The global carbon cycle. Click to enlarge.

The natural greenhouse effect is being intensified as humans alter the global carbon cycle. Forests, soil, oceans, the atmosphere, and fossil fuels are important stores of carbon. Carbon is constantly moving between these different stores, that act as either “sinks” or “sources.”

A sink absorbs more carbon than it gives off, while a source emits more than it absorbs. Before the Industrial Revolution, the amount of carbon moving between trees, soil, oceans and the atmosphere was relatively balanced.

Burning fossil fuels tips this balance. Oil, coal and gas combustion introduce at least 6 billion tonnes of carbon to the carbon cycle every year -- carbon that was stored underground, separated from the atmosphere for millions of years.

Forests and soils store large amounts of carbon.

Living forests absorb carbon dioxide and, through photosynthesis, convert it to biomass. Forest soils also store large amounts of carbon in their organic layer. Deforestation alters the carbon cycle by eliminating trees and disturbing forest soils, releasing the carbon stored in both to the atmosphere. Forest fires and decomposing wood waste and wood products also add significant amounts of carbon dioxide to the atmosphere.

Until the 1980s, Canadian forests were a sink, according to the Canadian Forest Service. Through increased fires, insect infestation and harvesting, they have now become a net source. Modern farming practices also disrupt the carbon cycle. Soils, which contain about 75 per cent of carbon found on land, are excellent sinks. Once cultivated, the amount of organic matter that soils contain drops by 20-50 per cent.

Intensive farming releases carbon from the soil to the atmosphere.

Intensive farming, erosion and salinization accentuate the problem. Government and industry should be cautioned against exploiting provisions of the Kyoto Protocol related to forests and sinks as a way to avoid meeting commitments to reduce greenhouse gas emissions. Although many projects, such as tree planting, may enhance carbon sinks and biodiversity, they could also diminish the impetus for long-term solutions to climate change: energy conservation and efficiency, and renewable energy sources. Unless fossil fuel use is reduced, air quality-related health impacts will also not be addressed.

See the David Suzuki Foundation report Taking Credit for more information about the science and policy of sinks.