The human impacts of global climate change aren’t just about rising sea levels, storms, droughts, and floods. If present trends continue, researchers say, the current century will also see large numbers of human deaths from excess heat — not just in the tropics, but in relatively temperate climes such as the United States.
In fact, says Eunice Lo, a climate scientist at the University of Bristol, UK, some US cities could see killer heat waves producing thousands of excess deaths.
In a study published in the journal Science Advances, Lo’s team compared weather data to day-to-day death rates in 15 major US cities, calculating how many people in each city could be expected to die from heat-related causes on any given hot day.
They then ran weather simulations for the US under three future climate scenarios: one in which the international community achieves the Paris Agreement’s most ambitious goal of limiting this century’s average global warming to no more than 1.5 degrees Celsius from pre-industrial times; another in which it holds the rise to two degrees; and a third in which the world remains on its current trajectory toward an average temperature rise of three.
The scientists then ran 900 simulations of North American weather under each setting, using their heat-and-mortality data to tabulate the number of heat-related deaths expected to occur in the target cities during each simulated year.
They then picked out events they describe as one-in-30-year hot summers — not the worst possible heat waves, but the type of once-a-generation ones most relevant for city planners.
If climate is stabilised at either of the two Paris Agreement goals, the number of heat deaths these summers produced in each city increased, but not enormously. But at three degrees warming, it shot up.
Worst was New York City, where at three degrees, a one-in-30-year hot summer increased the number of deaths by more than 2700, compared to what would happen at 1.5 degrees.
But Miami and Los Angeles could also each see approximately 1000 more deaths at three degrees than under either of the Paris Agreement goals.
Even Seattle, which lies only 200 kilometres south of Canada, could see hundreds of extra heat deaths in the worst-case scenario.
Lo sees this as yet another wake-up call regarding the need for progress toward meeting the Paris Agreement’s goals.
“We desperately need to immediately and drastically further reduce greenhouse gas emissions,” she says. {%recommended 8596%}
But she is also aware of the irony that her study found that global warming poses deadly risks in a country whose president, Donald Trump, recently pulled out of the Paris Agreement.
“The Paris Agreement is something that could prevent hundreds to thousands of heat-related deaths in their own country,” she says.
“Some people think about climate change as something that affects places that are far away, or [that] it would just affect future generations and doesn’t really matter now,” she adds.
“This research highlights that in the US — the biggest economy of the world — there are people who are going to be affected adversely. It’s not something that only happens to people we don’t know, or who are really far away.”
Not that America is the only country with cities that will experience heat-wave-related deaths if global warming is allowed to continue.
“This paper is US focused, as are most heat-health studies, due to data limitations,” says Ethan Coffel, a climate scientist at Dartmouth College, New Hampshire, US, who was not part of the study team.
“But the US is not among the most vulnerable regions for heat deaths.”
Developing countries in the tropics probably have far higher rates of heat death, “and climate change impacts will be even larger there”.
Lo agrees. “Pretty much all places in the world will face extreme heat,” she says. “I’ve looked at regions in the UK, and the big picture is the same.”
And, she notes, “we have data for other countries that we haven’t looked at [yet]”.
Lo’s primary interest is in highlighting the urgency for action to achieve the Paris Agreement’s climate goals. But her study also highlights the need for local and regional planners to prepare for the type of hot summers that future climates will bring.
“That’s very important in terms of preventing heat-related mortality,” she says.
In part, this can be done by increasing each city’s ability to handle killer heat waves via advance warnings, public education about the risks, and increased provision of public cooling places for those who would otherwise be left to swelter.
But some of these aren’t without their own problems.
“The obvious adaptation to heat— using more air conditioning — is projected to drive big increases in electricity demand,” Coffel says.
“[That] will increase carbon dioxide emissions unless that new electricity is produced with zero-carbon sources.”
Vivek Shandas, an urban climatologist at Portland State University in Oregon, US, notes, however, that no such problems apply to a simpler approach: using “urban greening” to provide shade and evaporative cooling to mitigate the heat of urban “microclimates.”
And that’s not the only way in which alterations to the physical, built environment can be used to mitigate urban heat, his team reported recently in the journal Atmosphere.
These findings are only from a model, he admits, “[but] we’ve validated them through a series of local trials.
