In late spring of 2015, a BBC camera crew accompanied a research team to the remote steppes of central Kazakhstan to film an extraordinary event in the animal kingdom. Tens of thousands of female saiga were gathered to give birth on the open plains over the course of just ten days—as they’d done every spring since the Ice Age. It’s one of the most spectacular migrations and mass birth events in the world.

The saiga, an antelope that has survived for millennia and once ran with the wooly mammoths, give birth in mass numbers in open spaces in order to offer their calves the best chance for survival. Wolves, the saiga’s main predator, can only capture and kill a relatively small percentage of the calves in such a mass-birthing process. Like horses, which were also once flight animals in the wild until they were largely domesticated, saiga antelope calves are born quite large and well developed and fully capable of outrunning predators within a few days of birth.

Once, hundreds of thousands of saiga roamed the plains of Russia and other countries within the former Soviet Union. Over time, however, the saiga had been hunted and poached almost to the point of extinction. As the Soviet Union collapsed, hunters killed saiga antelope in record numbers for meat, while poachers killed males of the iconic species for their horns (used in traditional Chinese medicine).

By the first decade of the 21st century, the saiga antelope’s numbers had diminished worldwide to a mere 50,000. The animal was listed on critically endangered lists. But an intensive effort by wildlife conservation and saiga research scientists—including several who accompanied that BBC crew in the spring of 2015—had successfully brought the saiga population back to healthy, annual numbers. By 2015, the saiga population in the world had stabilized to about 300,000.

So the research team that came to the plains of Kazakhstan fully expected to witness a mass-birthing event that rivaled those of years past, when saiga were plentiful and roamed widely across the grasslands. What the team witnessed instead was the mass die-off of an estimated 200,000 saiga almost overnight. The corpses were littered over hundreds of miles. Every single animal died over a period of several days.

The mass die-off was captured by the BBC team and transfixed global media coverage for several days. The images of the saiga females’ and calves’ corpses strewn across the open plain in Kazakhstan were hard to ignore. They were everywhere in the media during the die-off.

What no one could answer was why it had happened. Mass die-offs occur every so often among an animal species. Saiga had gone through die-offs on a smaller scale in the past, but earlier die-offs had been local and could be traced either to a toxin introduced into the local environment where they had died or to an infectious disease that had taken weeks or months to cause the mass deaths.

In this case, in Kazakhstan, 200,000 saiga died over a relatively broad area . . . and essentially overnight. Infectious disease made no sense (because it happened so quickly). A toxin made no sense (because the die-off wasn’t local or contained).

So what caused it? Why did this mass death happen? That was the question that researchers, conservation societies, and journalists asked for months following the die-off, with no good answers. There were crazy speculations, ranging from the Blood Moon and aliens to signs of an imminent apocalypse.

Finally, a year after the event—as a vastly diminished herd of saiga were gathering on the plains again for another mass-birthing event—at least a partial answer emerged from accelerated research and studies. The Saiga Conservation Alliance had sent a research team in and had analyzed tissue samples for months.

“The mysterious death of 200,000 critically endangered antelopes in Kazakhstan last year was caused by a bacterial infection, according to a new report by the Saiga Conservation Alliance,” CNN reported on April 15, 2016, nearly a year after the spectacular mass die-off.

“Several labs used tissue samples collected from the carcasses during the die-off, and confirmed that the deaths were linked to bacterium Pasteurella multocida. This pathogen caused hemorrhagic septicemia in the saiga population,” CNN reported. “The findings are surprising because this bacterial infection, although known to affect animals such as buffalo, cattle and bison, had never been documented to affect an animal group with a 100% mortality rate.”

Every other news report said essentially the same thing. Something had triggered the bacteria, which is usually dormant and not lethal. The bacteria had somehow become deadly to every saiga across the entire steppes. And this virulent bacterial infection, which strikes small percentages of grasslands creatures like buffalo and cattle, had managed to afflict a “100% mortality rate.”

None of the follow-up media stories addressed still-unanswered questions. Why a 100 percent mortality rate? Why did these usually harmless bacteria become virulent? Was there a trigger of some sort—either an environmental change or an internal change in the saiga’s immune system that made it susceptible to the bacteria? Or was it a combination of both?

Several of the initial researchers involved with the conservation efforts surrounding the saiga and the first BBC filming of the mass die-off in the spring of 2015 that stunned the world now have a more definitive answer. They talked about the underlying cause in a first-person account in The Conversation in December 2016 that explains in some depth what went wrong in the plains of Kazakhstan.

“In exploring these questions, our research is a Russian doll; as we take off a layer of explanation we find more questions within,” wrote E. J. Milner-Gulland, Eric Morgan, and Richard Kock. Milner-Gulland is the chair of trustees of the Saiga Conservation Alliance that reported on the proximate cause of the die-off a year after it had occurred. Morgan and Kock also received research funding to study the saiga event.

