Climate change is already afflicting human health worldwide, exposing tens of millions of elderly people to excess heat while possibly reducing the ability of hundreds of millions of workers to do their jobs, according to an expansive new synthesis from The Lancet, one of the world’s oldest and most widely cited medical journals.

The report examines dozens of statistics from around the planet and finds that the long-predicted effects of climate change have already become a reality in many places. Heat waves now last longer, reaching more people and broiling more territory, than they did in the 1980s and 1990s. In the United States, this spike in warmth is lengthening the allergy season, sometimes by weeks, and helping infectious diseases to spread.

But one of its techniques is questionable. Its findings about the global economy raised eyebrows among academic economists who more formally study climate change.

“The human symptoms of climate change are unequivocal and potentially irreversible—affecting the health of populations around the world today,” warn the paper’s coauthors, who are drawn from nonprofits and 24 different academic institutions. “Whilst these effects will disproportionately impact the most vulnerable in society, every community will be affected.”

The report, dubbed “Countdown 2017,” represents the first coherent attempt to unify many of the different climate-related threads in medical and global-health research. In 2015, a commission of researchers—also convened by The Lancet—warned that climate change could “undermine the last 50 years of gains in public health.” They also called for more comprehensive tracking of important climate signals.

This report, which will now be updated annually, is the result. The coauthors have pulled together figures and estimates from across the medical literature and attempted to tabulate single, worldwide figures for many values.

Some of its techniques may have significant weaknesses. The Lancet report makes an eye-popping assertion about the global economy, arguing that climate change has already significantly harmed labor capacity around the world. Between 2015 and 2016—which are the second- and first-hottest years ever recorded—it argues that “outdoor-labor capacity” fell by 2 percent. Since the year 2000, outdoor-labor capacity has fallen by 5.3 percent overall, it claims.

Academic economists who study climate change were very doubtful of this estimate. “I would back way off the claim that [the data] show any of these things, since they don’t have or use any actual data on labor,” said Solomon Hsiang, a professor of public policy at the University of California, Berkeley, and a member of the Climate Impact Lab.

The Lancet authors, he said, used a technique drawn from a 2013 Nature Climate Change article to estimate “labor capacity.” They effectively interpolated temperature data with conclusions from U.S. military and industrial experiments, from 1996 and 2003, that researched how much work soldiers could do in different temperature conditions. But their findings do not align to a type of labor measurements that economists usually track.

“The numbers presented are (in the most generous interpretation) actual measurements (although still not sure what kind) from a few military lab experiments using U.S. soldiers as subjects and then extrapolated to people across Africa, Latin America, the Middle East, and South Asia,” Hsiang told me in an email.

He continued: “There’s lots of ways this could go wrong, since we don’t know if people adapt, we don’t know if U.S. soldiers are good proxies for normal people in Indonesia, and we don’t have any idea how the “labor-capacity” index these guys put together actually connects to “labor productivity” or earnings as any economist would recognize it.”

Climate change will almost certainly harm global labor output, but a lack of empirical observations around the world makes the kind of measurement The Lancet is attempting difficult. The controversy points to one of the most difficult aspects of finding climate change’s effects on human life: While scientists can measure weather worldwide, measuring the fingerprint of climate change on all human activity is far more complicated.

The Lancet report makes other broad claims about global public health that were more widely accepted. It finds that many more older people experience heat waves now than did two or three decades ago. The report says that roughly 175 million more people older than 65 worldwide were exposed to excess heat in 2015 as compared to several decades ago. On average, 125 million more older adults are exposed to heat than were in previous decades.

In the United States, an additional 14.5 million people older than 65 were exposed, a number larger than the population of Pennsylvania.

Howard Frumkin, one of the coauthors of the report and a professor of public health at the University of Washington, said that heat’s health effects are insidious. While a heat wave may only seem to directly kill a couple thousand people via heat stroke, researchers find that hotter months—when studied after the fact—have many more deaths than would otherwise be expected.

“Heat does a lot of things to people. It disrupts sleep and contributes to sleeplessness. It triggers violence—crime goes up during heat waves. It may trigger self-harm. And there’s very strong evidence that occupational injuries rise during heat waves,” he told me. Heat appears to function as a general stressor on people, wearing down the most vulnerable among them.

The European heat wave of 2003, one of the worst natural disasters ever experienced, is estimated to have ultimately killed more than 70,000 people.

The report also examines climate-related migration from around the world. It finds that a minimum of 4,400 people have definitively been forced to leave their homes because of climate change. “The total number for which climate change is a significant or deciding factor is much higher,” it adds (and it exempts events like the Syrian Civil War, which some experts think climate change helped aggravate).

While this number may seem small, it presages tens of thousands more relocations to come. It also shows how poorly documented most of the relocations are: Most of the 4,400 come not from inundated islands or low-lying coasts in the tropics, but from indigenous villages in northern Alaska.

The report does pull out a spot of good news: Despite a 44 percent increase in the number of extreme-weather events since 2000 (as compared to the decades before that), there’s been no equivalent rise in the number of deaths. Frumkin said that suggested that—if it plans ahead—society may be able to adapt to some of the consequences of climate change.

A separate report from The Lancet pulls out specific findings about how climate change has already altered the public health of the United States. The allergy season here is getting much longer: Nebraska’s ragweed season has extended by 17 days since the early 1990s, and Minneapolis has seen it lengthen by 21 days. The ragweed season in Kansas City, Missouri, extended by 23 days and it now nearly encompasses a quarter of the year.

And certain insect-borne infectious diseases are already showing climate-related spikes. Doctors found three times as many Lyme cases in the United States were diagnosed in 2016, compared to 1990. And mosquitoes that carry dengue fever now transmit the virus 5 percent more effectively than they did a couple decades ago.

Public-health groups in the United States used the report to make a larger point about the direction of American climate policy. “The report emphasizes the scale of the threats to human health—and makes clear that reducing dangerous climate pollution is critical to protect health and save lives,” said Harold P. Wimmer, the president of the American Lung Association, in a statement.

“The U.S. Environmental Protection Agency must listen to this latest warning and implement strong tools, including many already in place, to fight climate change and protect the health and safety of all Americans. We urge the EPA to halt its efforts to repeal the Clean Power Plan,” he added.

Frumkin said it made sense to link the report’s conclusions to recent policy changes. “In my view, and the view of most health professionals, the current administration’s policies are nothing short of tragic,” he said. “They’re misguided; they will cause more suffering and deaths in the population; and they will only delay us in our attempts to tackle climate change.”

“The impacts of climate change are not some distant future event. They’re happening now,” he added.