Scientists from 99 nations have issued a strong warning that says a great increase in air pollution will cause severe climate changes. A new study by the United Nations says global warming is happening faster than expected. The study says human activity is responsible for the increase in global warming. The inter-governmental panel on climate change released the report Monday at a scientific conference in Shanghai. It says future changes in weather will raise temperatures and sea levels. It also says these changes will reduce the water supply in many areas, causing decreased farm production.