Sceptical confusion about the carbon cycle

Self-proclaimed climate change ‘sceptics’ are resorting to desperate measures in their efforts to confuse the public about the causes and consequences of global warming.

Their latest tactic has been to try to muddy the water about the undeniable rise in atmospheric concentrations of greenhouse gases due to human activities, which was briefly discussed in the Horizon programme ‘Science Under Attack’.

The programme’s presenter, Sir Paul Nurse (2001 winner of the Nobel Prize for Physiology and Medicine and new President of the Royal Society), interviewed Dr Robert Bindschadler, Chief Scientist at NASA’s Hydrospheric and Biospheric Sciences Laboratory, about the likely causes of the unequivocal rise in global average temperature over the past few decades.

Dr Bindschadler indicated that human activities emit the equivalent of about seven billion tonnes of carbon into the atmosphere each year, whereas natural sources, such as volcanoes, only produce about one billion tonnes.

Christopher Booker, whose weekly column in The Sunday Telegraph regularly recycles the content appearing on ‘sceptic’ blogs, attacked Dr Bindschadler’s statements, describing them as “mind-boggling” and “a grotesque misrepresentation”.

Mr Booker claimed that natural sources account for more than 96 per cent of annual emissions of carbon dioxide.

So who is right? Well, this discussion about emissions, of course, relates to atmospheric concentrations of carbon dioxide and other greenhouse gases. Carbon dioxide levels varied between about 180 and 300 parts per million during the 650,000 years prior to industrialisation, as recorded in air bubbles trapped in ice in Antarctica. But since industrialisation began in the 18th century, the concentration of carbon dioxide in the atmosphere has increased from about 280 to 390 parts per million, i.e. a rise of about 40 per cent.

As the measurements on Mauna Loa, Hawaii, show, there is a large natural exchange of carbon dioxide between the atmosphere, oceans and land, known as the carbon cycle.

In the northern hemisphere, during the winter and spring, there is a net transfer of carbon dioxide to the atmosphere (raising the concentration), and in the summer and autumn, this transfer is reversed (decreasing the atmospheric concentration). Globally each year, the land and atmosphere exchanges about 120 billion tonnes of carbon, while the oceans and atmosphere transfer about 90 billion tonnes of carbon between them|. In general, this natural carbon cycle is more or less in equilibrium, such that there is no significant net change in the amount of carbon absorbed in the atmosphere, oceans and land.

But we also know that human activities, such as burning fossil fuels, producing cement and destroying rainforests, have disturbed the natural equilibrium of the carbon cycle by emitting a further 7 billion tonnes of carbon into the atmosphere each year|. The land and oceans absorb about 45 per cent of this, but the remainder stays in the atmosphere and leads to the annual increase in concentrations which has been recorded in the measurements from Mauna Loa and elsewhere around the world.

There no serious dispute that the rise in atmospheric concentrations of carbon dioxide and other greenhouse gases since industrialisation is due to human activities. Some ‘sceptics’ have attempted to blame volcanoes|, but the United States Geological Survey and others| have pointed out that they produce about 100 times less carbon dioxide than human activities each year.

Yet Mr Booker appears to want to create doubt in the minds of his readers, when really there is very little doubt among the scientists.