Heat waves would increase in frequency as well as severity, Ebi said.
The researchers studied the impacts of one-in-30-year heat waves, or events that would have a 3.3% chance of happening each year. But as the world warms, the odds shift.
"In an average lifetime, this is the kind of heat wave you'd expect to experience maybe two to three times," Ebi said. "Of course, with warming, that 1-in-30 event becomes much more frequent."
Limiting warming to 3.6 degrees Fahrenheit would save 279 lives in each extreme Seattle heat wave. Keeping temperatures to an increase of 2.7 degrees Fahrenheit would avoid another 105 deaths.
The highest mean temperature (averaging day and night measures) in Seattle was about 83.5 degrees Fahrenheit from 1987 to 2000, according to the study data. But, if global temperatures rise 5.4 degrees, models predict Seattle could see that figure rise to about 97 degrees Fahrenheit.
By that time, mean temperatures are expected to exceed 83.5 degrees more than five days a year, on average.
"No one needs to die in a heat wave. All those deaths are preventable," Ebi said. "Rapidly reducing our greenhouse gas emissions will save lives in Seattle."
The numbers reflect averages and estimates. The study also did not consider adaptations people might make, like increased use of air conditioning. Nor do they consider population or demographic changes.
"There would be the same number living in Seattle and they'd all be the same age as they are today," Ebi said of the scientists' models. "There are ways the study is overestimating risk and ways the study is underestimating risk. It's not possible to tell how that would balance out."
About as many people in Seattle would die in each severe heat wave as in Houston, a much warmer city with a population more than double Seattle's.