The World Resources Institute (WRI) looked at what might happen in nearly 1,000 major cities if temperatures remain on track to rise 3 degrees Celsius above pre-industrial levels.

They found that the impact on these cities -- and the 2.1 billion people who inhabit them -- would be dire compared to a scenario where global warming is restricted to 1.5C.

"At 3 degrees C of warming, many cities could face month-long heatwaves, skyrocketing energy demand for air conditioning, as well as a shifting risk for insect-borne diseases -- sometimes simultaneously," the authors wrote.

In 2015, nearly 200 nations agreed in Paris that the world must strive to limit global warming to 1.5C to avoid the most disastrous consequences of climate change.

Taken together, the world's climate pledges and commitments today would only cap warming at 2.9C, according to the latest UN assessments.

"The difference between 1.5 degrees C and 3 degrees C has life or death consequences for billions of people worldwide," said Rogier van den Berg of WRI, a US-based think tank.

- Poorest hit hard -

Their report -- slated for publication in April but delayed for a revision -- underscores the particular risk for fast-growing cities in low-income countries.

By 2050, two-thirds of the world's population will live in cities and more than 90 percent of that urban growth will occur in Africa and Asia.

"People living in low-income cities will be the hardest hit," the authors wrote.

The IPCC, the world's leading scientific authority on climate change, has also been examining the specific threat for cities posed by rising temperatures and is devoting a major upcoming report to the issue.

Robert Vautard, co-chair of an IPCC working group, said cities had "very specific climate problems" and most "were not yet built, so there is real potential for transformation at the root".

Across the world's largest cities, WRI estimates the longest heatwave each year could last 16.3 days on average under a 1.5C scenario, but 24.5 days at 3C.

Their frequency is also likely to rise, from 4.9 heatwaves per year in the average city to 6.4 per year.

This would in turn spur enormous demand for air conditioning and energy.

In Johannesburg, demand for air conditioning at 3C would be 69 percent higher than at 1.5C, placing extra strain on a city that already suffers water and electricity shortages.

- 'Wake up call' -

Hotter cities would also provide optimal conditions for mosquitoes that carry potentially life-threatening arboviruses like dengue, zika and chikungunya.

At 3C of warming, 11 of Brazil's largest cities could see high arbovirus risk for at least six months of the year.

In Rio de Janeiro, this would rise 71 percent from 69 days of peak transmission to 118 days per year.

Conversely, rising temperatures may reduce the number of peak malaria days around the world, though cities in temperate zones in Europe or North America could witness an increase.

Again, the poorest cities -- those with the least means to adapt to climate change -- are the most exposed.

Freetown, Dakar, Sierre Leone and other capitals in sub-Saharan Africa "could suffer heatwaves of more than a month" with an average of seven such events each year.

"This data should serve as a wakeup call... now is the time to start preparing cities for a much hotter world while doing everything we can to slash emissions," said van den Berg.