Point of no return

One of the less well-reported effects of the 2003 heatwave in Europe was its impact on the continent’s carbon emissions. Europe-wide monitoring systems showed a 30% drop in primary productivity (basically plant growth) across the continent, as plant photosynthesis began to shut down in response to the twin stresses of high temperatures and crippling drought. From the deciduous beech forests of northern Europe to the evergreen pines and oaks of the Mediterranean rim, plant growth across the whole landmass slowed and then stopped.

Instead of absorbing carbon dioxide from the air, the stressed plants instead began to emit it; around half a billion tonnes of carbon was added to the atmosphere from European plants, equivalent to a twelfth of total global emissions from fossil fuels. This is a positive feedback of critical importance, because it suggests that as temperatures rise – particularly during extreme heatwave events – carbon emissions from forests and soils will also rise, giving a further boost to global warming.

A similar and even more dramatic positive feedback is predicted to emerge from Amazonia as the tropical forests there begin to die back in a warming climate, releasing billions of tonnes more CO2 into an already overloaded atmosphere. Warmer soils also lose carbon due to increased respiration by bacteria, and even greater quantities may reach the atmosphere through this route worldwide.

Polar regions too hold the potential for global warming tipping points. One may already have been passed. Global warming is amplified at the poles because as ice melts, revealing a darker ocean surface, more solar radiation is absorbed, ramping up temperatures still further. In the last decade, the northern polar ice-cap has reached all-time lows – with September 2004’s record being swiftly broken by a still greater melt extent in September 2005. Even if greenhouse gas emissions stopped tomorrow, it is unlikely that this ice would return.

In Siberia, huge areas of frozen peat bog are already melting, and will release billions more tonnes of carbon dioxide and methane as they warm, potentially pushing the planetary climate past a point of no return after which global warming becomes virtually unstoppable. It is unclear how much this warming can be limited by human emissions reductions – Siberia is already one of the most rapidly heating areas on the globe.

The Greenland ice cap provides another tipping point – and one which holds enough water to raise global sea levels by 7 metres, enough to flood major coastal cities around the world like New York, London, Mumbai and Shanghai. Studies suggest that less than two degrees of average global warming will be enough to tip Greenland into irreversible melt.

Many different groups – including campaigning NGOs like Friends of the Earth and WWF and inter-governmental bodies like the EU – have identified two degrees (above pre-industrial temperatures) as an absolute limit above which global warming must be avoided. Although climate impacts would still be dramatic, from melting glacial ice to biodiversity losses, some of the most catastrophic changes like the loss of the world’s coral reefs and the rapid melting of Greenland will hopefully be avoided with such a limit. The target would also – the groups hope – make passing global warming points of no return much less likely.

However, avoiding two degrees of warming with any high degree of probability requires stabilising CO2 concentrations in the atmosphere at 400ppm (parts per million), according to the International Climate Change Taskforce. This in turn requires urgent cuts in emissions. Currently CO2 concentrations are rising so fast that the limit will be breached within fifteen years – a sobering thought indeed.