Michael McCarthy: Pollution and debris can be dealt with - but at a price

The only practical way to get rid of that water is to send it down into the Gulf of Mexico, which is an important fishery; but it may be possible to do that on a controlled basis, so there is not a single enormous pollution "pulse".

Once the floodwaters have gone, there may be land that has been contaminated - which will have to be subjected to remedial techniques - and there will be a substantial amount of debris. When thousands of homes have been flooded to the rooftops, countless numbers of ruined televisions, electrical appliances and household goods will have to be disposed of somewhere.

But all of this can be done - at a cost. It is not clear that either the bayou country of Louisiana, with its crayfish and rice cultures, or the Gulf of Mexico itself will be damaged beyond repair. This is because many ecosystems are resilient, having evolved over millions of years in conjunction with the natural violence that the earth sometimes produces, and have remarkable powers of recovery from even from pollution, if the latter is not continuous.

It may seem counter-intuitive to say so, but despite its enormous human and political impact, Hurricane Katrina may not prove a long-term environmental disaster. Some scientists, for example, believe that Prince William Sound in Alaska, the site of the US's largest oil spill in 1989, when the tanker Exxon Valdez leaked 11 million gallons of oil after running on to a reef, has recovered completely. It is only fair to record that some scientists do not believe this, and the issue is complicated by the fact that Exxon, which has paid $900m (£490m) in damages, is liable to another $100m if long-term damage can be proved by next year.

Real environmental catastrophes are something different. They occur when a whole habitat disappears, or when the workings of an ecosystem are permanently disrupted. An example of the first would be Amazonian deforestation - the clear-cutting of old-growth rainforest that has evolved over millenniums and cannot replaced. An example of the second would be the collapse of the Grand Banks cod fishery off Newfoundland in 1992, which has not recovered. Because of overexploitation, the fishery appears to have undergone "regime shift", where it has stabilised at lower productivity.

Taking out whole habitats or ruining whole ecosystems infallibly produces environmental disaster, but dumping huge amounts of pollution or dealing out violence to nature can sometimes be repaired - not that one would want to take the risk.