Dead clams and clams covered rocks in the Pacific Northwest, their shells gaping open as if they had been cooked. Starfish were baked to death. Sockeye salmon swam lazily in an overheated Idaho river, which resulted in wildlife officials moving them to cooler areas.

The combination of exceptional heat and drought that hit the western United States and Canada in the past two weeks has killed hundreds of millions of marine animals and continues to threaten countless freshwater species, according to a preliminary estimate and interviews with scientists.

“It just feels like one of those post-apocalyptic films,” said Christopher Harley, a marine biologist at the University of British Columbia who studied the effects of climate change on coastal marine ecosystems and calculated the death toll.

Such extreme conditions will become more common and intense, according to scientists, as climate change, fueled by humans burning fossil fuels, has devastating effects on both animals and humans. Hundreds of people died last week when the heat wave parked over the Pacific Northwest. A study by an international team of climate researchers found that such extremes would have been virtually impossible without global warming.

Just before the heat wave, when Dr. Harley looked at the breathtaking weather forecasts, thinking of how low the tide would be at noon, and the exposed clams, starfish and barnacles were baking. When he went to the beach on one of the hottest days last week, the smell of putrefaction immediately hit him. The scientist in him was excited, he admitted, as he saw the real effects of something he had studied for so long.

But his mood quickly changed.

“The more I walked and the more I saw, the more sobering things got,” said Dr. Harley. “It just went on and on.”

The dead sea stars, usually the most noticeable creatures in tide pools, hit him particularly hard. But the obvious mass sacrifices were mussels, an ecologically important species that feeds starfish and sea ducks and creates habitat for other animals. Dr. Harley estimated the losses for the clams alone at hundreds of millions. When you factor in the smaller creatures that live in the mussel beds – barnacles, hermit crabs and other crustaceans, various worms, tiny sea cucumbers – the deaths run to easily over a billion, he said.

Scientists have only just begun to consider the domino effects. One concern is that the sea ducks, which eat mussels in winter before migrating to their summer breeding grounds in the Arctic, have enough food to survive the journey.

“At least it’s something we think about,” he said.

Species that live in intertidal zones are resilient, he noted, and the clams on the shady north side of the boulders appear to have survived. However, when these extreme heat waves get too frequent, the species have no time to recover.

As the heat wave over the Pacific Northwest has subsided, temperatures are sustained across much of the American West. Another heat wave now appears to be building, which is only compounding the ongoing drought.

This means that biologists monitor river temperatures with alarms. Salmon make an extraordinary migration, often hundreds of miles, from the inland rivers and lakes where they were born, out to sea, and then back again to spawn the next generation. A network of long-standing dams in the western states already makes the trip dangerous. Now that climate change is exacerbating heat waves and droughts, scientists see conditions bleak without intensive intervention, which carries its own risks.

Heat wave hits North America

With suffocating heat hitting much of western North America, experts are concerned about human safety and power outages.

    • Western Canada: Canada broke a national heat record on June 27 when the temperature in a small town in British Columbia reached nearly 116 degrees Fahrenheit, breaking an 84-year-old record by nearly 3 degrees, with dangerously hot weather expected to continue for a few more days.
    • Pacific Northwest USA: A heat dome has enveloped the region, which has brought temperatures to extreme levels – with temperatures well above 100 degrees – and created dangerous conditions in a part of the country that is not used to oppressive summer weather or air conditioning.
    • Heavy dry season: Much of the western half of the United States is suffering from severe drought of historic proportions. Conditions are particularly bad in California and the Southwest, but the drought extends into the Pacific Northwest, much of the Intermountain West, and even the Northern Plains. The extreme heat exacerbates the drought.
    • Growing energy shortage: Power outages have increased more than 60 percent since 2015, even though climate change made heat waves worse, according to a new study published in the journal Environmental Science & Technology.
    • The base temperatures rise: New baseline data for temperature, rain, snow, and other weather events show how the climate has changed in the United States. One key takeaway: the country is getting hotter.

Heat wave hits North America

With suffocating heat hitting much of western North America, experts are concerned about human safety and power outages.

    • Western Canada: Canada broke a national heat record on June 27 when the temperature in a small town in British Columbia reached nearly 116 degrees Fahrenheit, breaking an 84-year-old record by nearly 3 degrees, with dangerously hot weather expected to continue for a few more days.
    • Pacific Northwest USA: A heat dome has enveloped the region, which has brought temperatures to extreme levels – with temperatures well above 100 degrees – and created dangerous conditions in a part of the country that is not used to oppressive summer weather or air conditioning.
    • Heavy dry season: Much of the western half of the United States is suffering from severe drought of historic proportions. Conditions are particularly bad in California and the Southwest, but the drought extends into the Pacific Northwest, much of the Intermountain West, and even the Northern Plains. The extreme heat exacerbates the drought.
    • Growing energy shortage: Power outages have increased more than 60 percent since 2015, even though climate change made heat waves worse, according to a new study published in the journal Environmental Science & Technology.
    • The base temperatures rise: New baseline data for temperature, rain, snow, and other weather events show how the climate has changed in the United States. One key takeaway: the country is getting hotter.

“We are already in critical temperatures three weeks before the greatest warming occurs,” said Don Chapman, a retired fisheries biologist who specializes in salmon and steelhead trout, of conditions along the Snake River in Washington, where four dams will be the subject of long-standing Controversy. “I think we are headed for disaster.”

The salmon plight illustrates a wider threat to all species of species as climate change worsens. Many animals were already struggling to survive because human activities were destroying their habitats. In extreme heat and drought, their chances of survival decrease.

As an emergency response, Idaho Fish and Game Agency workers have begun catching a variety of critically endangered sockeye salmon at the Lower Granite Dam, putting them in a truck and, as a stopgap measure, driving them to hatcheries to decide what to do next do is. (Idaho rangers first attempted trucking the adult fish during a 2015 heat wave. It was done on a variety of runs for young salmon for a variety of reasons.)

In California’s Central Valley, Jonathan Ambrose, a biologist with the National Oceanic and Atmospheric Administration, said he wished he could do something similar. The Chinook salmon he oversees historically spawned in the mountains. But since the Shasta Dam was built more than three quarters of a century ago, they have adapted by breeding right in front of the mammoth structure that they cannot cross. The critical problem this year is that the water there is likely to be too warm for the eggs and young animals. Previous efforts to secure state or federal funding for transportation at the dam have failed.

“We have a mortality rate of maybe 90 percent, maybe even more this year,” said Ambrose.

Elsewhere in California, for the first time since the state built the Iron Gate Fish Hatchery on the Klamath River in 1962 to make up for lost spawning habitat, state biologists are not releasing the young salmon they reared for having it would probably die. Instead, they are distributing a million young salmon among other hatcheries in the area that they could house until conditions improve.

“I want to find the positives, and there are some, but it’s pretty overwhelming right now,” said Dr. Harley, marine biologist from the University of British Columbia. “Because if we get too depressed or too overwhelmed, we won’t try any more. And we have to keep trying. “