rice farmers in Assam, India Rising temperatures increasingly threaten the health and productivity of rural populations whose livelihoods require outdoor work, like these rice farmers on Majuli Island in Assam, India. (Photo: Biju Boro/AFP/Getty Images)

Climate change is insidious, easily lulling people into apathy with its seemingly slow pace and sporadic harm. The climate has always changed, doubters often correctly point out, incorrectly implying today's changes are either normal or benign.

In fact, Earth's atmosphere is changing more rapidly than humanity has ever seen. It hasn't held this much carbon dioxide in at least 800,000 years — about 600,000 years before our species existed — and possibly not since the Pliocene Epoch, which ended 3 million years ago. CO2 is key to life on Earth, but it also traps heat, and this relatively fast surge of CO2 in the sky is wreaking climatic havoc around the world.

The problem is not merely that climates are changing; it's that they're changing too quickly for many species and ecosystems to adapt. That includes famously at-risk wildlife like polar bears, pikas and coral reefs, but also many of the highly successful animals who are fueling this crisis in the first place: humans.

The effects of our CO2 emissions are well-known, yet they're often framed more as a threat to "the environment" than to ourselves, as if the two are separate. Many other species do face graver risks from climate change, including extinction, but that doesn't mean we're out of the woods. Human health still hinges on ecological health, and climate change doesn't have to drive us extinct to put us through hell.

coal-fired power plant A coal-fired power plant in North Dakota releases CO2 emissions. (Photo: Andrew Burton/Getty Images)

That's the message of a landmark new report published in the Lancet, one of the world's oldest and most renowned medical journals. The report is part of the Lancet Countdown, an international research project on climate and human health, and comes as a major two-week U.N. climate summit kicks off in Bonn, Germany.

It's also part of a broader focus on the health impacts of climate change. The American Public Health Association (APHA) declared 2017 the "Year of Climate Change and Health," for example, and the theme of its annual meeting in Atlanta this month is "Creating the Healthiest Nation: Climate Changes Health."

As that theme suggests, our fates aren't sealed just yet. On top of citing dire threats to public health, the Lancet report adds a hopeful tone, noting that comprehensive climate action could be "the greatest global health opportunity of the 21st century." Even if that action wasn't already needed to avoid ecological disaster, it would still offer a unique chance to save human lives and strengthen our species.

"I know that we can take this critical diagnosis from climate scientists — of a climate change-fueled public health emergency — and accelerate solutions that improve the health and well-being of billions," Christiana Figueres, chair of the Lancet Countdown advisory board and former executive secretary of the U.N. Framework Convention on Climate Change, recently wrote in the Guardian.

For more about the dangers (and opportunities) that climate change presents for human health, here are a few highlights from the new report:

Heatwaves

Earth is warming up unusually quickly, with the global average temperature now rising at a pace unseen in at least the last 1,000 years. But as the Lancet report explains, many people experience much more than the global average change. When temperature change is weighted by human exposure rather than surface area, the warming from 2000 to 2016 was 0.9 degrees Celsius — significantly higher than the area-weighted change of 0.4 degrees over the same period.

As average temperatures climb higher, the danger of heatwaves is rising, too. The number of vulnerable people exposed to heatwaves since 2000 has grown by about 125 million, the report finds, with a record 175 million people exposed to heatwaves in 2015. The average length of individual heatwaves has also increased since 2000, by as much as a full day when weighted by human exposure.


Aside from direct health effects like heat stress and heat stroke, exposure to extreme heat can worsen existing health risks like heart failure, the report points out, and in vulnerable populations it can also increase acute kidney injury from dehydration. These changes are especially dangerous to elderly people, children younger than 12 months, and people with chronic heart and kidney disease.

Labor capacity

The effects of extreme heat carry a high economic cost, and not just because of health care. Rising temperatures are making it harder for humans to work outdoors in many places, threatening the health, productivity and livelihoods of people like farm workers. Between 2000 and 2016, the average labor capacity of rural populations fell by 5.3 percent due to rising temperatures, according to the report. This effectively removed more than 920,000 people from the global workforce, it adds, with 418,000 of them in India alone.


