Isn't global warming just 2 °C and isn't that really small?

Some people insist that small sounding changes in global temperature don't matter, like here for example. On the face of it this sounds sensible: 19 °C or 21°C doesn't matter much, either way I'll wear a t-shirt.

There are 3 problems with even small sounding global warming. Firstly, 2 °C is a very optimistic assessment: we are currently on course to double CO2 twice soon after 2100. If the skeptical Dr Roy Spencer is correct here then we’re on course to get more like 3.4 °C, but if most climate science is correct then we’ll get 6 °C.

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 global temperatures haven't been ~2 °C higher than today in 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”.

Vostok temperature reconstruction

Posted by MarkR on Wednesday, 27 October, 2010

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