From today Zanzibar is banning plastic bags. It may be true that Zanzibar once tried to ban trousers, on the ground that they are non-Islamic. But the plastic bag is even more emblem-atic of consumer society than Oxford bags. And for such a flimsy object, it causes even more damage.

Discarded shopping bags are tumbled like tumbleweed across the islands. But unlike tumbleweed, they do not compost or burn. They provide micro-habitats for malaria-carrying mosquitoes. They block gutters and drains, choke animals and fish, pollute the soil, and have become the national flower because they flap on every thornbush. They can take up to a thousand years to decompose. Penalties for those caught with carrier bags are not flimsy. Offenders against the law risk a jail sentence of six months, and a fine of up to 2,000 dollars (£1,500).

An ocean to the east swirls the North Pacific litter vortex. This is an garbage archipelago of plastic that can swell to twice the size of the United Kingdom, and become solid enough to walk on. It is composed of plastic dumped from ships or blown from the land. It is made up of billions of plastic bags, beach toys, used condoms, toothbrushes and the rest of the detritus of our age. Plastic does not decay like organic material. It threatens marine life and the ecology of the planet.

This is a modern paradox that a thing as flimsy as a plastic shopping bag should have become a wake-up call for preserving the planet. But for want of a horseshoe nail, the battle was lost. And the trivial plastic bag is easier to identify with than the ozone layer or switching off electric devices on standby. Very few people anywhere do not use plastic bags to carry their shopping, and those few are old-fashioned creatures with canvas bags.

But the tide is turning against plastic elsewhere than in Zanzibar. Taiwan and Singapore are moving to ban free plastic bags. In South Africa they are a blot on the veldtscape. This year the Republic of Ireland introduced a tax of 15 cents on a plastic bag in an attempt to curb litter. It has been a success. The Irish Government says that its “plastax” has reduced the use of plastic bags by 90 per cent and raised millions of euros in revenue. Bangladesh has banned polythene bags, after it was found that they were blocking drainage systems, and were blamed for the disastrous floods.

Even in the lands of extreme liberty, the message is percolating. British supermarkets used to shower free plastic bags on their customers. They saved time at the checkout and identified goods that had been paid for. Now the environmentally aware stores ask customers whether they want a plastic bag, in tones inviting the answer “no”.

It will take time to re-educate the planet to environmentally unselfish behaviour. Sticks and carrots are needed, and above all, education and sensitive intelligence. But, paradoxically, the plastic bag is a simple place to start. From little acorns mighty oaks grow. Little platoons of shoppers reducing their petty dependency on carrier bags can make the Earth a safer, greener and healthier place.