Infectious diseases

Although climate change is robbing many species of valuable habitat, it can have the opposite effect on others. That unfortunately includes some arthropods that can transmit dangerous diseases to humans, from ticks that carry Lyme disease to mosquitoes that carry various viruses or parasites.

The Lancet report focuses on two notorious mosquito species: Aedes aegypti and Aedes albopictus. Both can spread several viruses to humans — including dengue, yellow fever, chikungunya and Zika — but the report cites particular concern about dengue. It's a "fast-emerging, pandemic-prone viral disease," according to the World Health Organization (WHO), and its incidence has risen 30-fold in the last 50 years. With up to 100 million dengue infections across 100 countries every year, the WHO estimates nearly half the world's population is at risk.

mosquito larvae Rising temperatures and changing weather patterns are helping to expand the range of many disease-carrying insects, like the Aedes mosquito larvae pictured above. (Photo: Sam Droege/USGS Bee Inventory and Monitoring Lab/Flickr)

Due to climate trends, the global capacity for Aedes aegypti and albopictus to transmit dengue has respectively risen by 3 percent and 5.9 percent since 1990, the report finds. Compared with 1950 levels, their "vectorial capacity" for dengue has increased by 9.4 percent and 11.1 percent, respectively. (Mosquitoes and other arthropods are known as "vectors" for the diseases they spread.)


Other diseases

While research into climate change and public health tends to focus on infectious disease, "the health effects from non­-communicable diseases are just as important," the report's authors write. This includes a wide range of climate-affected conditions like heart, kidney and respiratory disease, including acute and chronic breathing trouble from worsening air pollution and airborne allergens.

As agriculture is disrupted by stronger storms, longer droughts and rising temperatures, undernutrition may be "the largest health impact of climate change in the 21st century," the researchers add. Warming trends have been shown to reduce global wheat and rice yields by 6 percent and 10 percent, respectively, for each 1-degree Celsius increase. And as climate change threatens the viability of many human communities, the report also cites "the often-unseen mental health effects of extreme weather events or of population displacement."

CO2 emissions may not directly threaten human health, but on top of their indirect dangers, they tend to come from sources that also emit more directly harmful pollutants like sulphur dioxide, nitric oxide and fine particulate matter. "As such," the report explains, "well-designed actions to reduce greenhouse gas emissions will improve ambient air quality and have associated benefits for human well-being."


Treat 'the cause and the symptoms'

These are just a few of the many ways climate change can threaten human health. The report also touches on others, like the wide range of ailments and injuries caused by weather-related disasters — which grew 46 percent more frequent from 2007 to 2016, compared with the 1990-1999 average. According to an earlier report from the Lancet Countdown, "an additional 1.4 billion drought-exposure events and 2.3 billion flood-exposure events will occur by the end of the century, showing clear public-health limits to adaptation."

Some degree of human-induced climate change is inevitable — since we're still emitting CO2 that can linger in the atmosphere for centuries — so adaptation is key to minimizing the damage. But as Lancet Countdown co-chair Hugh Montgomery says in a press release, adaptation alone won't be nearly enough.

"We are only just beginning to feel the impacts of climate change. Any small amount of resilience we may take for granted today will be stretched to breaking point sooner than we may imagine," says Montgomery, director of the Institute for Human Health and Performance at University College London. "We cannot simply adapt our way out of this, but need to treat both the cause and the symptoms of climate change."


That's a daunting task, but the Lancet report argues there are "clear reasons for optimism." While global CO2 emissions are still dangerously high, for example, they have stalled in recent years — and without the kind of economic downturns that led to some previous dips. Coal use has declined in favor of less carbon-intensive natural gas, and renewable energy is becoming more affordable at a faster pace than many experts predicted. The capacity for renewable electricity generation grew by 20 percent from 1990 to 2013, the report points out, and in places where renewables replaced fossil fuels — especially coal — morbidity and mortality rates decreased.

There's still a long road ahead, but research like this can be a valuable reminder that climate change is not just some abstract or arcane issue about polar bears. It's a wide-ranging illness disrupting ecosystems all over the planet, and the species responsible for causing it is probably also the only one capable of curing it.

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.