Not much of a silver lining

It’s been a mixed news week for greenhouse gases. After last month’s ‘revelation’ that levels were dangerously high, there is a glimmer of hope that it’s not too late to act. But crucially, a new piece of work shows the window of time for dealing with the problem is becoming increasingly small.

A natural offset to warming

On an Australian Broadcasting Corporation program called Lateline it was suggested that greenhouse gases were now beyond the level considered likely to cause dangerous climate change. Although the amount of greenhouse gases in the atmosphere is dangerously high, this is just part of the story when it comes to warming our planet. Other factors also play a role in controlling the temperature of our atmosphere and some of these help to cool things down - a phenomenon known as ‘Global Dimming‘. One example is sulphur dioxide given off during volcanic eruptions; this can react with moisture in the air to form a fine mist of sulphuric acid droplets. These aerosols have an impact on the world’s climate by directly reflecting heat back from the Sun. But they also attract water in the air to form clouds, which in turn can also reflect heat back out to space. The result is eruptions can significantly cool the planet; the more sulphur that’s emitted, the colder it gets. When the Philippines volcano Mount Pinatubo erupted in 1991, the amount of sulphur given off helped cool the world’s surface by 0.5°C during the following year. The same sort of effect kicks in with the burning of coal. As more coal-fired power stations are being used to generate electricity – China alone is opening the equivalent of two coal-fired power stations a week – some of the associated sulphur dioxide gas forms aerosols, helping to offset the direct warming effect of the greenhouse gases and giving us a brief respite from the worst effects of climate change. But there’s a sting in the tail. Aerosols generally have a shorter lifetime in the atmosphere than most greenhouse gases. A molecule of carbon dioxide hangs around in the atmosphere for around 100 years whereas many aerosols usually only last for several days before they’re stripped from the air by rainfall. The result is as we start to clean up the world’s power stations, the cooling effect from aerosols will lessen, allowing the full warming effect of the greenhouse gases to become apparent.

If this wasn’t bad enough, a new paper just published in the Proceedings of the National Academy of Sciences shows things are getting worse. Josep Canadell of CSIRO in Australia and colleagues have looked at the changing concentration of carbon dioxide in the atmosphere. Their results are disturbing. In the last seven years, the growth in atmospheric carbon dioxide has increased by 35 percent. This isn’t just from a growing world economy. Since 2000, the global economy has also become less efficient in its use of fossil fuels. But more worryingly is that the world’s natural sinks seem to be soaking up less carbon dioxide. One of the big players is the Southern Ocean. This region has played a major role in amplifying climate change in the geological past; during ice ages, vast amounts of carbon dioxide was pulled down from the atmosphere and pumped into the deep ocean. Today, stronger winds around Antarctica are now mixing up the ocean water, bringing old gas to the surface and slowing the ocean’s uptake of atmospheric carbon dioxide. If present trends continue, this will amplify the effects of any greenhouse gases we’re pumping into the atmosphere. Some recent estimates suggest that by the end of this century, the world’s reduced ability to take up carbon dioxide could warm the world by an extra 1.5°C. It’s not a good sign for the future. Our leaders need to act, and quickly.