More people are projected to die from extreme heat in coming years, and new research is beginning to understand how. The findings suggest that heat may often be overlooked as a cause of death, giving the public a skewed picture of the risks they face in a warming world.

Scientists at the University of Hawaii at Manoa reviewed medical literature to identify ways in which the body responds to heat and how organs are affected. They calculated that there are 27 ways, physiologically speaking, for a person to die from extreme heat.

Their findings, the researchers say, suggest that more people are susceptible to heat-related deaths than previously thought, going beyond traditionally vulnerable populations such as the elderly and people without air conditioning. They believe the findings, released this week as the UN climate conference got underway, should trigger greater concerns over the immediate physical threats from climate change.

"What we're understanding is that the human body is actually very sensitive to heat, and that suggests pretty much everybody's at risk," said Camilo Mora, the paper's lead author. "It's not just the elderly. It's not just the poor. It's everybody."

Mora believes this could motivate faster action on climate policy. "The attitude is: If it's killing someone else, I'll deal with it tomorrow," he said. "This is coming at our doors right now."

Risks Go Beyond Heat Stroke

Mora, a climate scientist, delved into the subject after finding few records that identified or explained the underlying mechanisms of heat-related deaths. Part of the challenge, he said, is that climate scientists and medical researchers operate in their own silos and have very little grasp of work in another field, making it difficult to find relationships or patterns between medical outcomes and climate-related events.

"We need to have more opportunities to reach across these disciplines so we can get an understanding of how bad this is," he said.

Usually, heat-related deaths are reported simply as heat stroke, but that likely underestimates the risks. The American Public Health Association, which has been intensifying its focus on climate-related health risks, says heat may often be overlooked as a cause of death.

"We do a terrible job attributing morbidity and mortality to root causes," said Georges Benjamin, executive director of the association. "What [this research] is saying is we don't capture the full range of things because we're not very good at collecting the data. Death certificates are notoriously inaccurate. So unless someone decides to do a record review after an event—unless they do an in-depth analysis—we're very likely to miss the full scope of the impact of a heat wave."

In reviewing the literature, Mora and his colleagues identified five heat-induced physiological mechanisms, including inadequate blood flow to organs and toxic cells, and seven organs—brain, heart, lung, kidneys, liver, intestines and pancreas—that those mechanisms can impact. They then found medical evidence that 27 heat-induced interactions are possible.

The researchers noted there was a 2300 percent increase in deaths from heat waves from 2001 to 2010, with average global temperatures up less than 1 degree Celsius from pre-industrial times. Heat waves killed 70,000 people in Europe in 2003 and 10,000 people in Russia in 2010.

In an earlier study, Mora and colleagues found that heat waves will pose a danger to 74 percent of the global population by the end of the century—up from 30 percent today—if nothing is done to address climate change.

The new research "emphasizes the heightened risk even under the optimistic target of allowing the planet to warm up another 1 degree Celsius," the authors write.

Combating 'Optimism Bias'

These numbers, the researchers say, should stoke an awareness that climate change isn't an abstraction, but a reality that could have a direct personal impact on people's health.

People often have a reduced sense of concern—an optimism bias—that "basically, climate change may be bad but will not affect me," the authors write, noting that international climate reports tend to focus more narrowly on heat stroke in vulnerable populations. "This narrative can feed our optimism bias because heat stroke alone oversimplifies the many physiological ways by which heat waves kill and thus falls short of depicting our high sensitivity to heat; likewise, the suggestion that only some sectors are at risk could generate a false sense of security for those who are not in any of those vulnerable groups."

Tom Matthews, a lecturer in physical geography at Liverpool John Moores University who has written about communicating the dangers of heat stress, explained that researchers typically calculate the impacts of heat waves by subtracting the number of "normal" deaths from the number of "excess" deaths during a heatwave and attributing the difference to the heat.

The report's finding, Matthews noted, could help break "the myth that excess heat can only kill through 'heat stroke'."

The research, published Thursday in the American Heart Association journal Circulation: Cardiovascular Quality and Outcomes, follows the UK medical journal The Lancet's recent projections of the impacts of climate change on public health.

"The evidence is clear that exposure to more frequent and intense heat waves is increasing, with an estimated 125 million additional vulnerable adults exposed to heat waves between 2000 and 2016," the Lancet report, "Countdown: Tracking Progress on Health and Climate Change" said.

The report noted that the frequency of weather-related disasters has risen 46 percent from 2000 to 2013 yet there's been no clear upward or downward trends in deaths. That, the report found, suggests "the beginning of an adaptive response to climate change. Yet the impacts of climate change are projected to worsen with time, and current levels of adaptation will become insufficient in the future."