Capturing the greenhouse gang
Box 1 | Natural carbon capture
Trees
Many power stations and other greenhouse gas polluters have already started to pay for the establishment and management of tree plantations to 'offset' their carbon dioxide gas emissions. The idea is that if polluters continue to emit carbon dioxide, they should balance this pollution by capturing and storing it elsewhere.
Trees absorb carbon dioxide during photosynthesis. They use energy from the sun to drive a reaction between carbon dioxide and water to form carbohydrates, releasing oxygen as a waste product. Some of the captured carbon becomes stored in the trunk, branches, leaves and roots of the tree, where it might stay for decades. Even if the tree is cut down, carbon dioxide might still be kept out of the atmosphere if the wood is used to make long-life products, such as houses.
Scientists also believe that, in some conditions, the increased concentration of carbon dioxide in the atmosphere could help stimulate plant growth, accelerating its uptake. This is because the greater availability of carbon dioxide, an ingredient of photosynthesis, will act like a fertiliser. Other resources, such as water, nitrogen and trace elements, would also need to be available. Research has shown that the fertiliser effect tends to level off in natural ecosystems within a few years due to these other limiting factors. Other effects of climate change (eg. higher temperatures) can work against the fertiliser effect resulting in decreased growth in some regions.
The Kyoto Protocol, an agreement between nations designed to reduce the greenhouse gas emissions, allows countries to count carbon dioxide sequestered in tree plantations against their overall greenhouse gas emissions, but there are risks. Fire, disease, drought and poor management can all cause premature tree death. If a tree rots or burns, all its good carbon-capture work is undone because carbon dioxide, methane and other greenhouse gases are released to the atmosphere. Even carbon dioxide in well-managed, disaster-free plantations is likely to re-enter the atmosphere within decades or, at best, centuries.
Phytoplankton
Other organisms absorb and store carbon dioxide. Phytoplankton use a carbon-based compound, calcium carbonate, in their skeletal structures. They are abundant in the world's oceans and play a significant role in the global carbon cycle. Some scientists have speculated that stimulating the growth of phytoplankton by adding a micronutrient, iron, to phytoplankton 'dead spots' in the ocean could help remove large quantities of carbon from the atmosphere. When the phytoplankton die, their skeletons sink to the bottom of the ocean, where they can remain for many centuries. The impact of iron fertilisation of the oceans on carbon dioxide levels and the environment is subject to debate.
Crops
According to some scientists, even agricultural crops can help sequester carbon dioxide. As they grow, many plants form tiny, rigid particles called phytoliths. Phytoliths are made of silica and sometimes calcium oxalate or other minerals; these substances are deposited in and around cells as the plant grows, eventually forming microscopic sacks. Inside the sacks are small fragments of organic material – including carbon. Since these sacks are made of minerals that don't break down, the carbon trapped within them cannot escape. When the plant dies, its phytoliths remain in the soil, often for thousands of years, forming a long-term carbon pool.
Phytoliths are present in most soils of the world, but agricultural crops can produce them in large quantities. An Australian study found that the organic carbon trapped in phytoliths by a variety of sugar cane commonly used in New South Wales, for example, was more than 30 times higher than that observed in natural vegetation communities. Other sugar cane varieties sequester even higher amounts. Scientists suggest that the selection of crop varieties that yield high levels of phytolith carbon could make a substantial contribution to reducing the concentration of carbon dioxide in the atmosphere.
Box
Box 2. Clean technologies for fossil fuels
Related sites
CO2 tree capture – how much carbon dioxide do trees really capture? (Catalyst, 19 April 2007, Australian Broadcasting Corporation)
Forests, wood and Australia's carbon balance (Forest and Wood Products Australia)
'Plantstones' could help lock away carbon (New Scientist, 7 January 2008)
CO2: Don't count on the trees (New Scientist, 27 October 2007)
Climate myths: Higher CO2 levels will boost plant growth and food production (New Scientist , 16 May 2007)
Tree farms won't halt climate change (New Scientist, 28 October 2002)
Iron fertilisation of the oceans (Nova: Science in the news, Australian Academy of Science)
Farming the climate (ScienceAlert, 9 February 2007)
Posted July 2008.