Humanity is heading for a sharp drop in living standards by the middle of the century unless it stops its massive depletion of the Earth's natural resources, according to a report issued Tuesday. Titled "Living Planet Report 2002," the study said there was so much pressure on water supplies, forests, land, and energy sources that within 150 years the planet's riches could be exhausted and temperatures pushed inexorably upwards. WWF figures show while rich nations draw heavily on Earth's resources, people in poor African states eke out an existence without using all that is available to them within their national borders.