Global warming: why is it a bad thing?

"Climate change: Prepare for global temperature rise of 4C, warns top scientist" stated the Guardian's headline on Thursday. It was good to see these issues on the front page, where they should be, and the article was intelligent. Though I did feel that illustrating it with a picture of some plants, captioned "Drought-resistant plants...would become more common in British gardens" rather undermined its seriousness. However, inside, a table showed what the expected impacts would be. This is nicely set out, but still confusing in some ways. So I looked at its source, the Stern report, and found a much clearer version on page 57.

Clearly, there is a lot of uncertainty around these predictions, in that we can't be sure that any given impact will be as predicted, less than predicted, or indeed a lot worse (for example reports that the Arctic is losing ice so fast that the North Pole could have no ice cover in 5 years).

One of the problems getting most people to take global warming seriously is that the difference between, say, a summer's day of 20C and one of 23C is not dramatic. However, if global temperatures rise on average by just 1C, the disappearance of glaciers in the Andes will threaten water supplies for 50 million people, there will be 80% bleaching of coral reefs, including the Great Barrier Reef, and according to one study, we can expect 10% of land species to go extinct. At 2C, crop yield would start to decline in Africa, with potentially 20 - 30% decrease in water availability in Southern Africa and the Mediterranean.

Recent work by climate and policy experts modelled plausible scenarios. The most optimistic was that in 2010, climate-related disasters led to a dramatic change in international policy-making and a cap on total fossil fuel extraction, leading emissions to start falling as early as 2017. This scenario still led to a 2.89C rise in global temperatures by 2100. The next most optimistic was that current negotiations lead to a Kyoto style agreement for developed countries, with others joining as they achieve "first world" status. Under this, emissions continue to rise until 2030, and there would be an expected 3.31C rise in temperatures. At 3C, 1 - 4 billion more people are expected to suffer water shortages, and some models predict collapse of the Amazon rainforest.

The most pessimistic scenario was where governments make commitments on climate change, but then backtrack or fail to comply with them (i.e. as has happened so far). This has 90% probability of a 4.85C rise by 2100. At 5C, according to the Stern report, London and New York would be at risk from flooding, marine ecosystems would be seriously disrupted by acidity and many hundreds of millions in China and India would suffer from water shortages as Himalayan glaciers disappear.

Finally, one thing that has been given little emphasis in public debate is that somewhere well before 5C, positive feedback could kick in (i.e. triggering runaway global warming). For example, the Siberian permafrost melts, releasing methane, or tropical rainforests collapse, releasing carbon dioxide. One recent estimate put the 'tipping point in just 100 months' time.

So, not good. Next week I'll try and get back on the theme of 'positive things to do in the face of all this'.