A consortium of international scientists found that the recent Pacific Northwest heat wave was implausible if not for climate change, but also that temperatures soared so high that they exceeded what scientists thought were statistically likely today.
“Although this was a rare event, it would have been virtually impossible in the past,” said Sarah Kew, a researcher with the Royal Netherlands Meteorological Institute, during a news conference Wednesday, adding that the heat wave was estimated to be 150 times more likely because of human-caused greenhouse gas emissions. Kew added that heat waves are expected to become more common and more intense in the future.
Kew is one of 27 scientists who are studying the event as part of the World Weather Attribution group, which assesses the fingerprint of climate change on extreme events.
Ultimately, scientists pegged the heat wave — throughout the Pacific Northwest — as a 1-in-1,000-year event in today’s climate, acknowledging that the number was a very rough estimate.
With another 1.4 degrees Fahrenheit of worldwide warming, which the scientists said the world could reach as soon as the 2040s, the analysis suggests a heat wave of this magnitude could be expected roughly every five to 10 years.
The impacts of global warming are a moving target as more greenhouse gases are emitted each year, and the new analysis raises questions over how well climate models are grappling with the pace of change. The analysis could also indicate that there are ingredients spurring heat waves that climate scientists do not understand yet.
The heat wave — which began June 26 in Seattle — sent temperatures at Seattle-Tacoma International Airport above 100 degrees for a record three days. Emergency services were overwhelmed with patients, roads buckled due to warming and temperatures primed the Washington state landscape for wildfire. Normal high temperatures for Seattle in late June hover around 74 degrees.
The heat wave now is blamed for hundreds of deaths in British Columbia, Washington and Oregon, and researchers say it will take months, or even a year, to determine how many people died who would not have otherwise.
Seattle exceeded its previous highest temperature mark by five degrees on June 28 — reaching 108. Portland set its new record three times, ultimately reaching 116 degrees. The previous record, before this year, was 107. Temperature records were shattered throughout British Columbia, where the municipality of Lytton set a new national record for high temperature at 121 degrees. A wildfire burned through town just days later, devastating residents.
Geert Jan van Oldenborgh, of the Royal Netherlands Meteorological Institute, said he was unaware of another instance in which record temperatures made such towering leaps, suggesting there were two possibilities that could explain the observations.
The first possibility is that a series of improbable variables aligned — on top of an already warmer world — to send temperatures soaring.
“The earth is a huge place and really weird things happen that are improbable,” he said. “It could be that the people in this region were unlucky.”
He said the second possibility is that the earth has “crossed a threshold that makes these heat waves suddenly more likely,” adding that scientists are investigating the impacts of drought on heat waves and whether the jet stream is changing its patterns.
“Everyone is really worried about the implications of this event,” he said. “We don’t understand heat waves as well as we thought we did.”
The scientists plan to spend the next several months examining climate patterns for new clues on what ingredients could be behind extreme patterns of heat like this.
The scientists synthesized the results of 21 climate models to evaluate the recent heat wave. They focused their inquiry on the hottest temperatures in the Seattle, Portland and Vancouver areas, excluding analysis of mountainous areas and coastal rainforests.
The researchers kept a “brutal schedule,” said Friederike Otto, of the Environmental Change Institute at Oxford University, to try to provide the analysis quickly. Otto said a paper based on the analysis will be submitted soon for peer review, but that researchers have not yet decided on which scientific journal.
Climate scientists who were not involved in the research say the work holds up to scrutiny.
“This is clearly the most comprehensive and rigorous analysis of this event that can be generated on this time scale and I applaud the authors,” said Noah Diffenbaugh, a professor and climate scientist at Stanford University.
Nick Bond, Washington state’s climatologist, said he was impressed with the quality of the analysis, calling the estimate that such an intense heat wave could come every 5 to 10 years for the Pacific Northwest in as little as two decades’ time “a sobering finding.”
Karin Bumbaco, the state’s assistant climatologist, said the study’s simple focus — on hottest temperatures — does limit its findings, adding that if the researchers had included more aspects, such as heat wave duration or overnight temperatures, the study might have resulted in different answers.
“I think that the recent event was still exceedingly rare no matter how you define it and agree with their conclusion that increasing greenhouse gases were responsible for some portion of the recent event and that these types of events will become more common in the future,” Bumbaco said in an email.
Diffenbaugh said climate change is moving so rapidly that it can be difficult for climate scientists to pinpoint what the climate is now and to make estimates about future likelihoods. Estimates are often coarse and conservative, and their shelf life is limited as historical records drop.
“I think it would be wrong to think this level of heat won’t happen for another 1,000 years,” Diffenbaugh said. “This level of heat is more likely now than if global warming hadn’t occurred, and it’s more likely in the future.”