The combination of extreme heat and drought that has scorched the Western United States and Canada over the past two weeks has killed hundreds of millions of mussels, clams and other marine animals, the New York Times reports.

The big picture: An estimated 1 billion small sea creatures died during the heat wave in the Salish Sea at the end of June, according to marine biologist Chris Harley, per the Washington Post.

• The sea creatures' deaths coincide with the heat wave that hit the Pacific Northwest last week, which led to more than a hundred human deaths.
• A study by an international team of climate researchers said the heat wave would have been "virtually impossible without human-caused climate change."

Driving the news: Mussels attach themselves to rocks and other surfaces, but they generally can't survive temperatures over 100 degrees for extended periods of time, CNN reports.

• Ken Fong, head of the marine invertebrates stock assessment research program for Canada’s department of fisheries and oceans, called the incident a "perfect storm," per the Washington Post.
• "A very low tide in the afternoon in the Strait of Georgia that happened to coincide with the hottest part of the day, exposing the sea animals to the worst of the extreme heat," the Post reports.

What they're saying: "It was a catastrophe over there," Harley told CNN. "There's a really extensive mussel bed that coats the shore and most of those animals had died."

• "What worries me is that if you start getting heat waves like this, every 10 years instead of every 1,000 years or every five years, then it's you're getting hit too hard, too rapidly to actually ever recover," Harley said. "And then the ecosystem is going to just look very, very different."

Go deeper: Welcome to our hellscape summer