Carbon Sinks and Greenhouse Warming

The Carbon Cycle - Kevin Saff

Carbon sequestration techniques may soon be needed to supplement natural carbon sinks as a means of 'mopping up' excess atmospheric carbon dioxide.

A natural carbon sink is defined as anything that absorbs more carbon than it produces, whereas a natural carbon source produces more carbon than it absorbs. Natural carbon sources include the respiration and decay of living organisms, emissions from volcanoes and bushfires and gaseous exchange between the oceans and the atmosphere. Natural carbon sinks include the soil, oceans, forests and solid earth.

Although the time taken for carbon to cycle between the atmosphere, the surface waters of the ocean and living things is relatively brief, a large amount of carbon is locked away in the form of coal, oil and natural gas or as inorganic carbonates on the ocean floor. It takes millions of years and the presence of volcanic activity for carbon to be released from carbonates, while the carbon in fossil fuels remains trapped indefinitely.

Why Can’t the Oceans Absorb All the Excess Carbon Dioxide?

Industrial activities involving the burning of these fossil fuels have consequently upset the natural carbon cycle. These, and other human activities such as cement production and land use change, have added around 5.5 Gt (gigatonnes: 1 Gt = 1 billion tonnes) per year to the atmosphere (see above diagram). According to studies made by Samar Khatiwala, the oceans only absorb about 20-35% of all man made carbon dioxide emissions and may have in fact decreased their rate of absorption by around 10% since 2000.

Moreover, as atmospheric temperatures increase, the solubility of carbon dioxide in the oceans will decrease, thus reducing their effectiveness as carbon sinks. This is supported by studies showing that even in present conditions around 40% of all anthropogenic (human induced) carbon dioxide is absorbed in the cold waters of the Southern Ocean near Antarctica.

Indeed, even if more carbon dioxide does dissolve in the oceans, the resulting increase in acidity may adversely affect marine life. As a consequence, relying on the oceans to soak up excess carbon emissions is not a realistic option.

How Reliable are Forests as Carbon Sinks?

While plants take up carbon dioxide as a means of manufacturing organic compounds in photosynthesis, it should be noted that a forest or an expanse of grassland will only have a net ‘sink’ effect while it is growing. When large areas of vegetation reach maturity, the carbon released through respiration and decay will eventually balance any carbon taken up in photosynthesis. The uptake of carbon dioxide by forests can therefore range from 1.0 to zero Gt per year.

In addition, when sudden bushfires or deliberate burning off of this vegetation occur, large amounts of the carbon that has been stored in the plant material is released all at once into the atmosphere. Attempts to reforest or ‘afforest’ (create new tracts of forest) large areas of land as an attempt to gain carbon credits may therefore only offer a temporary carbon storage solution.

Organisations such as ‘Fern’ use this argument to question the wisdom of the Kyoto Protocol decision in 2001 to allow nations to offset their industrial carbon emissions with tree planting. Indeed, they also point out that increasing the area of forested land will interfere with the livelihoods of farming communities and could also reduce the earth’s ability to reflect heat (known as its ‘albedo’).

Until recently, scientists believed that enhanced atmospheric carbon dioxide levels would promote the growth of vegetation (as a result of increased photosynthesis), which would subsequently result in greater carbon sequestration. Various studies, including those made by Dr Richard Norby, however, have shown that limiting factors such as nitrogen levels in the soil will restrict the extent of plant growth under greenhouse conditions.

Soils as Potential Carbon Sinks

Carbon, in the form of humus, is taken up by soils, and is a key ingredient in soil organic matter. Estimations of the amount of carbon held in soils range from 700-1000 Gt .When soils are tilled, however, organic matter previously protected from microbial action is decomposed rapidly because of changes in water, air, and temperature conditions.

This carbon loss can be reduced by using alternate farming methods which include ‘no till’ farming, crop rotation, the use of manures and mulches and winter cover crops. Such methods cannot, however, be relied upon to help the soil act as a sink for excess greenhouse carbon.

Man Made Methods of Carbon Sequestration

In light of the above, scientists are consequently using available technology to devise their own methods of carbon sequestration. In terms of increasing the capacity of the soil to absorb carbon, one solution may lie in the development of ‘biochar’, a product formed from the pyrolysis of biomass. In this process, green waste is heated in the absence of oxygen to produce a carbon rich substance – ‘biochar’- that can be added to soils as a fertilizer.

Other sequestration methods being explored include geosequestration (storing carbon deep in the ground), mineral carbonation (converting atmospheric carbon dioxide into carbonates) and ocean sequestration. All techniques are currently in the experimental stage, and will not offer a solution to global warming in the short term. As a consequence, immediate reductions in greenhouse gas emissions would appear to be a more logical line of action to take.

References

Bevan, P., 2007 ‘Soils Offer New Hope as Carbon Sinks’, dpi.nsw.gov.au

Fern, 2010, ‘What Are Carbon Sinks?’, fern.org

Khatiwala, S., 2009, ‘Reconstruction of the History of Anthropogenic CO2 Concentrations in the Ocean’, nature.com

Parliament of Australia Library, 2010, ‘Carbon Sequestration’, aph.gov.au

University of NSW, 2010, ‘Trees Unreliable Carbon Sinks’, sciencealert.com