Currently, 3.9 billion people live in cities, representing 54% of the world's population. Cities, as hubs of fossil fuel-based economic activity, emit over 70% of global energy-related greenhouse gas (GHG) emissions. The world's 50 largest cities are collectively the third largest emitter of energy-related GHGs, after China and the U.S. In many North American cities, transportation accounts for the largest share of emissions, while industry and buildings are major sources in many Asian cities. The rate of urbanization is accelerating in the world's most populous countries, with associated rapid and high-volume production of energy- and carbon-intensive building materials to construct urban infrastructure.
Impacts of climate change are already being experienced in cities, from severe storms damaging infrastructure, to droughts and floods, intensified heat waves, worsening smog, and other ecological and human health impacts. Nearly 80 million Chinese city dwellers live in coastal zones at risk for sea-level rise, compared to 30 million in India and 20 million in the U.S. Both as drivers of climate change and sites vulnerable to climate impacts, cities are at the forefront of pursuing energy-efficient and low carbon development.