Washington, Aug 31 - Megacities, with populations exceeding 10 million, that consume energy, food, water, and other raw materials and release wastes are contributing to global climate change, researchers say.

More than half the world's population today lives in cities, and the world's largest urban areas are growing rapidly. The number of megacities has grown from just three in 1975 to about 20 today.

Charles Kolb, from the Centre for Atmospheric and Environmental Chemistry, (CAEC) said: 'Carbon dioxide and other pollutants in megacities make them immense drivers of climate change.'

'They impact climate on both a regional and global level because these long-lived greenhouse gases are dispersed around the world.'

Kolb said that the most highly polluted megacities are in developing countries.