The typical image used to represent the process of global warming is a power station, belching out black smoke. But an equally valid image would be an oil palm sitting serenely under a tropical sky. Rainforests are being cleared across south-east Asia, West Africa and South America to make way for palm oil plantations, which produce the world's cheapest vegetable oil. Yet deforestation is one of the greatest drivers of climate change. The destruction of the planet's rainforests is responsible for 20 per cent of global carbon dioxide emissions, as hardwood trees that have locked up carbon for decades are felled and burned.

Tropical deforestation might feel like something that is remote from our daily lives in Britain. But the reality is that the consumer choices millions of us make every day are contributing to the destruction of these forests. Half of all packaged food products sold by our supermarkets are made with tropical palm oil.

But this is not an exclusively British phenomenon. Food manufacturers across the world are helping to drive demand for palm oil. And in so doing they (and we) are adding to the forces of destruction assailing our precious, carbon-storing rainforests.

Palm oil cultivation does not need to involve such rampant destruction. If planted on marginal land, its environmental impact can be minimal. And many Western companies signed up three years ago to a commitment to use Asian palm oil from sustainable plantations, rather than the variety produced by rainforest clearance. But as we reveal today, their record in following through on these commitments has been miserable. Relatively few have made serious efforts to ensure that their palm oil is sustainably sourced. Although British manufacturers have generally been better than those in the rest of Europe, their achievement is nothing to boast of. The food industry as a whole has failed to make a decisive shift to sustainable palm oil.

Failure threatens on other fronts too. As we reported this week, the fate of a global deforestation treaty that will be presented to international delegates at the Copenhagen climate change summit in December is hanging in the balance. As presently framed, this treaty would grant Western subsidies to poor nations that cut down virgin rainforests and replace them with palm oil plantations. This is the opposite of what is required. Subsidies from rich countries to encourage developing nations to preserve their rainforests are undoubtedly needed. They will encourage sustainable economic growth in some of the poorest nations in the world while protecting a common international environmental resource. But there can be no question of subsidising palm oil plantations.

If this treaty is ratified in its present form it would be a disaster. Its effect would be to encourage the destruction of rainforests and accelerate the catastrophic release of carbon dioxide into the atmosphere that such felling would entail. Responsible governments need to ensure this treaty is modified before it reaches Copenhagen. But relying on high politics is not enough. Consumer pressure is also needed. At the moment, many food manufacturers are paying little more than lip service to their environmental commitments. Shoppers in the rich world should increase the pressure on such firms by boycotting products made with unsustainable palm oil.

The threat of environmental disaster that hangs over us comes in many shapes; and few loom larger than the shape of the oil palm.