Once they’d peeled back the layers, the underlying causes started to become clear. They aren’t especially pleasant answers, but they should also give us pause, because they give us a vivid snapshot of similar events that will become more prevalent over time—not less—as Earth’s climate system changes. And future events might not always kill off only a particular animal species. Bacteria that can flip from dormant and harmless into virulence can also affect humans.

“We [went] back to old field notes from the Institute of Zoology in Kazakhstan for 1988 when a similar mass mortality occurred; reviewed research on mass deaths in other species; looked for differences in the vegetation composition between the 2015 die-off and in other years; and built statistical models to explore changes in temperature and rainfall over a range of different temporal and spatial scales,” the three researchers said. “We also tested tissue and environmental samples for a wide range of toxins, as well as other disease-causing agents, in case some underlying infection was involved. So far, the evidence points towards a combination of short-term but landscape-scale weather variation and physiological stress from calving causing a cascading effect of virulence. There’s no evidence for environmental toxins, other underlying infections or (as yet!) alien influence.”

The complex answer, it seems, is that the combination of temperature changes combined with extreme and erratic precipitation—both signatures of the new Earth system that will affect regions of the world more frequently from here on out—had the net effect of creating a “short-term but landscape-scale weather variation.” This change in the total environment, combined with physiological stress from calving, caused the ever-present bacteria in the saiga to become virulent and kill them all.

“The life cycles and transmission of many infectious agents—including those causing disease in humans, agricultural systems, and free-living animals and plants—are inextricably tied to climate.”

The saiga are prone to disease and birth-related mortality. What triggered the “100% mortality rate” in this case was a new climate system that created a temporary weather variation. This sort of thing is going to happen again, and more frequently, the researchers warned.

“There has been huge public interest in this event, both within Kazakhstan and globally,” the researchers said. “People want quick answers and they want us to find solutions so that this will never happen again. It seems, however, that we won’t be able to give the comfort that is wanted; in fact, it is likely that with climate change these types of events will become more rather than less prevalent.”

The research that they were pointing to is an important study in Science from the summer of 2013 that firmly linked climate and public health threats like infectious diseases.

“The life cycles and transmission of many infectious agents—including those causing disease in humans, agricultural systems, and free-living animals and plants—are inextricably tied to climate,” ecology and evolutionary biologists from five American universities reported. “Over the past decade, climate warming has already caused profound and often complex changes in the prevalence or severity of some infectious diseases.”

The world has already begun to change, they said, and the public health risks are starting to increase. “Climate change has already increased the occurrence of diseases in some natural and agricultural systems.”

While experts continue to debate how swiftly it will cause serious harm to humans, the impacts on animals are more clearly understood now. Animals are, in effect, harbingers of our own future.

“For example, although the effects of climate warming on the dynamics of human malaria are debated, climate warming is consistently shown to increase the intensity and/or latitudinal and altitudinal range of avian malaria in wild birds,” they reported.

The saiga antelope die-off was a highly publicized example of this new threat, layered on top of other stresses on the species. It’s a complicated story, with answers that have taken nearly two years to sort through. The good news for the saiga is that they have begun to repopulate. Their numbers doubled, to about 100,000, in 2016.

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But the overarching threat that created a “100% mortality rate” event for the saiga antelope in 2015 is with us now and will only increase. Events like the saiga die-off will happen again and more frequently. That’s what the study in Science shows. It sends a clear signal to every species on Earth. What the saiga story also illustrates is just how difficult it is to sort through myriad threats to species—including humans—even in simple ecosystems such as the open grasslands of Kazakhstan. Explanations for threats to iconic species like the saiga antelope are never simple, including poaching, land development, and, at present, a changing environmental landscape they may not be equipped to handle.

The extinction-level threats facing iconic species like the African elephant, the giant panda in China, African giraffes, or mountain gorillas in Africa are good illustrations. These species are in serious trouble. All face a multitude of dangers, some more widely known than others. Cutting across them sharply now is an environmental landscape that is shifting under their feet into something they simply may not be able to adapt to.

Other iconic species that schoolchildren read about in countless stories and tales are in serious trouble for all the same reasons—and serve as warning flares for us.

Mountain gorillas, for instance, are critically endangered. There are fewer than 900 of them currently in the wild, living in two national parks in central Africa. The species was discovered by us just a century ago. Since then, through a combination of uncontrolled hunting, war, disease, the destruction of its forest habitat, and illegal capture for trade, we decimated the species. The number of mountain gorillas fell to a low of just 620 in 1980 before conservation efforts brought the numbers back slightly, but the species’ newest threat—a changing Earth system that systematically conspires against them in ways that other threats do not—may be the most difficult. Human beings, driven by water scarcity and rising temperatures, are encroaching on the gorillas’ habitat. In northern Rwanda, for instance, people routinely head into the national park for water. Warming temperatures are pushing the mountain gorillas to higher elevations. As people edge farther into their habitat, mountain gorillas are driven even higher into the mountains, where they can suffer from exposure to harsher temperatures and find less to eat. While it’s unlikely that mountain gorillas will simply vanish for good—thanks to concerted efforts by wildlife conservation societies—the environmental threat is real and ever present.

