America's biggest cities could avoid hundreds of deaths during future heat waves if the world reduces its greenhouse gas emissions enough to meet the Paris climate agreement goals, a new study shows.

Even half a degree Celsius makes a difference in the number of lives lost.

"At the path we are on, toward 3 degrees Celsius warming, we get into temperatures that people have not previously experienced," said Peter Frumhoff, director of science and policy and chief climate scientist at the Union of Concerned Scientists and a co-author of the study published Wednesday in Science Advances. "The core point is, across these cities, thousands of deaths can be avoided by keeping temperatures within the Paris target."

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The scientists used detailed data on the deaths that occurred during past heat waves in 15 major U.S. cities, then applied climate models to show what future extreme heat waves would look like as global temperatures rise.

They compared three scenarios: If countries meet their Paris climate pledges, expected to result in about 3 degrees Celsius (5.4°F) of warming this century compared to pre-industrial times; if global warming is instead kept to 2°C; and if countries are able to cut greenhouse gas emissions enough to keep warming to 1.5°C.

With each of those baselines, they explored what temperatures would do to people in the kind of extreme heat wave expected to occur two to three times in a person's lifetime.

"Rising global temperatures mean more people in major U.S. cities will be exposed to extreme heat, and more heat-related deaths will occur. This is relevant to climate policy, especially with the next round of climate pledges taking place in 2020," said University of Bristol climate researcher Eunice Lo, lead author of the study. "This shows the substantial public health benefits of reaching the Paris goal."

Cities vary in their vulnerability to extreme heat for several reasons. Demographics, such as age and poverty, can mean a larger percentage of the population is at risk. Miami, for example, has a large proportion of elderly residents.

Some of the cities are also warming faster than the global average. And dense cities often have a stronger urban heat island effects.

How well the city is prepared also has an effect on survival rates. For the comparison, the study didn't factor in future changes in population, and cities might develop adaptation plans as the heat rises.

Europe and the Staggering Global Scale

The number of heat deaths becomes staggering if you try to project them out to a global scale, with hundreds of millions of people expected to be exposed to potentially deadly heat by 2050, said Dann Mitchell, a climate researcher at the University of Bristol. The challenge for doing similar studies for big cities in Asia, Africa and Middle East is lack of reliable health data.

Analyzing medical statistics is the tried and true way of accurately estimating heat deaths. The European Academy of Sciences recently released its latest estimates of heat wave impacts in a report for European Union policy makers.

Up to 132,000 additional deaths are projected in Europe by the end of the century if the average global temperature rise passes 2°C, the academy's scientists wrote, warning that action to limit climate change is "imperative" to protect public health in Europe.

In Paris and London, cities with extensive public health data, capping global warming at the Paris target of 1.5°C could reduce the number of additional heat deaths between 15 percent and 22 percent, the report concluded.

Robin Fears, EASAC's biosciences programme director, said southern Europe would see some of the greatest effects of rising heat, and that the impacts are exacerbated by population growth and urban development. As everywhere else, the poorest people, including migrant communities in southern Europe, are among the most vulnerable, he said.

What Cities Can Do: Adaptation & Resilience

Heat waves are silent killers. They are deadlier most years in the U.S. than tornadoes, floods, hurricanes and earthquakes combined, but the impact is not as visually dramatic, like collapsed buildings or flooded streets, said Julie Arrighi, a climate expert with the International Red Cross Red Crescent Climate Centre.

People die, one by one, often without a realization across the community of what's happening on a city scale until after the heat wave is over and the sobering statistics are evaluated, she said.

At the same time, heat deaths are among the most preventable deaths from natural disasters because heat waves are forecastable, and there are many things people and communities can do to protect themselves, she said.

Many cities have developed heat action plans, but not all have been implemented. "Those plans need to be scaled up. You need to make sure you can reach most vulnerable populations. Some research estimates that 5 billion people could benefit from simple improvements to early warning systems for heat and cold," she said.

Fears, the EASAC expert, said investments like making sure senior living and care centers have air conditioning would help address vulnerabilities among the elderly.

Arrighi, who helps coordinate global heat resilience efforts with the Global Disaster Preparedness Center, said there's a widespread misperception that heat risks are not a problem in the Global South because it's already hot there.

"That has left a lot of places unprepared for the future because it's not true. You hit thresholds that the body can't adapt to," she said.

Rapidly growing cities, especially in Africa, need to also be thinking about long-term heat prevention by ensuring a lot of green spaces and reflective surfaces in the urban environment, as well as considering regulatory issues around worker safety in connection with heat extremes, she said.

One model other cities could follow is in Philadelphia, where one of the first comprehensive heat wave early warning systems in the U.S. was based on an existing neighborhood crime-watch program, said University of Washington public health researcher Kris Ebi, a co-author of the Science Advances paper.

"Essentially, nobody needs to die in a heat wave and people do, so we're falling short in a lot of different dimensions," Ebi said. "The fact that we see so many deaths and hospitalizations shows we're not doing enough."