It is now widely believed that the increased atmospheric concentration of greenhouse gases (GHGs) is contributing to the process of climate change and global warming. Most OECD countries, under the 1994 United Nations Framework Convention on Climate Change, committed themselves to stabilise emissions of GHGs at 1990 levels by 2000, and further agreed to implement the 1997 Kyoto Protocol, which specified the levels of emissions for the target period 2008 to 2012.
It is not only the contribution of agriculture in the climate change process, but also the impact of climate change on agriculture that is of concern to farmers and policy makers. Monitoring the role of agriculture as a source and sink for GHGs is of importance to policy makers, in view of the need for countries to assess domestic strategies, and to meet international obligations to reduce GHG emissions. Data on the specific contribution of agriculture as both a source and sink of GHG in relation to climate change, relative to other sectors in the economy, can help to develop appropriate policies.
The greenhouse gas indicator measures the gross agricultural emissions of three gases: carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), expressed in CO2 equivalents. The share of agriculture in OECD total national gross GHG emissions in CO2 equivalents is below 10 per cent, although for methane and nitrous oxide agriculture contributes a major share in the emission of these gases, about 40 and 60 per cent, respectively. For a few OECD countries the contribution to national total GHG emissions is above 20 per cent, which is largely a reflection of the greater importance of the agricultural sector in the economies of these countries.
Livestock farming and the use of inorganic fertilisers are key sources of methane and nitrous oxide gases. The trend in agricultural emissions of GHGs has declined since the early 1990s for most OECD countries. This is mainly explained by a reduction in cattle numbers and the use of fertilisers. For a few countries GHG emissions have been rising, because of an overall expansion in crop and livestock production.
The work to date on agricultural GHG indicators focuses on emissions, because as yet there are no systematic estimates of agriculture's role as a sink for GHGs across OECD countries. Agriculture's capacity as a GHG sink is enhanced by improvements in management practices, such as tillage practices, crop cover and residue management.
The development of a comprehensive net GHG balance indicator would address both GHG emissions and removals. A number of OECD countries have begun to measure soil carbon fluxes and agriculture's capacity to act as a GHG sink. Research in Canada, for example, shows that net CO2 emissions from agricultural soils in Canada have been considerably reduced by converting from conventional tillage to no-till systems, increasing cover cropping and improving crop residue management practices. A study in France calculated net CO2 emissions from changes in agricultural land use. Overall the French research showed emissions exceeded removals, with an increase in CO2 by converting grassland to other uses and clearing forests, while agricultural land left uncultivated acted as a CO2 sink.