Catch that carbon
examines ways of cutting
emissions of carbon dioxide by growing forests and burning hydrogen
It is some 300,000 years since our ancestors first began to deplete the world's stocks of carbon. It started when they learned to control fire. For a long time they must have used dead and fallen wood, only gradually switching to chopping down trees, then to digging for coal, and now to drilling under the oceans for petroleum and natural gas. The large releases of energy from this stock of carbon have permitted our species to build up ever more elaborate social institutions and to shape the world to our own ends.
But meanwhile carbon dioxide (CO2) has built up in the atmosphere, absorbing long wavelength radiation emitted from the Earth, and leading to global warming. We now need to 'sequester' carbon - to remove it as CO2 from the atmosphere and set it aside, preferably as molecules of reduced carbon, i.e. hydrogen-containing molecules such as sugars, cellulose and hydrocarbons, to augment the currently rapidly depleting stock. The time has come to deploy the institutions we have built up with the help of all the energy gained from reduced carbon to promoting the further production of such carbon, to cutting the rate at which we are burning it up, and to guiding CO2 to destinations other than the atmosphere.
Increasing the growth of plant matter is an effective way of stepping up the rate of production of reduced carbon. This is now well below its potential over large areas of the Earth, particularly in the developing countries. Many of these areas are undulating lands, unsuitable for cultivation; others are too dry to sustain agriculture. If only we could organize ourselves, every hectare of these lands could support several hundred tonnes of plant matter above and below ground, removing a great deal of CO2 from the atmosphere in the process. The barriers to such an enhancement of productivity are not so much technological as human. For most of these lands support people who subsist by gathering living resources with their own hands, or through low-input agriculture and animal husbandry. Many have no choice but to harvest wood or graze their flocks on these lands, demands which are incompatible with dedicating them to building up plant biomass.
Enhancing biomass production only becomes possible in such areas if it also helps these people sustain their livelihoods. One way of doing this is to use the biomass as an energy source. Trees would be periodically harvested; if they were cut after 10 years' growth, 10 per cent of the area of the energy plantation could be harvested every year.
Many factors need to be taken into account in working out the efficacy of carbon sequestration through energy plantations. If the relatively young trees harvested in such a plantation are only 20 per cent of the size of fully grown ones, it will only sequester 20 per cent as much carbon as a forest totally dedicated to sequestration. Furthermore the plantation would consume some energy, for instance in replenishing soil nutrients. Yet, on the other hand, its wood will provide energy that might otherwise have been generated by a thermal power plant, thus adding to the CO2 in the atmosphere.
As might be expected, energy plantations are a desirable option where the land is fertile and can support high rates of growth but is currently unused. They are not particularly cost-effective where productivity is low, or where the land already supports a good forest stand. Clearing away existing forest to create such plantations is not an attractive choice for sequestering carbon, especially where the growth rate is unlikely to be high. Nor is raising energy plantations on degraded lands that are unlikely to support high rates of tree production. But such rehabilitation of degraded lands may be highly desirable as a component of rural development programmes.
Such rehabilitation could usefully be modelled on the traditional system of land use in the northeastern Indian state of Manipur on the borders of China and Myanmar. Here uncultivated lands were put into two categories: 'supply forests' and 'safety forests'. Supply forests, covering about 40 to 60 per cent of the land, were harvested for firewood or small timber in a regulated way. Safety forests, some 10 to 30 per cent of the land, were traditionally protected as sacred sites - but often continued to be safeguarded even after the loss of religious beliefs because they served as firebreaks at the time of slash-and-burn agriculture. Left untouched, they supported biological communities in a state of equilibrium and constituted a dispersed system of refugia - areas where populations of organisms can survive even if they are exterminated elsewhere - that sustained the diversity of many groups of organisms. Remnants of such safety forests survive even in thickly settled, intensively cultivated tracts of the southwestern coast of India. Here primeval stands of evergreen dipterocarp forest extending over several hectares still occur in villages such as Iringole in the Ernakulam district of Kerala. I have asked villagers and workers from non-governmental organizations whether it might not be possible to dedicate 10 per cent of the land dedicated to energy plantations on degraded lands to raising such safety forests, perhaps in patches of 0.5 to 2 hectares, stocked by indigenous plant species and left permanently untouched. Many have reacted enthusiastically. These refugia could
both serve to sequester carbon to the maximum extent possible in the prevalent environmental regime and contribute towards the conservation of biological diversity.
A second approach to sequestering carbon arises from new ways of extracting energy from reduced carbon compounds. All of these - whether in petroleum, natural gas or kitchen wastes - contain hydrogen, and it is now technically possible to isolate this and use it as a clean fuel that produces water as its only by-product. The process of isolating the hydrogen will simultaneously produce almost pure CO2. This can be banished to underground storage such as in partially depleted petroleum reservoirs or deep coal beds. Driving CO2 underground in this way could also perform other useful functions, by forcing otherwise inaccessible petroleum or coal-bed methane reserves to the surface. If the release of CO2 to the atmosphere is taxed, as in Norway, such possibilities become all the more attractive.
These are then two, in some ways diametrically opposite, avenues for sequestering carbon. One is the traditional, tried and tested route of growing plants for energy production, leaving some areas untouched as refugia for biological diversity. The other is the, as yet commercially untested, possibility of using hydrogen as a fuel, while storing the concomitantly produced CO2 in geological reservoirs. Both pose exciting challenges which were the subject of a recent brainstorming session of the Science and Technology Advisory Panel of the Global Environment Facility.
Professor Madhav Gadgil is a professor in the Centre for Ecological Sciences at the Indian Institute of Science, Bangalore, India, and Chairman of the Science and Technology Advisory Panel of the Global Environment Facility.