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Unpredictable weather: why the climate is not a model citizen

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Richard Cable | 15:13 UK time, Tuesday, 9 June 2009

One of the awkward things about global warming is that there are no absolutes. No one can say definitively what the climate will do next. Anyone who thinks they can will probably end up looking like one of those TV scientists from the 1950s who said we'd all be holidaying in space and flying around in hover cars by now.

But why is it so very difficult to state anything with complete confidence about the behaviour of our climate? Even the Intergovernmental Panel on Climate Change (IPCC), which periodically publishes 'the largest and most detailed summary of the climate change situation ever undertaken' is only prepared to say that human beings are 'very likely' to be the source of the problem. They are hedging for a reason.

Admittedly, it's a little firmer about the temperature itself, stating: 'Warming of the climate system is unequivocal.' But then that's a bit like saying that, today, it is hot. It doesn't tell you very much about tomorrow.

The weather is chaotic. Chaotic systems are infinitely complex and inherently unpredictable, (although not, as some suppose, random). The climate is simply 'big, long weather' - the atmospheric conditions of a region charted over a period of time - and is therefore also infinitely complex and inherently unpredictable.

The IPCC attempts to predict this unpredictability by using climate models - fiendishly complex computer simulations of the Earth's climate that explore 'emissions scenarios'. Each of these scenarios looks at different levels of emissions, and from them the IPCC draws conclusions about where we might be heading.

The models are not without their critics. In order to accurately model a chaotic system, you arguably have to be able to describe the starting conditions of the system and understand pretty perfectly how each of the elements in that system will act upon every other element in that system.

But we don't yet fully understand key issues, such as to what degree carbon dioxide warms the atmosphere or how clouds form and disperse, and can't yet accurately predict even complex human systems that themselves act on climate, like population growth and economic development.

With this in mind it's hard to see how a computer model with so much potential error in its starting conditions can accurately extrapolate what the climate will be doing in 100 years. That's not to say they never will, although anyone who has ever relied on a British weather forecast for the next 24 hours will instinctively take any predictions with a pinch of salt.


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