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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

What the science says...

A few degrees of global warming has a huge impact on ice sheets, sea levels and other aspects of climate.

There are 3 problems with even small sounding global warming. Firstly, 2 °C is a very optimistic assessment: if the skeptical Dr Roy Spencer is correct here then we’re on course to get more like 3.5 °C. If most climate science is correct then we’ll get 6 °C by doubling CO2 twice.

Secondly, if we cause a ~2 °C warming, some scientists think feedbacks such as melting permafrost releasing more greenhouse gases might kick in. Ice and sediment cores suggest we haven’t been this warm in at least 600,000 years so we’re not sure – but this could trigger a lot more warming.

Finally, 6 °C, the actual “best estimate” for eventual global warming from current CO₂ trends still sounds small. But heating isn’t distributed evenly: as we came out of the last ice age, the temperature in northern countries rose by more than at the equator. When you average over the entire world it turns out to have only been about 6 °C global warming: for people living in Northern Europe and Canada it’s the difference between walking around in a t-shirt and a mile of ice over your head.

The graph below is the temperature calculated over the past 400,000 years in Antarctica from the Vostok ice core. The tiny peaks are a bit like today and the tiny troughs would force hundreds of millions from their homes. A few degrees of warming might sound small, but it can mean a lot and this is why scientists look at what the impacts of warming will be, rather than just saying “it doesn’t look like much so it can’t matter”.

Updated on 2010-10-28 by MarkR.

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