The fact that we are having profound and far-reaching effects on the world’s climate is no longer in serious doubt. As a result of human activities concentrations in the atmosphere of so-called greenhouse gases, chiefly carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are currently at levels unseen for at least the last 650,000 years, and are rising at unprecedented rates. Around two-thirds of the increase in greenhouse gases in the last 150 years or so can be ascribed to the burning of fossil fuels. Most of the rest is a result of changes in land-use and a small proportion is the product of burning calcium carbonate to produce cement. Land-use change – most notably deforestation – results in an increase in greenhouse gases chiefly through the release of carbon stored in biomass.
The greenhouse gases emitted as a result of human action enter the carbon and nitrogen cycles. As a result of these cycles, not all the greenhouse gases produced through human action remain in the atmosphere: it is estimated that nearly 30% of these emissions over the past 150 years have been absorbed by the oceans and just under 30% by terrestrial ecosystems.
The Intergovernmental Panel on Climate Change believes that in order to avoid the worst effects of climate change, at the very minimum greenhouse gas concentrations need to be stabilised at 445–490 parts per million carbon dioxide equivalent. The current concentration is around 430 parts per million CO2e. At current rates of CO2 emission alone, the threshold of 445 parts per million CO2e will be reached in a mere seven years, even sooner if the accelerating output observed in the first few years of the present century continues.
Stabilising greenhouse gas concentrations can be achieved either by reducing the rate of emission, or by increasing the rate of absorption of the gases or both. Reduction in emissions from fossil fuel use is clearly of paramount importance. Carbon capture technologies that store the greenhouse gases produced at concentrated emission points such as power stations offer some hope for reducing rates of increase in emissions although their likely overall impact in the short or medium term remains uncertain.
But the management of fossil fuel use and adoption of carbon capture technologies will not in themselves be sufficient to prevent serious climate change in the next few decades. The management of carbon in living systems has a vital role to play: even with drastic cuts in fossil fuel emissions, current land-use practices would still lead to significant increases in greenhouse gas concentrations. Such management has two fundamental components: ensuring that existing carbon stocks held in natural ecosystems and in agricultural areas remain secure; and attempting to increase the rate at which carbon is sequestered in these systems.
Some aspects of the carbon cycle are at present effectively beyond direct policy control or technological intervention – notably the behaviour of the oceans in mediating the carbon cycle and global climate (large-scale fertilisation experiments are being undertaken to try to improve carbon fixing through oceanic photosynthesis, but there can be little human influence on the physical and purely chemical role of the ocean in the carbon cycle). Similarly, warming at high latitudes will lead to at least partial melting of the permanently frozen deep soil layer or permafrost there, releasing a proportion of the vast amount of carbon stored in the permafrost into the atmosphere. At present there are no technologies to prevent this happening: the only certain avoidance measure is to prevent the warming in the first place. Overall, there also appears to be relatively little scope at present for actively increasing carbon storage in most natural or largely natural ecosystems.
There are, however, many areas where appropriate policies and direct interventions could have major impacts. Large amounts of carbon are stored in peat soils worldwide and in remaining tropical moist forests. Protection of these from drainage and clearance would greatly help to slow down the rate of increase of greenhouse gases as well as delivering valuable benefits for biodiversity. Of particular importance are the tropical peat-swamp forests of South-East Asia – ironically under threat of clearance for biofuel production, despite the fact that their value as a carbon store hugely outweighs any possible carbon benefits to be gained from the biofuel crops that are replacing them.
Agricultural systems offer many opportunities for active carbon sequestration and reduction of emissions. They often have highly depleted soil carbon stocks, which could be replenished through the adoption of appropriate techniques, such as conservation tillage and integrated nutrient management using compost and manure. Overall, if best management practices were widely adopted, it is believed that the agricultural sector could become broadly carbon-neutral by 2030.
Not only is this technically possible, it is also economically feasible. Indeed, the IPCC has concluded that at an appropriate level of valuing or costing carbon emissions (US$100 per tonne of carbon dioxide equivalent), in 2030 the agricultural sector would be second only to building as potentially the most important sector for contributing to mitigation of climate change. At this level of carbon pricing, forestry and agriculture combined become more important than any other single sector. Even at lower carbon prices, the two sectors still retain high importance in mitigation.
There still remain many challenges to effective implementation. The greatest potential for increasing carbon storage in agricultural systems is in the developing world, where lack of knowledge and access to appropriate technologies are major barriers to change. Overcoming such barriers will need a commitment to capacity-building on a very extensive scale. Incentive-led systems, to encourage for example the planting of biofuels on marginal lands, need to be very carefully planned and executed if they are not to have adverse impacts on local livelihoods, on biodiversity or even on carbon stocks themselves.
If the global community can rise to these challenges, the Earth’s living systems can play a vital role in the struggle to avoid dangerous climate change. Not only that, but measures to manage ecosystem carbon can offer great potential benefits for biodiversity and soil fertility. This opportunity to contribute to so many important environmental goals should not be missed.