Original article by Alison Colls, Environmental Change Institute. Read this fascinating introduction to carbon sequestration: what is it, different types of sequestration, and why we need to consider it seriously.

Introduction

One way of contributing to a reduction in greenhouse gases in the short term could be a process known as carbon sequestration. This involves the deliberate removal or storage of carbon in a place (a sink) where it will remain. What follows is an outline of some of the different approaches which are being, or have been tried.

Photo from Berkeley National Laboratory

Sections in this article

• The impetus for sequestration

• Ocean sequestration

• Geological sequestration

• Forest sequestration

The impetus for sequestration

By signing up to the Kyoto Protocol, developed countries (though notably not the USA) have agreed to reduce their emissions of greenhouse gases collectively by 5.2% from 1990 levels between 2008 and 2012.   The UK is legally committed to a reduction of 12% by 2008-2012, although the Government has also set a higher domestic target of 20% by 2010.

The Kyoto protocol is a start but more dramatic reductions will be required in the long term, if climate change is to be stabilised.  According to The Royal Commission on Environmental Pollution, the UK will need to reduce its emissions of carbon dioxide from burning fossil fuels by 60% over the next 50 years. Sequestration could have a significant role to play in meeting these targets.

Ocean sequestration

Scientists are exploring the possibility of sequestering carbon into the oceans.

One approach is to enhance the natural oceanic uptake of carbon dioxide (CO₂) by microscopic plants called phytoplankton.  The theory is that fertilising the world's oceans with iron (a micro-nutrient) would increase biological productivity and enable phytoplankton near the sea surface to absorb more CO₂, thereby removing it from the atmosphere and storing it in the ocean's interior. However, there are major concerns about the impact such fertilisation would have on marine ecology, as well as doubts about how effective it would be at increasing the long term storage of CO₂ in the oceans.

Another approach is to inject liquid CO₂ into the deep ocean.   Model studies suggest that CO₂ injected at depths of 1500m, could, with careful site selection be stored for several hundred years.  Longer-term storage could be achieved by injecting CO₂ at depths greater than 3000m, where the density of liquid CO₂ exceeds that of water. In theory, this would enable liquid CO₂ to be injected into a hollow or trench on the seabed, where it would remain as a submarine lake.  In the long term, it might also be feasible to produce solid CO₂ blocks which would sink to the ocean floor, with little impact on the ocean environment.

There is a great deal of uncertainty about the permanence and stability of carbon stored in the deep ocean and many concerns about the impact it would have on the sensitive deep ocean environment.

Geological sequestration

A possible alternative way of sequestering carbon dioxide would be to bury it underground. Collecting and compressing carbon dioxide from power plant exhausts, and then burying it in, for example, old oil wells at sea, deep deposits of briny water or depleted natural gas fields, is technically feasible. The Norwegian energy company Statoil has practised the technique in the North Sea since 1996, as part of a European research project. They extract carbon dioxide from the natural gas being produced and then inject it into porous sandstone some 3,000 feet below the floor of the ocean.  To find out more, read the article ‘Geosequestration' in this section (for a link, see ‘Where next?').

Forest sequestration

Although geological sequestration may be important, it is biological sequestration of carbon in plants and soils that may have the greatest impact during the immediate few decades.  The world's forests represent an important carbon sink, and just as deforestation can reduce this sink, so afforestation can be used to increase this sink. On this basis, the Kyoto Protocol makes provision for developed countries to take into account forestry activities and land-use changes to meet their commitments.    

Forests could be incorporated into an emissions trading system by allocating credits, for the amount of carbon sequestered. Satellite remote sensing could be used to determine (and later verify) initial areas of land use, as well as the extent, rates of change, and locations of activities that result in forest cleaning and regrowth. 

Changes in other carbon stocks, such as soils, are much harder to verify because they are so variable over time and space.  Despite this uncertainty, the soil carbon stock is greater than the carbon stored in vegetation.  Consequently soil conservation is important for minimising the oxidation (caused, for example, by soil erosion or ploughing) and subsequent emission of soil carbon to the atmosphere. 

There is increasing concern that planting more trees should not be seen as the solution to the problem of increasing atmospheric CO₂. Forests are only sinks when they are expanding in area or growing. In the absence of major disturbances, newly planted or regenerating forests only continue to uptake carbon for 20-50 years, depending on species and site conditions.  It is also feared that unless global warming is gradual enough, forest decay and fires may turn forests into a global source of carbon. Furthermore, it has been suggested that an increase in forests could alter the mean global albedo (reflectivity of the land surface) and lead to more radiation being absorbed, warming up the earth!

Where Next?

On climatex.org

You may be interested to read the following related articles in this section:

‘Geosequestration' by Andrew Chadwick, The British Geological Survey, and ‘Lower Carbon Fuels' and ‘The Carbon Cycle' by Alison Colls.

You can also read an interview with Tina Fawcett a former researcher at the Environmental Change Institute about garbage, recycling, and carbon in the UK, in ‘Don't throw the climate out with the garbage'.

There are articles about the many solutions that exist to combat climate change: see for example ‘Introducing Solar Photovoltaics', ‘Wave Energy: an Overview', and ‘Be Your Own Generator'.

External links

The IPCC Special Report on Land Use, Land Use Change and Forestry

An article by James Kastings on The carbon cycle, climate and longterm effects of fossil fuel burning on the US Global Change Research Information web site