Over the past 200 years or so, human beings, through burning of fossil fuels and changing land-use patterns, have added about 400 Gigatonnes of carbon into the atmosphere (1 Gt = 1,000 million tonnes). But it turns out that the present atmosphere contains only about half of this stuff, meaning the other half must have been absorbed by the Earth. The solution to this mystery lies in carbon sinks, which chiefly include the growth of forests and oceans’ ability to absorb carbon dioxide. Without this removal of carbon from the atmosphere, the present atmospheric concentration of carbon dioxide — which stands at 370 ppm (parts per million) — would be significantly higher (about 450 ppm).

It sure looks as if carbon sinks are putting a brake on ever-rising carbon emissions, delaying the speed and extent of climate change. Moreover, it is quite feasible for individual countries to regulate their carbon dioxide inventory by tinkering with carbon sinks. But many a jumble needs unscrambling before the actual significance of carbon sinks in checking climate change — and how they could be rationally used to meet individual countries’ commitments to cap emissions — can be assessed.

For instance, one needs to know how long the natural uptake of carbon dioxide will continue. To what extent can we induce ‘human-made’ carbon sinks by planting more forests or changing agricultural practices? How accurately can we measure the effectiveness of individual carbon sinks in reducing atmospheric carbon dioxide levels? This is important both to understand the main components of natural carbon sinks as well as to verify the success of human-induced carbon sinks, if and when they are implemented.

Oceans lick off carbon dioxide from the air by trapping it chemically on the surface. But oceans are lazy — it takes about a thousand years for an ocean to recycle its water from the deep to the surface. This means that before long, water on the surface becomes saturated with carbon dioxide, thus allowing forever less absorption of the greenhouse gases (GHGs). Indeed, models of the vertical movement of ocean waters predict that in a warmer climate the sinking of surface water further reduces the future role of the ocean as a carbon sink. Land sinks, by contrast, are more dynamic, and hence more amenable to human doctoring. For this reason, they are the focus of current negotiations to help individual governments achieve Kyoto emissions reduction targets.

Two factors determine the importance and size of a terrestrial sink — land-use changes and concentration of carbon dioxide in the air. The latter boosts a sink by stimulating photosynthesis. However, while some environmental changes can boost the role of land as a sink, others will ultimately diminish the overall land sink. For example, large stocks of carbon are currently preserved in frozen soils of the polar regions. Climate warming would melt these soils, stimulating breakdown and release of this ‘locked up’ carbon to the atmosphere and so form a carbon dioxide source that would offset carbon dioxide sinks elsewhere. Indeed, most researchers predict the overall role of land as a carbon sink to diminish over the next few decades. Some say it could disappear altogether as early as 2050.

So it is for good reasons that the Kyoto Protocol stresses so much on the reduction of carbon dioxide emissions. However, while the role of natural carbon sinks is not addressed in the protocol, it does accept that artificial manipulation of carbon sinks can help governments reach national emission reduction targets in the relatively short term.

But the way it is worded at the moment, the exact nature of human-induced carbon sinks recognised by the protocol as potential contributions to meeting carbon emission reductions are both limited and ambiguous. For instance, Article 3.3 of the protocol, which sets out the guidelines for using carbon sinks, refers only to ‘afforestation, reforestation and deforestation’ as allowable activities. But the extent to which this includes management of existing forests and other current carbon-sequestering management practices, such as reduced ploughing of agricultural land, is not yet clear.

Furthermore, all changes in carbon stock in these projects must be verifiable. In other words, it must be possible to independently measure the amount of carbon sequestered. But while carbon stocks in vegetation on land are fairly straightforward to assess from ground and satellite surveys, those underground, which can represent up to 90 per cent of the total carbon stocks in some forest systems, are far harder to determine.

Then there is the question of non-permanence, where sinks can become net sources of carbon for a variety of natural and human-made reasons. But sequestered carbon must be for all time to come, if the climate is to benefit. Critics therefore caution that rules for accounting for CDM sink projects must ensure that carbon either remains sequestered forever or that any new releases are equivalently made up for elsewhere.

Measuring emissions is another thorny issue. Since emissions and removals of GHGs are almost never directly gauged for reasons of money and feasibility, ways must be found to cope with inaccuracies with respect to forestry CDM projects. Indeed a recent study published in the journal Nature suggests that the conversion of grassland into shrub-land only incorporates a small amount of carbon. The researchers found that in some cases the amount of carbon stored in the soil actually decreased. The results suggest that the US carbon sink has been significantly overestimated, and could have serious implications on the use of new forest plantations to combat climate change.

A related issue is that of setting emission baselines so that the impact of a CDM project can be estimated in terms of scenarios that do not include the project. If not properly checked, the gains could be easily undone, either by deforestation elsewhere or by monocultures of plantations that would have happened anyway, being commercially lucrative. Carbon could also leak from locations in which it has been stored or sequestered as a result of natural disturbances — such as fires, storms or the effects of insects — or of human activity.