The planet can support around two billion humans but there is a problem - there will be seven billion by the year after the London Olympics and heading for a peak of around nine billion by 2050.

There are two ways to try to cope with this in the face of relentless climate change and the imminent decline in oil supplies - even though it is likely that our numbers will decrease through disaster, warfare, disease and starvation.

We must both adapt the way we live and look for new technologies - though the latter is how we got into the mess in the first place.

Previous efforts with technology have not always worked the way we intended. Artificial fertilisers have ruined the stability of many soils and while DDT killed mosquitoes for a time, it did the same to many other creatures.

Man-made CFCs, for refrigeration, almost destroyed the ozone layer, and fire-retardant asbestos has had a major health impact. BSE was the result of feeding animals to other animals instead of vegetation, and the effect of mobile phones is possibly still unknown.

However, "geo-engineers" are unrealistically optimistic and they have worked out technical interventions to slow down climate change and allow us to carry on with business as usual.

One idea is to mimic a volcanic eruption - sulphur injected into the stratosphere reduces temperatures though the risk of increased acid rain and ozone layer damage is considerable.

A remarkably expensive idea is to put 600,000 square miles of mirrors into space to reflect the sun's radiation and reduce it by a significant one per cent.

Increasing cloud cover by spraying water vapour in the air would be cheaper but it could seriously alter weather patterns.

A better idea involves synthetic "trees" that scrub CO2 from the air so it can be collected and stored underground though it may use up more energy and produce as much CO2 as it saves.

Plankton take CO2 out of sea water and they thrive in iron-rich waters so dumping iron filings in the sea would lead to plankton blooms. The side effects are unknown and the case may be overstated.

One recommendation is particularly interesting because of who made it and why. Two very reputable scientists, the head of the Science Museum and ex-Antarctic Survey boss Chris Rapley and creator of the Gaia concept, James Lovelock, suggest deep ocean pipes could bring up cold water and mix it with warm surface waters. The stimulated plankton would absorb CO2 that would fall to the bottom in time.

Clearly, these two are not confident that mankind can use political persuasion to change.