Oceans clearing greenhouse gases faster than expected

Greenhouse gases over the tropical Atlantic are disappearing faster than expected, according to the first comprehensive measurements taken in the region.

British scientists working at the Cape Verde Observatory on the volcanic island of São Vicente believe chemicals produced by sea spray and tiny marine organisms are speeding up natural processes that destroy the gases.

Detailed measurements taken over a year revealed that levels of one greenhouse gas were substantially lower than climate models predicted.

The observatory, which was recently set up by British, German and Cape Verdean scientists, has given researchers an unprecedented ability to study climate change in one of the most remote regions in the world.

In the tropics, intense UV rays in sunlight trigger reactions that effectively scrub greenhouse gases from the air. Without this natural cleaning process, atmospheric levels of the gases and other pollutants would be substantially higher than they are.

One of the most important cleaning reactions destroys ozone, which contributes to global warming at lower altitudes. A byproduct of the reaction, called hydroxyl, cleans the air even more by breaking down methane, the third most abundant greenhouse gas in the atmosphere. The study, led by scientists at Leeds and York Universities, revealed ozone levels over the tropical Atlantic were 50% lower than expected.

Ozone is a byproduct of burning fossil fuels, and is carried down to the tropics from the UK and Europe on trade winds. As the air crosses the ocean, the harsh sunlight reduces ozone levels from around 50 parts per billion (ppb) to 10 ppb, the scientists report in the journal Nature.

To check their readings, the team used a research plane to measure ozone levels at different altitudes over the ocean. Their readings confirmed a major loss of ozone in the remote region.

"At the moment this is a good news story, more ozone and methane being destroyed than we previously thought. But the tropical Atlantic cannot be taken for granted as a permanent sink for ozone," said Alastair Lewis, who led the study at the national centre for atmospheric science in York.

"It may mean that there are sources of methane and ozone that we aren't aware of that the atmosphere has been working away on without us realising it. Or perhaps we've been overestimating how quickly ozone is removed in other parts of the world," Lewis added.

Instruments at the observatory later pointed to an explanation for the rapid destruction of ozone over the ocean. The sensors picked up bromine and iodine oxide, which are produced by sea spray and plankton, and join forces to attack ozone.

Lewis said the discovery should give renewed impetus to programmes that aim to reduce atmospheric methane and low-level ozone. "It's an incentive to get on with cutting our emissions of these gases, because if we do, the atmosphere will scrub them away quicker and we will get the benefits sooner. Carbon dioxide lasts for hundreds of years, while methane lasts only a year or two in the tropics," said Lewis.

Climate scientists will use the findings to fine-tune computer models that attempt to predict future warming. "We now have to get to grips with why this wasn't spotted earlier and ultimately put this into our models so we can improve their accuracy," said Lewis.