The world's cities face the brunt of climate change but some are starting to respond vigorously to the threat, experts say at a conference here staged ahead of the June Rio summit. More than half of the world's population of seven billion currently lives in cities and by 2050, this is expected to increase to 70 percent, or around 6.4 billion, according to UN figures.
More than 60 percent of the increase will occur in Asian cities - and nearly half of the growth will happen in cities that currently have 500,000 inhabitants or fewer.
It means that cities will face unparalleled challenges when climate change starts to bite, scientists said on March 26 at a meeting on the world's environment ahead of the June 20-22 summit. "Cities are emerging as first responders. They are on the frontline, both in the cause and effect of climate change," said Cynthia Rosenzweig, who heads the Climate Impacts Group at the NASA Goddard Institute for Space Studies. The hazards facing cities are many.
By 2100, or sooner, heatwaves, droughts, storms and floods are expected to become more frequent and last longer. Cities built on deltas or on the coast will face rising seas, which threaten homes and drinking water. That raises mighty questions about water supplies, drainage and flood defences and the resilience of homes, offices, factories and transport systems.
In 2003, one of the hottest summers on record killed around 35,000 people in Europe. Some climate scientists predict that by the 2040s, more than half of the continent's summers will be warmer than that of 2003. Alex de Sherbinin of the Earth Institute at New York's Columbia University pointed to a dangerous phenomenon called the urban heat island.
Cities can hold pockets of heat that are up to four to six degrees Celsius (7.2-10.8 degrees Fahrenheit) greater than in the surrounding countryside. The warmth comes from the reflected radiation from treeless streets sealed in heat-trapping black tarmac; from buildings in "street canyons" which block cooling breezes; and from heat discharged by air conditioning ducts.
Those most at risk are the elderly, battling heat stress and air pollution, and the poor, who cannot afford to cool their homes or move elsewhere, he said. All cities will be challenged by shifting climate, but some will be more exposed or cope better than others, said Stephen Tyler, working with a group called the Asian Cities Climate Change Resilience Network (ACCCRN).