Since the 1990s, many in climate policy have held up the idea of 2° as a line not to be crossed. When they say 2°, they mean 2° C warming of the Earth’s average surface temperature, above preindustrial levels. It’s a bit arbitrary and simplistic, but gave policy-makers something to hook onto. Increasingly, however, the validity of this benchmark is being questioned.

In February this year, a paper from the Union of Concerned Scientists warned we are already on track for warming way beyond 2°. Timed for the Doha talks in December 2012, another Nature Climate Change paper argued there’s no way we’ll meet the 2° target without radical emission reduction policies.

One element of the 2° controversy is urgency of action, and how much we will have to do to avoid the benchmark. This can overlap with activities of climate sceptics, or at least it may serve the same interests. As sceptic activity shifts from denial and towards delay and/or dilution of concern, we see people agreeing climate change is happening, just arguing it’s not happening that fast and/ or we don’t need to do much. We might also question the ethics of the idea of 2°, and if society is up for the necessary public debate on such targets, or how to limit global warming. Scientist James Hansen of the Earth Institute at Columbia University, for example, has argued repeatedly, 2° is still way too much and, as Chris Shaw of the University of Sussex argues, we never really had a public debate on this.

If you want to read more: Brad Plummer’s primer is clear, albeit depressing. Plummer’s title ‘how the world failed on climate change’ offers a sense of where he stands on the issue.