This is a review of how the increasing population and green house gas emissions of the world's growing cities are changing the global climate and some of the most sensitive regional environments. Meteorological , environmental and social science studies show how cities are increasingly vulnerable to climatic and environmental hazards , and can become even more so as mega-cities grow to 50 -100 km in size , with populations of more than 30million. Experiences around the world show that practical actions to meet the policy objectives of mitigation and adaptation to climate change, resilience against hazards and long term sustainability and security are more effective when they are appropriately integrated. But community initiatives responsive to local environments will always be very important.