, to fill an Olympic-sized swimming pool - into the porous rocks at a depth of about one kilometre. Then, researchers will assess the effectiveness of the 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.

If this 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 United States over a 20- to 50-year period, according to McGrail. Across the nation, deep geologic formations such as saline aquifers and coal layers could sequester 150 years' worth of worldwide power-plant emissions, possibly providing a rock-solid solution to one of the world's most pressing problems.

The world needs carbon sequestration, says Schrag - and soon, on an enormous scale. The challenge, he notes, is to ensure that carbon capture and sequestration technologies are ready when serious political action on climate change is finally taken. And according to King, that time is looming. "It's beginning to dawn on people," he says, "that they can change the planet in ways larger than the planet can change itself."

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