Extreme weather events tied conclusively to humans

In a rare and likely unprecedented step, a peer-reviewed scientific study has definitively concluded that certain extreme weather events could not have occurred without human-caused climate change. The record-high global average temperatures recorded in 2016 couldn’t be attributed to natural variability alone, say the authors of a 13 December report by the American Meteorological Society (AMS).

Meanwhile, NOAA reported that land surface temperatures in the Arctic this year are the second-highest, behind 2016, since records began in 1900, and the maximum extent of sea ice is the smallest ever. Both announcements were made during the fall meeting of the American Geophysical Union (AGU) in New Orleans.

Stephanie Herring was lead editor of the sixth annual AMS report that examines the impact of climate on extreme weather events of the previous year. She said the record-setting global heat in 2016—the warmest since modern record keeping began and nearly 1 °C above the mid-20th-century average—could not have occurred without a century of human-caused climate change. Climate was also a necessary contributor to the record heat wave that occurred over much of Asia that year and to the anomalously warm surface temperatures of the Bering Sea, part of a larger area of warm water in the eastern Pacific Ocean known as “the blob.”

“From a technical perspective, we have evidence now that climate change is pushing events beyond the threshold that could be achieved with natural variability alone,” Herring, a climate scientist at NOAA’s National Centers for Environmental Information, told reporters at the AGU meeting.

Scientists have been reluctant to attribute any single extreme weather event to climate change alone, preferring to maintain that human activity increases the frequency of such events, including the most intense ones. Over the six years of reports, the AMS team has determined that climate change had at least a partial role in 89 of 131 extreme weather events, including 21 of 27 in 2016.

Jeremy Mathis, director of NOAA’s Arctic Research Program, told reporters at the meeting that agency scientists are “fairly confident” of a connection between the warming Arctic and some extreme weather events at lower latitudes, including cold snaps, droughts, and hurricanes. But he cautioned that no single event can be conclusively tied to the Arctic changes. Improved observations in the polar region are needed to validate the association that models have drawn, he said.

The Arctic is warming at twice the global average rate, and the impacts of that warming are being felt throughout the Northern Hemisphere. The higher temperatures have created imbalances in the jet stream that allow polar air to migrate into lower latitudes. “The door to the refrigerator has been left open and the cold is spilling out,” Mathis said.

Although the growth of atmospheric greenhouse gases has been a contributor to the Arctic warming, Mathis said the primary cause has been Arctic amplification. Rising temperatures have created a positive feedback loop, as the loss of reflective sea ice and snow cover leaves darker open water and landmasses that absorb more sunlight, accelerating the warming and melting.

At +1.6 °C, the mean annual air surface temperature anomaly measured on land north of 60° N latitude during the year ending on 30 September was the second-highest value since records began in 1900. Cooler summer temperatures slowed the rate of summer sea-ice loss, yet at 4.6 million km², the minimum was still 25% smaller than its 1981–2010 average. Each of the 10 lowest minimum sea-ice extents has occurred during the past 11 years.

The unusually cool summer of 2017 slowed the rate of loss of the Greenland ice sheet to below that of the 1981–2010 average. But the net loss of the ice sheet, a major contributor to sea-level rise, continued.