Giant pandas, like mountain gorillas, are still with us now largely because of conservation efforts and zoos. The giant panda population, which is native to central China, has largely been decimated by habitat loss from logging, agriculture, and infrastructure expansion. What’s left of the pandas’ habitat is now protected through a network of nature reserves.

But in an unprotected world, pandas would likely vanish as a species over time. They have a finicky digestive system. They have to eat anywhere from 20 to 40 pounds of bamboo a day to stay alive. A study several years ago, however, found that up to 100 percent of the bamboo in pandas’ native habitat was under threat from the changing climate—meaning that pandas literally would have nothing left to eat if they weren’t being protected in nature reserves.

African elephants are the largest species walking on Earth. They wander through nearly every country in Africa. There were once ten million elephants in Africa before human colonization. Now there are only 350,000 of them left, with an ongoing species rate decline of eight percent a year. Some experts now genuinely fear that African elephants, one of the continent’s most iconic species, could disappear in a generation.

African elephants are now vulnerable to extinction for many of the same reasons that other big, iconic species are in serious trouble—uncontrolled hunting, habitat loss from development, poaching, and disease.

The changing landscape in Africa is a grave threat to them. Elephants’ DNA varies only modestly and adapts to environmental changes slowly over long periods of time. For this reason, most experts believe it may not be able to adapt in time to the changes that are beginning to transform Africa.

But the biggest problem facing the African elephant is precisely the same problem that is confounding human populations across the continent—water scarcity.

“The biggest concern for elephants is their need for large amounts of fresh water, and the influence this has on their daily activities, reproduction and migration,” WWF said in a special report on the extinction threat facing African elephants. “Other threats—like poaching, habitat loss and human-elephant conflict—remain, and have the potential to increase due to the effects of climate stressors on humans and resulting changes in livelihoods.” Their cousin, the Asian elephant, faces similar stresses. They require extensive amounts of water to live—up to 60 gallons a day. For this reason, Asian elephants tend to remain close to freshwater sources. As these are beginning to dry up in the 13 countries where roughly 50,000 Asian elephants now roam, they will find it hard to adapt. And like their African cousins, they, too, adapt slowly to changing landscapes and will likely run out of time. The habitats of Asian elephants are just 15 percent of their historical range today.

Perhaps the most surprising recent development was the announcement in December 2016 that giraffes in Africa were undergoing a “silent extinction” through a combination of poaching and the rapid expansion of farmland. Africa’s changing landscape could finish them off, conservationists warned.

Virtually unnoticed, giraffe numbers have plunged by up to 40 percent since the 1980s, the International Union for Conservation of Nature reported in its annual Red List of endangered species. The overall population of the world’s tallest land animal had fallen below 100,000 (from roughly 163,000 three decades earlier).

It was the first time the group had ever listed the giraffe as vulnerable to extinction. The reason, it said, was because the giraffe’s steep decline in numbers in sub-Saharan Africa had largely gone unnoticed.

“Whilst giraffes are commonly seen on safari, in the media and in zoos, people—including conservationists—are unaware that these majestic animals are undergoing a silent extinction,” said the group’s giraffe specialist, Julian Fennessy.

Other conservation experts swiftly weighed in on the sobering news. The same water scarcity and food security problems that were causing such troubles for humans in sub-Saharan Africa were likewise causing giraffes serious trouble. That, combined with war and civil strife, was trapping giraffes inside a tightening noose.

“People are competing for fewer and fewer resources and the animals are worse off . . . especially with civil strife,” Craig Hilton-Taylor, who runs the group’s Red List, told Reuters. He said that drought and Africa’s changing landscape were making things even worse for giraffes.

Other iconic species that schoolchildren read about in countless stories and tales are in serious trouble for all the same reasons—and serve as warning flares for us. The African lion population, for instance, has now fallen 90 percent. Western black rhinos are already extinct. Experts believe that the entire population of wild rhinos are likely to go extinct in the next 15 years.

And a surprising study published in the Proceedings of the National Academy of Sciences of the United States of America in December 2016 showed that cheetahs—the fastest land animal on Earth—likewise simply haven’t been able to escape the same multitude of threats facing other iconic species.

There are now only 7,000 cheetahs left in the world—down from a population of 100,000 a century ago. Habitat loss is mostly to blame for the cheetahs’ demise, the researchers said, as they’ve lost more than 90 percent of their historical range. The changing landscape and climate system in Africa could finish the job.

Whether the vanishing of iconic animal species, a dome of heat–killing waves, Category 6 superstorms, or extreme weather events that create a new and yet unrecognized category of refugees, all are realities of climate change and real-world consequences we cannot ignore.

From This is How the World Ends by Jeff Nesbit, courtesy Thomas Dunne Books. Copyright 2018.