Carbon emissions could be lost in the trees and seas

March 2, 2001

Tennessee scientists are exploring methods to combat global warming by 'locking up' carbon. Steve Farrar reports.

Could genetically superior poplars that suck more carbon dioxide from the air than their wild counterparts, or slicks of deliberately fertilised, greenhouse-gas guzzling microbes, help stave off climate change? As consensus grows as to the reality and possible consequences of green house gas emissions, scientists at the Oak Ridge National Laboratory, in Tennessee, United States, have been exploring a host of scientific and technological responses to an increasingly urgent problem.

At the heart of the dilemma is the conflict between increasing global energy requirements - projected to rise three or fourfold over the next 50 years - and the need to reduce the associated emissions of greenhouse gases to reduce the risk or at least mitigate the worst effects of climate change.

Some ORNL scientists are looking to develop energy-efficient technologies to reduce the need to burn fossil fuels. Others are studying alternatives such as non-polluting fuel cells. As co-leader of the centre for research on enhancing carbon sequestration in terrestrial ecosystems (CSITE) at the ORNL, Gary Jacobs is exploring a third approach -locking up carbon in vegetation and soil while colleagues look to lock it into underground rock formations and oceans.

Jacobs admits that carbon sequestration is fraught with uncertainty and at best will only buy mankind more time to find an overall solution. "We're studying the deliberate manipulation of the carbon cycle (the movement of carbon through the biosphere, atmosphere, hydrosphere and lithosphere) when we don't fully understand that cycle yet," he says.

Nevertheless, his cost-no-object estimates suggest this has the potential to hold carbon dioxide levels down until at least the last few decades of this century. "The earth's vegetation and soil could act as a huge natural scrubber for carbon dioxide emissions from industrial sources and land-use changes," Jacobs says.

It is a controversial suggestion that groups such as Friends of the Earth regard as inherently unreliable, given the complexities of the carbon cycle. They call simply for cuts in greenhouse-gas emissions. Jacobs agrees while contending that if forests and agricultural land were better managed to protect grassland, replant trees and prevent soil erosion, it would result in a significant dent in the carbon content of the atmosphere.

Furthermore, Jacobs says that schemes to enhance the amount of carbon that is locked in the ground could be introduced within a decade. In one such project, ORNL plant geneticists are breeding hybrid poplars and deep-rooted switchgrass that can fix more carbon dioxide from the air when they photosynthesise and selectively store it in their leaves, stems and roots.

These plants could be used to sequester carbon in the soil by channelling it to lignin-rich root cell walls that are less susceptible to being broken down by microbes. They could also be used as a source of energy, with fast-growing above-ground cell walls high in cellulose that can be processed into ethanol as a cleaner alternative to fossil fuels.

Other projects look at ways to produce and protect soils high in carbon-containing organic matter, from analysing ways to optimise the microscopic structure of soils to devising better agricultural practices. Jacobs has estimated that up to 4 gigatonnes of carbon - that is 4,000 million tonnes - could be held annually through such terrestrial sequestration for 50 years.

Another way to sequester carbon would be to store it in geological formations, injecting it, for example, into depleted oil or gas reservoirs or brine-filled layers.

Jacobs envisages that systematic geological sequestration could begin during the second or third decades of the century and at its peak might account for between 300 and 3,200 gigatonnes of carbon.

Finally, by 2030, research may have advanced sufficiently to approach the most controversial arena of carbon sequestration - the oceans. Estimates suggest many thousands of gigatonnes of carbon could be captured from the atmosphere here, yet the expense, scale of engineering required and leap in knowledge of the ecosystems involved make the task onerous and difficult to assess. Proposals include direct injection of carbon dioxide into deep portions of the ocean, and the fertilisation of parts of the ocean surface to stimulate large algal blooms that would then take up great quantities of carbon dioxide from the air.

Even if all the scientific, ethical, political and economic hurdles can be overcome to realise some of carbon sequestration's promise, the fight against global warming will be far from over. However, it might just provide sufficient breathing space for humanity to find a way to tackle the problem in a more responsible fashion.

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