Some ideas for interdisciplinary teaching

A Jumbo jet uses about 12 tonnes of fuel an hour – about 36 kg, or 12 gallons, for each passenger – to travel about 500 miles. Thus the fuel consumption is comparable to each passenger going a similar distance by car – one car each. Cars carrying passengers are considerably less wasteful! Buses, ships, and especially trains are incomparably better. Yet the Jumbo is the least profligate aircraft [update: no longer quite true, but the broad picture remains].

Airlines pay considerably less for their fuel than motorists – the cost of a plane ticket is usually less than the value of the share of the fuel used, at the prices motorists pay1. Considering the overall cost including damage to the environment, motorists pay too little – how much more so, airline passengers? What about financing the UN with a tax on plane fuel?

The Sun is 1.4 million km in diameter, and 150 million km distant. Thus its diameter subtends a little over half a degree of arc (about the same as the Moon, 3,500 km in diameter and 380,000 km distant). A mirror of metalized plastic film in orbit at an altitude of 1000 km would need to be 10 km across to give a full reflected ‘second sun’ to a single point on the Earth’s surface. If it were 20 km across it could shine fully on a 10 km circular area on the Earth. If it were orbiting at a greater altitude it would need to be correspondingly bigger. It might be difficult, but perhaps not impossible, to arrange such a mirror to rotate in such a way that it illuminated the same area for some time2. This could warm cold regions, lengthen growing seasons, or provide light at night – some of these applications could use the ‘part suns’ provided by smaller mirrors. Similarly, while between the Earth and the Sun, they would produce a shadow and a cooling effect – if there were sufficient numbers of them, they could have an appreciable effect on the climate, and perhaps be used to counter the greenhouse effect3.

Of course for the shadow and cooling effect you don’t have to do anything as exotic as putting a mirror in orbit. How about laying a sheet of aluminized plastic over a few thousand square kilometers of the Sahara? This would reflect a considerable quantity of energy back off the Earth‘s surface4, and from just where it is most unwanted. Would the local reduction in air heating be enough to cause a local weather effect – even rain? Already the Sahara is not completely rainless; the reduction in evaporation should at least allow the water table to rise. If the area covered had a hole in the middle, located at a low point in the topography, an oasis should develop there – or an existing one extend itself. Anyone got £25,000,000 for our first 5000 sq km project? That’s only a 40 km radius circle – about one two-thousandth part of the Sahara. If our price is about right, covering 10% of the Sahara would cost the people of the Earth about a pound each. It would be only fair for the richer countries who produce proportionately more carbon dioxide to contribute proportionately; the UK contribution would be about £5 a head.

If the UK funded the suggested demonstration project, it would cost us 50p each.

Governments (and large companies) frequently do dafter, more expensive things for less reason. Is it so daft, anyway? Try asking your pupils. If you can, suspend your own disbelief. At the very least, try not to show it. (If you’re actually enthusiastic, try not to show that, either. Be neutral.)

There’s lots of scope for pupil ideas and investigations, and with luck, a bit more excitement than some of the humdrum stuff engenders. Perhaps you can even get the Mathematics and Geography staff involved.

1 This isn’t even budget airline prices. It’s cattle class prices on the major airlines.

2 If so it would presumably also be possible to have two or more trained on the same area for a real hot spot.

3 There is (at least one!) potential problem with all of this: would the mirrors set off like sails on the solar wind? See The Wind from the Sun by Arthur C Clarke. (Which is greater, the pressure from the wind itself, or the pressure from photons? If the latter, does a coronal mass ejection make any difference? To what extent is the wind itself driven by photons, at various distances from the Sun?)

4 Is this true? Have you checked? How much of the heat of the sun arrives in the visible spectrum, and how much in infrared? Does aluminium actually reflect infrared well? Does aluminium even reflect visible light better than desert sand – it reflects specularly (like a mirror) whereas desert sand scatters the light, but does it actually reflect a higher percentage of it, or absorb more? Check! Or better still, get your pupils to check – preferably experimentally!