Today, for instance, over 200 million people in 350 cities face stifling heat where average daily peak temperatures hit 35 degrees Celsius (95 degrees Fahrenheit) for three months of the year, according to a study released by C40 Cities, a network of major world cities pushing climate action.
But by 2050, more than 1.6 billion people in 970 cities will face those conditions, researchers predicted.
The number of people who are both in poverty and battling brutal heat — usually without air conditioning — will rise tenfold, they said.
"This is a wake-up call," said Kevin Austin, deputy executive director of C40 Cities, at an international meeting in the South African city of Cape Town on adapting to climate change.
"The magnitude of people affected by heat will be (much) greater than today if we continue to increase greenhouse gases at this rate."
But cities can take action to directly curb the risks, besides working to cut emissions, he said.
In Seoul, for example, a major elevated thoroughfare through the center of the city has been removed, opening up access to the river and lowering urban heat in the area by at least half a degree Celsius, he said.
South Korea's capital also has planted more than 16 million trees and created shaded cooling centers for those without air conditioning.
"We want to encourage cities to adopt more of these solutions and implement them as quickly as possible. In the worst case scenario, they will need to do them quickly," Austin said.
More drought, less water
The research, carried out by the New York-based Urban Climate Change Research Network, looked at data from more than 2,500 cities and predicted likely conditions if emissions continue to rise at their current rate.
It found that Cape Town's ongoing battle with drought-driven water shortages could become far more common, with over 650 million people in 500 cities — among them Sao Paulo and Tehran — likely to see their access to water reduced by 2050.
Many thirsty cities are already aiming to set caps on water use per person, with Los Angeles pushing for 200 liters a day, Melbourne for 155 litres and Cape Town a dramatically reduced 50, Austin said.
The U.S. Geological Survey estimates that an average American today uses at least 300 liters of water per day.
Sharing advice on how to make cuts happen — including insights gained in Cape Town, which has slashed its water use by half in the face of extreme drought — can save cities time and help them make changes faster, Austin said.
But more cities "need to transition in a planned way, not in response to disaster," he added.
Cape Town Mayor Patricia de Lille said dealing with a crisis when it arrives leaves little room to maneuver.
"In a crisis like this there is no time to go by trial and error. You unfortunately have to get it right the first time," she said at the Adaptation Futures conference in Cape Town.
Power risk from floods
The C40 Cities study also found that by mid-century over 800 million people will live in 570 coastal cities at risk of flooding from weather extremes and sea level rise.
Flooding presents a particular risk to urban power supplies, with many power stations located in flood-prone areas - and everything from transportation to heating and hospitals at risk if power plants flood in cities from London to Rio de Janeiro, the study noted.
Decentralizing power systems - including by getting clean energy from a larger number of smaller power plants - could help cut the risks, researchers said.
But flooding risks may be coming faster than expected.
Patrick Child, the European Commission's deputy director-general for research and innovation, said a predicted one-meter (3-foot) rise in global sea level, once anticipated by 2100, is now expected by 2070.
Last year already saw the highest-ever documented economic losses from severe weather and climate change globally, he said.
Experts at the adaptation meeting also predicted that extreme weather could bring cascading problems for cities, with flooding, for instance, triggering everything from disease outbreaks to road failures, food shortages and closed schools.
Looking at just one type of problem — such as a health threats from extreme heat, or sea level rise — isn't enough to capture the risks, said Cynthia Rosenzweig, a climatologist at the NASA Goddard Institute for Space Studies and one of the authors of the report.
"In cities, all of these impacts interact with each other, and are all happening at the same time," she said.
Solutions also need combined approaches, with engineering efforts to cut flooding, for instance, working hand in hand with things like better protection of flood-absorbing wetlands, Rosenzweig said.
She said she hoped the research would help city officials prioritize what changes need to happen first to better protect their citizens from climate threats.
In cities "it's often overwhelming, with so many things to do," she said.