The Stern report makes recommendations that will allow for a temperature rise of around 3°C, but this is likely to be devastating for the planet. George Monbiot says that, "Two degrees is the point at which some of the most dangerous processes catalysed by climate change could become irreversible".
Stern includes the consequences of a 2°C rise as: a 20-30% decrease in water availability in some vulnerable regions; 15-40% of species facing extinction; and the potential for Greenland ice sheet to begin melting irreversibly, accelerating sea level rise and committing the world to an eventual 7-metre sea level rise.
But is he too cautious in his predictions?
The 2006 Conference of the International Association of Hydrogeologists heard that rising sea levels will also lead to the inundation by salt water of the aquifers used by cities such as Shanghai, Manila, Jakarta, Bangkok, Kolkata, Mumbai, Karachi, Lagos, Buenos Aires and Lima long before they are finally flooded.
It was predicted that the floating Arctic ice cap may melt by 2100, but increasing rates of melt now suggest there will be no Arctic icecap in the next five to 15 years, and the Arctic region will rapidly begin heating by as much as 12°C, triggering the irreversible meltdown of the Greenland ice sheet much earlier than expected.
The 2001 Intergovernmental Panel on Climate Change (IPCC) report predicted that a local warming of larger than 3°C, if sustained, would trigger the complete melting of the Greenland ice sheet. What is truly frightening is that temperature increase in the Arctic region has already been 3°C, five times the average global increase.
Positive feedback mechanisms triggered by a rise of 2°C are likely to push temperature increases towards 3-4°C. Positive feedback occurs when, for example, a rise in temperature triggers an event (such as the melting of Arctic floating ice) which then triggers a further rise (because the dark seas absorb sunlight and heat, whereas the white ice reflects the sun). This is the Albedo feedback effect, now being witnessed in the Arctic polar basin.
Other positive feedback mechanisms include:
As the Arctic warms, melting permafrost in the boreal forests and further north in the Arctic tundra triggers the release of huge volumes of methane, a powerful greenhouse gas, from thick layers of thawing peat.
Decreasing sulphate emissions. Sulphate gases from aerosols and burning fossil fuels are masking the effects of global warming because they have a cooling effect of 2–3°C. Any significant reduction in sulphate emissions — be it a reduction in the use of fossils fuels, a large economic downturn, or campaigns such as that by the European Union to reduce aerosol emissions (as sulphates are a source of acid rain) — will diminish the masking effect and result in higher temperatures.
A decreasing capacity of soils to absorb carbon dioxide. A recent study found that the calculated increase in carbon lost by British soil each year since 1978 is more than the entire reduction in emissions in Britain between 1990 and 2002, as part of Kyoto. It is thought that at 2-3°C the conversion of the terrestrial carbon sink to a carbon source will begin due to temperature-enhanced soil and plant respiration overcoming CO2-enhanced photosynthesis. This will result in widespread desertification and enhanced feedback.
As the Amazon rainforest turns to savannah (triggered by land use change interacting with warming, but predicted to become calamitous at 2-3°C) it turns trees back into carbon dioxide. The destruction of the Amazon rainforest will cause catastrophic water shortages across most of south America, and will turn the Amazon from being one of the world's great CO2 sinks to being one of the world's great CO2 producers as the rainforest rots and decomposes.
If positive feedback mechanisms, triggered by a rise of 2°C, push temperature increases towards 3-4°C, it is predicted the oceans may warm sufficiently to trigger the destruction of the ocean's algal mass — the world's largest carbon dioxide sink — which could result in making most of the world uninhabitable.