Climate change pushing weather extremes 'off the scale', says global cities group
"Almost every (C40 member) city is reporting extreme weather events that are off all the scale of previous experience, and ahead of all the modelling of climate change," Watts told the Thomson Reuters Foundation.
"Given that all the scientific models are failing to predict the pace that climate impact's actually having, how do you do good public policy?" he said on the sidelines of the C40's Women4Climate conference.
Nearly half of the 92 cities in the C40 network saw extreme flooding last year, according to Watts, who said an "optimism bias" was built into scientific forecasts.
Unpredictable events are making it increasingly difficult for cities to decide whether they should invest in expensive protection measures such as sea walls, or opt for flood plains instead of building luxury waterfront apartments, he said.
"In most cases, we've experienced something beyond what the model projected, whether that's for flooding, for extremes of heat, or just the switches in the violence of weather we're seeing," he said.
While floods are affecting many cities, the severe drought pushing Cape Town towards "day zero", when its taps could run dry, is sending a message to cities that they need to make themselves resilient, he said.
"It's a huge warning signal as Cape Town is a well resourced city... Yet they're still now going to be first city in the world that runs out of water completely, just because of the severity of climate impacts," said Watts.
"And they're not going to be the last unfortunately," he said.
Ten cities in the C40 network, including London, Mexico City and Sao Paulo, are on "danger watch" and could run out of water if rainfall is lower than average, he said.
Cities could help save scarce supplies be reusing water that flows into drainage systems and is otherwise lost, and by installing features such as permeable pavements to capture rainwater, he said.
"You've really got to plan for the worst now," said Watts.