Medical experts are worried about climate change too. Here's how it can harm your health.

As the world nears the end of what could be the hottest year in recorded history and heads into one predicted to be hotter still, a report underscores the health consequences of the warming climate.

The 8th annual report of the Lancet Countdown on Health and Climate Change, released Tuesday, describes a "grave and mounting threat" if we fail to reduce greenhouse gas emissions, especially given the evidence of worsening world health as the planet warms.

It outlines many ways this warming trend is already impacting the health of Americans. They include heat waves that stress young and old bodies and threaten to overwhelm hospitals; droughts and floods that endanger the food supply; the spread of disease to new areas, the extension and altered timing of allergy seasons; increases in air pollution and the growing scale of lethal fires.

"As we see temperatures continuing to rise and wildfires continuing to get worse, we're just seeing these really stark increases in impacts to health," said Naomi Beyeler, the lead author of the U.S. section of the report.

It's not too late to change the trajectory of global climate change, she and other experts say. But the world is getting close to the precipice.

"We have the tools at hand. We have the money at hand. We can do this," said Dr. Kari Nadeau, chair of the Department of Environmental Health at the T.H. Chan Harvard School of Public Health. Nadeau was not involved in the Lancet report, but praised its analysis and priorities. "It's really a matter of the political will."

Nadeau said she doesn't want to be a purveyor of doom, but climate change and its health impacts are no longer something coming in the future, it's something that is happening now.

"I hope people realize it's going to affect them, their children, their grandchildren and their friends," she said.

There is room for hope, and for improving public health if countries take action, according to Beyeler, of the University of California, San Francisco.

The Lancet report is one of several arriving ahead of COP 28, a meeting of the United Nations Framework Convention on Climate Change that begins Nov. 30 in Dubai.

Countries that committed to the Paris Agreement have pledged to try to keep global temperatures from rising more than 2.7 degrees above pre-industrial times.

That goal is still achievable, but only if governments, companies and banks stop "negligent" investments in oil and gas, according to the Lancet report. "Without profound and swift mitigation to tackle the root causes of climate change, the health of humanity is at grave risk," it says.

Also on Tuesday, the White House released the nation's Fifth National Climate Assessment and the U.N. Framework Convention on Climate Change released an analysis of the climate plans of member nations and found them lacking.

"The climate crisis is not just changing the planet – it is changing children," according to a report from the United Nations Children's Fund, released on Monday.

Who's to blame for climate change?Scientists don't hold back in new federal report.

Human-induced climate change is warming Earth

This year, more than nine in 10 people worldwide encountered high temperatures made much more likely because of human-caused climate change, the Lancet report found. It represents the consensus of more than 100 experts from dozens of research institutions and UN agencies. Topics include 47 indicators of household air pollution, financing of fossil fuels and engagement from international organizations on the health benefits of limiting climate change.

Among its findings:

• The heat was most extreme in the tropics, concentrating the impact on developing countries.
• Every country experienced some level of climate-driven heat, including the U.S.
• Heat-related deaths of people older than 65 years increased by 85% from 1990–2000, above the 38% increase that was expected if temperatures had not changed.

The unprecedented heat this year has sparked widespread alarm among many climate scientists. The global average temperature through October was the highest on record, nearly 2.6 degrees Fahrenheit above the pre-industrial average, the European Union's Climate Change Service said last week.

If the global average temperature rises by 3.6 degrees Fahrenheit above pre-industrial times by mid-century, the world could see a 370% increase in heat-related deaths and increasing food insecurity for more than a half-billion people, the report states.

Health consequences of climate change

Health consequences of climate change come directly from warming temperatures, melting ice that can lead to floods and expose new pathogens and droughts that affect the food supply and the likelihood of forest fires. Contagious diseases are likely to spread more, too, experts said, either through vectors like mosquitoes that can survive in new, warming regions or because people searching for new food sources are coming into closer contact with wild animals, passing on diseases like Ebola.

The increased intensity and frequency of wildfires have undermined air quality improvements since the passage of the Clean Air Act in 1970, Beyeler said, and have even led to reversals in some areas. "There's emerging evidence that smoke may be even more harmful to health than non-smoke particulate pollution," she added.

The scale of the exposures was greater this year than ever before, with tens of millions of Americans breathing in unhealthy air from Canadian wildfires.

This, combined with extreme heat events which especially harm older adults, placed an added burden on the health system, Beyeler noted.

Heat waves occur when temperatures remain elevated for several days in a row, including overnight. They're particularly dangerous because the overnight warmth doesn't give people, animals or crops any chance to recover, Karin Gleason, chief of the monitoring section at the National Centers for Environmental Information, told USA TODAY.

"If you don’t cool down several nights in a row there are higher mortality rates,” Gleason said. “Crops and plants and animals need that recovery overnight so they can deal with the intensity of the daytime highs the next day."

As countries in parts of Europe faced sweltering temperatures this summer, hospitals were "quite stretched," in treating victims of heat-related illness, said Joyce Kumutai, a research associate and climate scientist at the London-based Grantham Institute.

"We saw something close to COVID-era stretching of hospital facilities," Kumutai said.

What individuals can do

Everyone has a role to play in fighting climate change and safeguarding human health, said Titus Schleyer, a research scientist at the Regenstrief Institute, in Indianapolis, who is leading a summit this week focused on using data to fight the medical consequences of climate change.

"It's easy to be hopeless, but that's not going to get us anywhere. That just seals our fate," he said.

He hopes to use medical informatics to reduce the negative effects of climate change, by providing and analyzing large-scale data about the impacts.

"Data is crucial to understanding where is global warming going and what we can do short-term and medium-term," said Schleyer, whose conference this week is part of the American Medical Informatics Association's annual meeting in New Orleans. "We have only one big try and we've got to succeed."

People can consume fewer resources, cut back on airplane travel, recycle, compost and talk to public officials about taking climate action, Schleyer said.

At the community level, switching a single school bus from diesel gasoline to electric power "can improve a child's asthma who rides that busy every day by about 30%," Nadeau said. Within a month after the switch, the child will be 30% less likely to have an asthma attack and also less likely to end up in an emergency room.

Trees combat the "heat island effect" of so much concrete in cities. Investing $1 in planting city trees saves about $5 in emergency room costs, she said. "And you don't have to wait a lifetime to see those economic benefits."

Beyeler added that people can also help reduce pollution from cars by supporting safe walking and biking in their communities.

The Lancet report, Beyeler said, pushes people to see the connections as it tries to "highlight places where we can make progress on both (climate and health) goals at the same time."

Although a lot has gone wrong in the last 18 months, with heat waves and forest fires, Beyeler said, a lot has gone right, too. There has been more investment in renewable energy and away from fossil fuels, she said. At the same time, the federal government has renewed its commitment to COP28's climate goals and to reducing health and climate inequities.

"There is momentum to be built on," Beyeler said. "At the same time, even with that progress, the scale of implementation and action that's needed to get us from where we are now to where we need to be is still tremendous."

Contact Karen Weintraub at kweintraub@usatoday.com and Dinah Voyles Pulver at dpulver@gannett.com

Health and patient safety coverage at USA TODAY is made possible in part by a grant from the Masimo Foundation for Ethics, Innovation and Competition in Healthcare. The Masimo Foundation does not provide editorial input.