What constitutes 'safe' global warming?

Currently, global temperatures have warmed around 0.8°C from pre-industrial temperatures with around 0.6°C warming in the pipeline (Hansen 2005). What limit should we set on global temperatures? According to the European Union (EU), global warming should be restricted to 2°C from pre-industrial levels to be considered safe. Two degrees warming are expected to have significant impacts on ecosystems, water resources and global food production. To restrict global warming to around 2°C means we need to stabilise greenhouse gas levels at 450 parts per million CO2 equivalents.

A new study brings more empirical data to this question. It addresses the question: what will the world be like if it warms 2°C degrees from pre-industrial levels? One way to find out is to look at the last time global temperatures were at those levels. This was between 116,000 to 130,000 years ago in an interglacial period called the Eemian. Due to changes in the Earth's orbit, the amount of sunlight on the Northern Hemisphere was higher at summertime than today, triggering a series of changes that drove the planet to warmer temperatures. How warmer?

In a new study Does the Agulhas Current amplify global temperatures during super-interglacials? (Turney & Jones 2010), the authors use 263 estimates from ocean sediments and ice to reconstruct temperatures around the globe during the last interglacial. Globally, the world was around 1.9°C warmer than pre-industrial temperatures. In polar regions, temperatures were more than 5°C warmer while tropical warming was not so pronounced (similar to  warming patterns today). Global warming of 1.9°C is roughly the amount of warming expected in the more optimistic IPCC emission scenarios (to put this in perspective, we're currently tracking above the most pessimistic scenario). So the last interglacial gives us an empirical window into what our best-case future will look like.

What were conditions like at these temperatures? During the last interglacial, sea levels were 6.6 to 9.4 metres higher than current sea levels. Large parts of the Antarctic and Greenland ice sheets melted, with the southern part of Greenland having little or no ice. When we look at accelerating ice loss from the Greenland and Antarctica and wonder about its future trajectory, the past gives the answer. Metres of sea level rise.

There's a degree of uncertainty over the time frames involved. But several peer-reviewed studies, using independent methods, indicate we'll experience roughly 80cm to 2 metres sea level within this century (Pfeffer 2008, Vermeer 2009). The driving question from this study remains: can 6 to 9 metres sea level rise be considered safe?

Posted by John Cook on Monday, 4 October, 2010

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