Steps away from the public restrooms in Yosemite Village, a buzzy stop in Yosemite National Park’s iconic valley, sits a brown metal dumpster. Visitors reach up to open the trash chute. Their peanut butter jars and apple cores tumble into a sealed compartment. The slot slams shut. Then, they clip a tethered steel carabiner through a loop, which prevents less dextrous creatures from getting access. “USE CLIP,” reads a sticker on the chute. “SAVE A BEAR.”
“Bears have evolved to be these food-finding machines,” says Heather Johnson, a research wildlife biologist with the US Geological Survey Alaska Science Center and a member of the IUCN North American Bear Expert Team. Yet climate change is making it harder for them to find a meal in the wild. Bears prefer eating their natural foods—grasses, berries, pine seeds, and acorns. But droughts, for example, damage roots, shrivel berries on the vine, and force oaks to abort their acorns.
So bears are becoming increasingly likely to scavenge from people. They’re good at it. “I did my work in some of the wildest places in Colorado, about as far from roads as you could get,” Johnson continues. When natural food was scarce, the bears she studied “would beeline 20 miles as the crow flies to go to where there’s human developments, foraging on people’s orchards and trailer parks for garbage.” When bears seek out human food, that puts them at greater risk of conflict with people—one they are likely to lose.
The United States is home to roughly 300,000 notoriously omnivorous black bears; they’re the most common and widely distributed bear species in North America. (Yosemite has about 500 of them.) Black bears very rarely attack people; they are generally less aggressive toward people than grizzlies. Outliers exist: A black bear killed a man unprovoked in Tucson, Arizona, in June. But these bears are more often the ones that get hurt. Hunting for food, they venture into traffic or damage property, cause a nuisance, and get euthanized. “That’s why we’ve seen this population decline when we have this big flood of bears really seeking out human foods,” Johnson says.
Hotter seasons are also amplifying encounters between humans and wildlife, making run-ins more frequent. In her previous job with Colorado Parks and Wildlife, Johnson tracked the multiple forces driving human-bear conflicts, the most well-studied being hibernation. Bears hibernate when cold weather makes food scarce. But warmer winters means bears begin hibernating later and emerge earlier.
“If they’re awake for more of the year, they have more time to get into conflicts with humans,” agrees Gloria Dickie, a journalist and the author of Eight Bears, a book released this July about each of the world’s eight remaining ursine species. “It’s basically just more opportunities to die.”
Those effects compound when bears can access human food—be it trash from homes cozied up to the wilderness or from snacks packed in by campers. These extra calories shorten their hibernation. (Bears that hibernate less also appear to age faster.)
Female bears also naturally regulate their pregnancies based on food supplies. When high heat, drought, or some other climate event hits and makes food scarce, female bears can’t gain enough weight for successful pregnancies. They delay implantation of fertilized eggs and may not deliver. But in some places, human food is always an option, so females are still getting enough calories to carry their cubs, even when environmental conditions are bad. “They’re having cubs born into drought, and they then have to teach those cubs to also eat human food,” Dickie says.
Bears are, in fact, gifted teachers. “Bears are really smart,” Dickie says. “When a mom is going to look for food, she takes the cubs with her and teaches them: The dumpster is where you find food.”
Johnson has seen this turn deadly for bears in Colorado. In 2012, a late-spring freeze killed the flowers that had bloomed during warmer weather. That meant no fruit for bears late in the summer. Johnson’s team documented a huge increase in bears coming into town and searching for human food. “I mean, skyrocketing conflicts with people,” she says. The female bear population collapsed by 57 percent as a result.
Weird winters affect bears’ summers, too. In the northeast, floods can wake black bears from their dens. This year’s record-breaking snowpack has enticed Yosemite’s bears to stay longer in lower elevations, and floods pushed wildlife into the more developed parts of Yosemite Valley. Fuel build-up throughout the year has set the stage for more frequent and intense wildfires, which kill bears or coax them into human spaces in search of food or a new habitat. “With more climate variability, these things are expected to happen more frequently,” Johnson says.
While people in the US are most likely to happen across a black bear, climate change and human encroachment are problems for many bear species. North America’s brown bears, better known as grizzlies, also hibernate less and will forage for human food when they need to. Lingering snowpack and hot summers reduce the food available to Asiatic bears emerging from hibernation, increasing conflicts. Polar bears are the poster child for how warming threatens species. When they have less sea ice to hunt from, conflict with people may increase as they spend more time on land.
“It’s not just a consequence of long-term climate change,” says Briana Abrahms, a wildlife ecologist at the University of Washington. “It’s also a consequence of the increased severity and frequency of acute extreme events.”
In February, Abrahms reviewed the many pathways through which climate change and variability “percolate” into human-wildlife interactions that imperil people, animals, property, and crops. That included anything from livestock loss to car accidents. The paper covered 49 case studies on six continents and in all five oceans. “The most surprising thing was just how ubiquitous it is. We found cases in the Arctic to South Africa,” she says. “From animals as small as armadillos and even mosquitoes to elephants and whales.”
Abrahms wants to understand why these dynamics change, because the answer isn’t always clear. “In some cases, it’s actually not the animals that are changing what they’re doing,” she says “In a rare number of cases, it’s actually people that are changing what we’re doing.”
One example may be the spectacled bears in the Andes. Severe droughts pushed some Bolivian farmers to replace their agriculture with cattle ranching. When hungry bears struggled to find food in the forest, they killed livestock—and got killed as a result.
Around the world, more communities sprawl into bears’ home ranges. Hotter summers in cities also drive more people to parks and forests. Even when campgrounds have bear-proof food storage, local bears still make their rounds, sniffing out crumbs. The staff who manage parks, trails, and campgrounds find themselves on the front lines between bears and human visitors.
Historically, wildlife managers in Yosemite have had to kill bears who became so habituated to humans that they posed a threat. Dickie writes that it took decades before a new generation of animals, born in a less permissive era, learned to be more cautious around people. Now, bear education is a must for park visitors. They need to know that bears will sniff out everything from fish bones left on the ground to sunscreen in a tent. (In Eight Bears, Dickie finds a wooden plaque that reads: “At the root of the bear problem in Yosemite is the overlap in intelligence between the smartest bear and the dumbest camper.”)
To Abrahms, it’s crucial that we uncover how global climate change manifests itself, even deep in the woods. “Obviously, it’s concerning that climate change is exacerbating some of these conflicts,” she says. “But the good thing is that understanding that relationship can really help us predict when those conflicts are more likely to occur.”