Madeline Darnell, of Athens (Georgia USA), asks :-

The reports about the carbon dioxide in the atmosphere worry me. Is there any way people can take carbon dioxide out? I'm doing my best keeping all these trees in my back yard but that's not even a grain of sand off the beach.

Kim Currie, a marine carbonate chemist at NIWA and the University of Otago, responded.

You are right to be worried about the effects of rising atmospheric carbon dioxide concentrations, and the effects of this greenhouse gas on our climate and on our oceans through ocean acidification. Reducing emissions by cutting the use of fossil fuels such as coal and oil will slow down the rate of increase. However, in order to decrease the atmospheric concentration, the CO2 needs to be actively removed. This is difficult to do on a personal scale, because of the large amounts of CO2 that are needed to be sequestered to have any effect on atmospheric concentrations. Planting trees is one way that individuals can contribute. The tree will remove carbon dioxide from the atmosphere through the process of photosynthesis, and the carbon will be stored as long as the wood is not burnt or decomposed.

Most methods for CO2 removal would be done on a large scale and many such methods have been proposed. These are usually based on enhancement of natural processes, climate engineering, or removal of the carbon dioxide at the source. The process of removal must not produce more CO2 than is removed, some of the proposed methods are very energy intensive!

Artificial trees have been proposed where carbon dioxide is chemically removed from an air stream either passively or by actively pumping large volumes through a scrubber. The carbon dioxide is then stored in such a way as to remove it from the biosphere, atmosphere or ocean, either in the earth's crust or under the sea.

Carbon dioxide from industrial processes can be removed from the flue gases after combustion such as when burning coal in a power plant, or from the source material, such as from natural gas, before combustion. Either way, the CO2 needs to be collected ("scrubbed"), concentrated, transported to the disposal site, then sequestered. Alternatively, the carbon dioxide can be used in industrial processes such as ammonia production, refrigeration or in fire extinguishers.

Transportation to the storage site can be either via pipeline, or by road, rail or ship tanker. CO2 can be stored in in geological formations such as deep saline aquifers, or in gas or oil reservoirs. Injection into almost depleted oil or gas reservoirs can be used to increase the pressure and therefore to aid extraction of the remaining resource. Care must be taken to select a storage site that is geologically stable, to ensure that leakage does not occur, and the CO2 removal is permanent.

Fertilising the ocean to enhance uptake by phytoplankton has been investigated, however ocean processes at many locations mean that the carbon is not permanently removed, but is recycled in the ocean or redistributed via ocean currents.

Growing plant material which is then burnt under specific conditions produces biochar. The carbon dioxide is converted to charcoal (biochar) which will then be tied up in soils.

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