The consequences for Alaska were stark: dozens of whales died, as did thousands of common murres and tufted puffins, while sealife native to the tropics came up in nets pulled from sub-Arctic seas.

But an unusual mass of warm water nicknamed "the blob," which appeared off Alaska and hung around through 2016, didn't occur in isolation. In northern Australia in 2016, high ocean heat bleached hundreds of miles of corals, killed mangroves, and destroyed giant clams. Off New Zealand, an ocean hot spell wiped out black abalone and brought an oyster-killing disease.

Just as atmospheric shifts can bring droughts and nasty heat waves on land, shifts in weather or ocean circulation also can spark deadly marine