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Climate Change
and Carbon Dioxide Sequestration
The threat of climate change is one that hangs over all of us and is a threat that will also affect our children and grand-children unless action is taken to stabilise or even reverse some of the changes which are already taking place.
Atmospheric concentrations of carbon dioxide have risen to levels which are unprecedented in the recent history of the earth. Deep ice cores from Antarctica, which helps reveal the Earth’s climate history, show that carbon dioxide levels are the highest for at least 440,000 years. And - the amount of carbon dioxide in the air is likely to triple in the next 100 to 150 years. The amount of carbon dioxide in the air is likely to triple in the next 100 to 150 years. The result could melt some polar ice, raising sea levels and severely damaging low-lying areas. In addition, higher temperatures may damage crops and human health and cause other problems. Indeed, ecosystems around the world are already being affected by global warming. One of the Kyoto protocols seeks to limit the accumulation of carbon dioxide (CO2) and other greenhouse gases in the atmosphere. Sequestration (or “locking-up”) of carbon dioxide may hold one of the keys to meeting this objective. Our science correspondent assesses what this means and how it might affect us.
Carbon sequestration from afforestation
Most of us are familiar with the concept that actively growing trees use CO2
but that this ceases when they mature. In productive forests, trees are often
harvested before they mature so that sequestration is not halted. However, this
process is also reversible because of the vulnerability of forests to fires,
pests, changes in land use, and, in some parts of the world, the effect of climate
change itself. Thus the potential of forests to sequester the sorts of volumes
of CO2 to make a major impact is rather limited. For example, UK emissions are
currently of the order of 155MtC (million tonnes carbon) per year. Planting
trees to absorb just 2 MtC/year would require forest area to expand at about
30,000 ha/yr for next 20 years.
However, additional benefits can arise, such as that. from improved biodiversity and new recreational opportunities, though increased water uptake by trees can in some situations (e.g. in dry tropics) have impacts on supplies of groundwater and river flow.
Geological storage
Sequestration in suitable geological formations such as disused oil and gas
fields or unmineable coal measures, involves removal of carbon dioxide from
flue gases prior to combustion. The first such facility for storage of CO2 began
operation in 1996 in the Norwegian sector of the North Sea. Here, one million
tonnes of CO2/year are being stored in a deep saline reservoir. Although it
is still in a development phase, it is possible that this type of sequestration
might contribute in the future, provided concerns about reservoir security can
be met.
Oceans
The ocean represents a large potential sink for sequestration of CO2 emissions.
Carbon dioxide once captured could also be injected into the deep ocean for
disposal, at depths greater than 1500m, or at the sea bed (at depths greater
than 3000m), where a lake of liquid carbon dioxide would form. This technology
is not yet demonstrated and research is required into possible impacts on marine
life, and in particular, deep ocean species in and below the sea floor.
In the case of seabed storage, the environmental impacts will be concentrated on a very small fraction of the global seabed, but might be severe. More research is required to investigate the effects of increased oceanic CO2 concentrations on marine life.
The benefits
One of the key benefits of capture and storage of CO2 is that it would allow
continued use of fossil fuels in a climate-friendly way. As a result, society
would have more choice over the rate at 
which
it wishes to switch to other sources of energy. This could be important for
some major developing countries, such as India and China , both of which have
vast coal fields and are heavily dependent on use of these local fossil fuel
reserves. If they choose to burn their coal, we in the west are not in a very
good position to tell them not to, because its exactly what we did in our industrial
revolution.
Conclusions
Substantial reductions in CO2 emissions could be achieved whilst continuing
to use fossil fuels by application of technology for capture and storage of
CO2. This could be applied to power stations and many energy-intensive industrial
processes.
Capture of CO2 can be done using available technology, although there is scope for reducing costs and energy consumption. Storage of CO2 is feasible in natural underground reservoirs, which have sufficient capacity for many years' emissions, although the possibility of long-term leakage if aquifers are used certainly needs investigation.
Finally, questions still remain about the costs, safety, legality, environmental impacts, long-term effectiveness and public acceptability of such an approach.