Cloudy issues

In a country dominated by desert, the lush tropical rainforests of northern Queensland stand out. This fantastically rich ecosystem is located just inland from the Great Barrier Reef, and maintained by something like three metres of rainfall per year - as much as eight metres, some years. It is still too early be sure, but new research by Graham Jones and his team at Australia's Southern Cross University suggests a surpsigingly intimate connection between this rainfall and the nearby reefs.

One of the great uncertainties in the scientific debate around climate change is the exact nature of the interaction between cloud cover, rising temperatures and increasing pollution. Broadly speaking, the more clouds there are, the higher the Earth's albedo – that is, the more radiation gets reflected straight back into space – and the cooler the Earth gets. On the other hand, uncondensed water vapour and high ice-crystal clouds have the opposite effect, bouncing infrared radiation back towards the ground in the same way that carbon dioxide does.

Warmer seas lead to higher evaporation rates, increasing levels of both clouds and vapour, and it is not yet clear which effect will win out. If the increased albedo caused by clouds turns out to dominate then we have a negative feedback, putting a brake on climate change. If instead the warming effect of the vapour dominates, we are looking at a positive feedback loop, meaning that any heating of the Earth will tend to be self-reinforcing. There is a danger of runaway climate change if this and other self-catalysing mechanisms – the disappearance of polar ice caps (decreasing the Earth's albedo), the melting of the permafrost (releasing stored methane), and so on – prove to overwhelm any negative feedback effects. My brother made an excellent animation about positive feedback loops and tipping points in climate change, with extensive scientific references to back it up, which has been translated into 22 languages by volunteers.

Jones' research gives us yet another source of positive feedback to worry about. Algae, along with other marine organisms, produce a gas called dimethyl sulfide (DMS), which encourages clouds to form. Water vapour in the atmosphere will usually only condense into droplets and form clouds when it has something to stick to, such as specks of dust or soot – or the sulfate aerosols created by DMS when it oxidises in air. The same principle has been exploited with some success in cloud seeding experiments, and if the presence of algae-rich coral and other sources of DMS does indeed cause the increased cloud cover expected, its implications for the local and global climate could be profound. Jones suggests that the presence of coral reefs may help explain the lusciousness and location of rainforests in Queensland and around the world, though this is a long way from being confirmed by research.

At one time it was hoped that rising sea temperatures might lead to more productive algae, increasing the overall production of DMS and creating a negative feedback loop to help slow down climate change. Unfortunately, Jones' research suggests that the opposite is likely to be true. When temperature rises by just 2°C, some coral algae stop producing DMS at all – and of course with less DMS, the cloud cover will be reduced, more sun will get through, temperatures will rise, and levels of DMS will drop.

Perhaps in the future, a better understanding of the mechanisms involved might help us to mitigate climate change either through a global engineering project to increase the planet's albedo, or at least helping us to ensure local levels of rainfall. Until then, the prospect of algae choking in warmer seas is just one more cloud on the climate horizon.


I originally wrote this when I was applying to work at The Economist in 2010, so I made some effort to write in their house style. I'm not sure how well I succeeded; they didn't give me a job, in any case.

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