3 February 2007
It's a pretty grim conclusion: greenhouse gas reduction targets being talked about to stop climate change will not now avoid potentially catastrophic rises in global average temperatures, yesterday's report from the UN Intergovernmental Panel on Climate Change (IPCC), makes clear.
CARBON DIOXIDE LEVELS
The target for stabilising CO2 levels in the atmosphere which some scientists and politicians increasingly hope to aim for - an upper limit of 550 parts per million - would probably involve a rise of 3C, perhaps one as high as 4.5C, and almost certainly no lower than 1.5C, the report says.
But a 3C rise would bring about enormous damage to agriculture, weather patterns and ecosystems across the world with catastrophic effects on human society.
The figure of 550ppm represents a doubling of atmospheric CO2 compared with the level pertaining before the Industrial Revolution. The current CO2 level is about 382ppm, having risen from 315ppm 50 years ago, and is rising by more than 2ppm annually, with the rate increasing.
Leading scientists such as Professor Sir David King, the Government's chief scientific adviser, have suggested that 550ppm should be the upper limit the world community should allow the CO2 rise to reach.
Although 550ppm is seen as far too lax by environmenalists, other observers see it as an ambitious target for a world so wedded to carbon use. Yet yesterday's report makes clear that even in the 550ppm target were attained, the future looks bleak.
TEMPERATURE RISES
The report says that for the next two decades a warming of about 0.2C per decade is predicted. It says that even if CO2 emissions had been halted completely in 2000, the world would still be committed to a rise of about 0.6C, because of the time the climate system takes to react. The report offers a series of projections of future temperature increases, based on six different scenarios of global human, and in particular, industrial activity, and consequent emissions of CO2 from cars and power stations.
The best estimate for the low scenario, known as B1, is a 1.8C rise (on a range of 1.1C to 2.9C) and the best estimate for the high scenario (known as A1FI) is 4C, with a lower range limit of 2.4C, and the upper range limit of 6.4C - up susbtantially from the worst-case figure of 5.8C given in the IPCC Third Assessment Report, known as the TAR, published in 2001. As we report elsewhere, a 6C-plus rise would represent a true nightmare for the world.
FEEDBACK
What has informed these new projections is the increase over the past six years in knowledge of climate "feedback" mechanisms - by which a warming world becomes less able to absorb CO2, which further increases the warming, which further slows the CO2 uptake, and so on. The report says: "The new assessment of the likely ranges now relies on a larger number of climate models of increasing complexity and realism, as well as new information regarding the nature of feedbacks from the carbon cycle."
SEA LEVEL RISES
With regard to sea level rise, yesterday's fourth report (known as AR4) actually has a more optimistic upper-range prediction than that given in the TAR in 2001, predicting a worst case of a 59cm rise by 2100, compared to 88cm in the earlier report.
But hang on. Don't breathe that sigh of relief.
The report admits that the computer models currently being used to simulate future sea levels do not yet take account of feedback mechanisms. Furthermore, although the predictions include a contribution due to increased ice flow from the Greenland and Antarctic ice sheets for the period 1993-2003, they do not take account of the fact that these flow rates could increase as the world warms. If this contribution were to grow linearly with global average temperature change, the report says, the worst-case upper limit could increase by another 10cm to 20cm - and so back up to 79cm.
Sea-level rise approaching a metre would vastly increase the risk of catastrophic storm damage to low-lying regions such as Bangladesh or Egypt's Nile Delta.
ACIDIFICATION OF THE OCEANS
For the first time, the IPCC scientists address another issue which has emerged since the last report: the acidification of the oceans. Much of the CO2 we are pumping out is absorbed by the sea, but there has been so much of it that it is starting to react chemically with the water to produce carbonic acid. The pH value of the sea - the way in which its acid/alkaline balance is measured - has already decreased by 0.1 units since pre-industrial times, and could decrease by a further 0.14 to 0.35 units during the 21st century, the report says.
This steadily acidifying ocean will pose a severe threat to many of the small marine organisms which need an alkaline environment, to build their calclum carbonate shells, and could threaten the whole marine food chain of which they are the base.
The fourth IPCC report is most noteworthy for its greatly increased confidence levels. It says that the fact that global warming is taking place is now "unequivocal", saying this is evident "from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level". The IPCC scientists believe there is "at least a nine out of 10 chance" that this has been caused by human actitivities. In practice the report will end the debate about the fact and causes of climate change for all but the most perverse sceptics, and will serve as a forceful spur to unified action for a world community still divided over how to combat the greatest threat the Earth has ever faced.
The 600 meterological researchers who prepared the report used 14 different supercomputer simulations of the world climate system in institutes around the world, which were run more than 50 times.
Main points of the IPCC report
* "Best guesses" are that the global temperature will rise by 1.8C to 4C over the next century,depending on the level of the world's population and industrial activity. These are global averages and the local figures would be higher in high latitudes, such as Britain.
* Worst-case scenario is that with high fossil fuel use and strong economic growth, rise could be 6.4C, again, with higher rises nearer the poles.
* Stabilising CO2 levels in the atmosphere at 550ppm - which some experts think the world should aim at - would itself probably mean a rise of 3C, and possibly 4.5C.
* Temperatures in the next two decades are likely to rise by 0.2C per decade.
* Sea-level rise worst case scenario is 59cm by 2100, less than predicted in 2001, but this might be much higher when climate system feedbacks are factored in, and ice discharge from Greenland and Antarctica rises.
* That global climate change is occurring is "unequivocal."
* That human beings are responsible for it is "at least a nine out of 10 chance".
http://news.independent.co.uk/environment/article2211567.ece
