Saturday, January 15, 2005


Ever heard of global dimming? Maybe you’re more in touch than I am, but although I thought I kept abreast of environmental issues, until an hour ago I’d never heard of it (more here).

Airborne pollution comes in two types - greenhouse gases which give rise to well-known global warming effects, and particulates – tiny solid particles of soot, ash etc. These latter give rise to the visible pollution haze that hangs over our big cities and industrial areas; they contribute to respiratory disease but it now seems have an even more alarming effect, namely global dimming.

It works like this: the pollution particles provide sites for water vapour in the atmosphere to condense into droplets and so forming clouds, but these droplets are much smaller and more numerous than those that form in pollution-free clouds. And it turns out that clouds made up of smaller and more numerous droplets are more reflective than their clean cousins, so much more of the suns rays are reflected back into space. Less sunshine reaches the ground, or the oceans – hence global dimming.

Greenhouse gases cause global temperatures to rise, but global dimming has the opposite effect; it causes them to fall. It now seems these two forces have been working in opposition, yet most world climate models to date take no account of global dimming. As ever in our arrogant way, we thought that we knew all there was to know about the link between greenhouse emissions and global warming – but if global warming has been kept in check by global dimming, what happens when we alter the balance? Global warming turns out to be a much more powerful force than we imagined; take that check away, in effect take the brakes off, and suddenly we find that greenhouse emissions have a far greater impact that we first thought.

The thing about interdependent systems – such as those that support life on our planet - is that their behaviour is never simple or obvious. There is strong evidence, it seems, that the terrible African droughts of the 1970s and 1980s which led to the worst famines seen in modern times were caused, at least in part, by global dimming. The chain of causality goes something like this: The growth of industrial activity combined with limited pollution controls in Europe and North America in the middle of the last century gave rise to an increase in airborne particulates in the northern hemisphere, and particularly over the Atlantic ocean. The polluted clouds reflected more sunlight back into space and the temperatures dropped. This caused changes in air circulation patterns which took the rain-bearing clouds away from sub-Saharan Africa, hence the droughts. Simple, and catastrophic.

So what do we do? If we go back to allowing airborne particulate pollution so as to keep global warming in check, we risk disease and ever more terrible famine. It’s no good patting ourselves on the back over our success with clean-air policies if we don’t achieve equal success in reducing greenhouse emissions – that way, global warming will run away twice as fast as previous climate models suggested. The predictions may sound like scare-mongering, yet they are entirely plausible: in our lifetime we see melting of the Greenland ice-cap causing sea levels to rise 25 feet, submerging many of the world’s major cities; tropical rain-forests become tinder-dry, burn, and turn to savannah or even desert. The scariest part is that it may only take 20 years to pass the point of no return on the road to some of these changes. Time is running out fast.

There really is no option. We have to understand that our planet is not a stable, static system, but sits for ever on the edge of instability, with all kinds of opposing forces – climatic, tectonic, demographic – existing in a precarious balance. Humankind has become powerful enough to upset this balance; we have to stop treating our planet like a sewer.

What will it take before governments look beyond their own survival at the next election and start taking these issues seriously?

Doubtless there will be sceptics. They may even be right. But as a famous physicist once said: “If you doubt every new idea in science you will be right 90% of the time, but you will be wrong the only time it matters.”

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