One of the key international bodies working on global warming is the Intergovernmental Panel on Climate Change (IPCC). Every five years, they produce a series of reports incorporating the latest scientific information on climate change, its mechanisms, likely impacts, and options for adaptation and mitigation. The next report is due out this year - and according to a draft leaked to the Guardian, it sets no upper limit on the potential impact of climate change.
This is important. As Tim Flannery notes in The Weather Makers, the IPCC is by its nature an extremely conservative body. It has to be, when every word has to be fought over with outright deniers such as Saudi Arabia and the USA. So, when it says something, you expect the reality to be much, much worse. In their last assessment, summarised in the 2001 Synthesis Report, they said that, if current emissions trends continued, average global temperatures could be expected to rise by between 1.4 and 5.8 degrees, a rate of warming "without precedent during at least the last 10,000 years". Since then, models have improved, and begun to take into account some of the positive feedback loops in the climate (such as massive methane releases from melting permafrost or warmer oceans, or releases of soil carbon from the desertification of the Amazon) - resulting in expected warmings of up to 11 degrees. We're already seeing evidence that this is occurring, and not even the IPCC can ignore it. As a result, they've now raised their baseline estimate to between 2 and 4.5 degrees (a fairly important increase by itself), and noted that higher increases are possible.
What this also means is that the "large-scale, high-impact, non-linear and potentially abrupt changes in physical and biological systems" they warned about - things like melting ice caps or a slowdown in ocean currents, are also more likely. And this means that governments have to sit up and take notice. The risk assessments that underlie greenhouse policy have so far ignored these scenarios, considering them too unlikely to be considered a basis for policy. Now they're looking a lot likelier, lending support to a precautionary approach.