• The environment absorbs less CO2. About 50% of our current emissions are absorbed by the environment – roughly half of that by the oceans, the other half by plants on land. This uptake of CO2 by the environment may already be in decline.
• Reflection of sunlight drops. As snow and ice cover retreat – as cover is smaller in geographic extent, or seasonal cover lasts for less of the year – dark ground and even darker water are exposed, which absorb sunlight, further warming the earth.
• More CO2 and methane are emitted from nature. Soils, forests, peat, the seas, organic deposits in permafrost, and methane clathrates all emit some CO2 and methane. As the environment warms, “natural” emissions increase.

Less uptake of CO2, plus less reflecting of sunlight, plus more CO2 from nature, can add up to a self-sustaining cycle. Once begun, it might well not end until the uptake of CO2 by nature has largely stopped, there’s very little snow and ice left to reflect sunlight, and stored deposits of CO2 and methane are largely depleted.

When this has occurred, our planet would be a very different place. It might be 5-10C (9-18F) warmer, bereft of most of its living species, and substantially desertified, for a very long time to come.

In its 2007 Report, the Intergovernmental Panel on Climate Change, or IPCC, warned that global warming must be kept at less than 2C (3.6F) to give humanity a 50/50 chance of avoiding runaway climate change. This raises three questions:

• Is 50/50 a chance we want to take?
• Might the climate be more resistant to warming, perhaps giving us more breathing room?
• Might the climate be less resistant to warming? In particular, might runaway climate change have already begun?

This blog’s job is to help investigate these questions, from a science and technology journalist’s point of view. That is, not to do the scientific work to answer the question – no one person could do that – but to review the work that has been done, and new work as it comes along, with a view to answering this question.

I’ve already done a preliminary survey, and made some initial calculations, in my book, Runaway. My current hypothesis – and my personal belief – is that runaway climate change is inevitable, even if emissions from humanity stopped today.

I believe even more firmly that humanity will not do anything resembling halting emissions today, but will emit much more. Even if I’m wrong, and we still have a margin before runaway warming begins, we’re certain to bust right through it before we get a handle on our emissions. If and when we do so, we’ll then have to stop the mightiest genie ever let loose from any bottle, before it permanently changes the earth, and eliminates most or all of us in the process.