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Read more: 鈥�Instant Expert: Carbon capture and storage鈥�

Burning fossil fuels provides most of the world鈥檚 electrical energy. It also produces large amounts of carbon dioxide. Carbon capture and storage (CCS) offers one of the most direct and rapid ways of reducing CO₂ emissions right now. It involves capturing CO₂ from the exhaust gases of large emitters, such as power plants, natural gas production platforms and industry. The gas is then transported by pipe or ship and injected into porous rock to be stored deep underground for millennia. Using CCS to capture and store CO₂ emissions will be crucial to preventing atmospheric CO₂ concentrations reaching a level that will drastically change global climate and the acidity of the oceans.

The impact of carbon dioxide

The volume of CO₂ released by humans has overwhelmed the Earth鈥檚 natural carbon cycle. Concentrations of CO₂ in the atmosphere and upper layers of the ocean have risen significantly since industrialisation.

The effects are twofold. First, the atmosphere retains more heat, leading to global warming and regional climate change. The most recent predictions from climate scientists make a 2 掳C warming look inevitable by 2100, and a 5 掳C warming quite possible.

The other effect is less well known but very potent: ocean acidification. When CO₂ dissolves in water, it forms carbonic acid. Ocean acidity has been stable for the past 20 million years, but climate scientists predict that by 2100 our seas will be four times as acidic as they are now. The effects on marine ecosystems and the species that depend on them, including humans, could be catastrophic.

Halfway to disaster

How much CO₂ is too much? Nobody knows for sure, but sophisticated climate models suggest that if average global temperatures rise more than 2掳C then the consequences are likely to be dramatic, from more natural disasters to increased agriculture failures. The concentration of CO₂ in the atmosphere needed to force a 2掳C rise can be calculated: around 450 parts per million.

A simpler way to consider this threshold is to calculate the total mass of carbon that we would need to burn and emit as CO₂ to force a 2掳C rise. That turns out to be 1 trillion tonnes of carbon (or 3.6 trillion tonnes of CO₂). This is a useful concept, because it means we can estimate how much of this total fossil carbon 鈥渂udget鈥� we have already used. By looking to projected rates of fossil fuel combustion, we can also calculate how much time is left until the remainder of the budget is used up.

The results are shocking. The world is already around halfway through its budget of carbon emissions. To satisfy growing energy demand, the 鈥渢rillionth tonne鈥� of carbon is likely to be burnt within the next 20 to 30 years.

Reducing emissions

For the near future, we will continue to rely on coal, oil and gas for more than 80 per cent of our energy needs. It will take time to develop programmes of energy efficiency and renewable power generation. So unless we are willing to gamble on the future ecology of the continents and oceans, then there is no better way to rapidly reduce carbon emissions over the next 30 to 60 years than by capturing and storing CO₂.

Carbon capture and storage (CCS) is no panacea, but it could provide a major contribution to emissions reduction. The International Energy Agency calculated the cost-effectiveness of the various methods for reducing emissions, and concluded that CCS should provide one-fifth of the reductions required by 2050 (see diagram). Achieving the same cuts without CCS will cost massively more.

For this reason, CCS is an integral part of the low-carbon ambitions of rich industrialised nations, such as the members of the European Union, the US, Canada, Norway and Australia. It is also recognised as an important technology by rapidly industrialising nations like China, South Africa and Brazil.

For CCS to deliver, it will need to be installed on all new and existing power plants burning fossil fuels as well as on industrial sources of CO₂. CCS requires extremely large and complex equipment. Large-scale industrial development of CCS has to start now, in 2011, to make the required impact on CO₂ emissions 鈥� a challenge unprecedented in scale and cost.