A draft report on the probable impacts of climate change, written by the world's leading climate scientists, carries a stark warning - that the world may be in for some nasty shocks. The report, entitled "Climate change: Impacts, adaptation and vulnerability", is undergoing expert review and is unlikely to be published for some months.

Pulling the trigger

In its most disturbing section, it deals with the subject of thresholds. These are levels of environmental disruption or pollution below which no observable effect occurs. However, exceeding the threshold can trigger major climatic changes in short periods of time.

Acknowledging the incorporation of such thresholds in some climate models, the authors write: "In climate change, thresholds have been proposed which are much more worrying than this. Below the threshold there may be some impacts, but they will be smoothly varying with the change in the climate. Above the threshold something really nasty may happen.

Examples that have been given include the instability of the thermohaline circulation that drives warmer water to the North Atlantic and the collapse of the west Antarctic ice sheet. If the warm waters of the Gulf Stream ceased to flow, the British Isles would be plunged into severe winters.

The authors say other surprises could include the release of methane - a potent greenhouse gas - from frozen gas hydrates trapped in permafrost.

Best-laid plans

They go out of their way to warn that unpredictable impacts of climate change could have unforeseeable consequences. "Surprises can make even the most careful calculation of impacts way off the mark." And they say the speed at which the climate heats up is crucial. "Non-linear systems, when rapidly forced, are particularly subject to unexpected behaviour. A fast rate of change is likely to generate more 'surprises' which inhibit the effective adaptation of both natural and managed systems."

The report includes a number of more detailed impact assessments for particular sectors:

• A probable decrease of crop yields in the tropics and sub-tropics "raising enormous problems because of the serious potential for exacerbated deprivation".
  
  
• Drought and floods "are projected to become a larger problem in many temperate and humid regions".
  
  
• Soil degradation could be worsened by increased erosion caused by higher rainfall, while elsewhere more land could become desert.
  
  
• Existing stresses on fish stocks could be worsened as water temperature, salinity and entire ecosystems change.
  
  
• Insect species may expand their ranges towards the poles, increasing the risk of crop loss and, in Africa, there could be significant changes in tsetse fly distribution.

The report also says that the composition of the world's forests is likely to change because some tree species will not be able to move northwards (or to higher altitudes) to escape the encroaching warmth.

"Entire forests types may disappear" and there is likely to be "a net decrease in global biodiversity." Nor will humans escape unscathed. The report says land lost to rising sea levels is likely to affect about 70 million Chinese and the same number of Bangladeshis. The IPCC still faces criticism from those who do not accept that climate change is inevitable, or that it matters, or that it is caused to some degree by human activity. But this sober and detailed report - even while it remains in draft form - will command wide respect as an all too credible warning of what possibly awaits the world. (BBC News 18 November 1999).