The fact that it takes more than 50 litres of water to manufacture a single pack of supermarket lettuce sums up our society's wastefulness when it comes to the earth's resources. Much of the lettuce on the UK's supermarket shelves is grown in southern Spain because of the hot climate.

But it is not just in Europe that water is used inefficiently by intensive agriculture. The same is true in the developing world. And here it is having a terrible effect. Flower and lettuce farms in Kenya, serving the European market, are draining Lake Naivasha, threatening the livelihoods of local fishermen. Likewise, intensive use of water by a Coca-Cola plant in Rajasthan, India, has made it increasingly difficult for locals to irrigate their lands. Coca-Cola is not solely responsible for the problems - droughts are common in Rasjasthan - but the plant has certainly exacerbated the situation.

The appetite of Western consumers for out-of-season produce lies at the root of many of these distortions and injustices. Yet this is no clear-cut moral issue. In the immediate term, the Western world's desire for cash crops from developing countries is one of the few routes out of poverty in these lands. India is a good example of the conflicting demands of globalisation and sustainability. This is a country eager for foreign companies, like Coca-Cola, to create jobs on their territory. But, at the same time, record numbers of Indian farmers are committing suicide because of drought.

The way forward is greater responsibility from consumers. Our culture of convenience is the problem. Increasing amounts of our fresh food is sold washed and bagged. The bottled water industry transports millions of tonnes of water across national boundaries. In this type of market, it is unsurprising that suppliers will bring out-of-season produce to consumers, whatever the cost. Yet we all have the power to change this culture. By demanding locally produced food and eschewing produce that is harmfully produced in the developing world, we can send a message to suppliers.

There can be no doubt that the present system is unsustainable. It is already producing great strains in the poorest parts of the world. And the situation will only grow worse as climate change results causes more droughts and desertification. We will one day look back in amazement that we were foolish enough to waste so much precious water on a single salad.