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As temperatures at the North Pole approached the melting point this week and the Arctic ice thins toward the vanishing point, new research suggests that the extraordinary records being set year after year are a manifestation of manmade climate change.
One new study, published last week in the journal Weather and Climate Extremes, found that unusual heat that hit the Arctic in 2016 could not have happened without the increases in greenhouse gas emissions since the Industrial Revolution, which has set off a feedback loop of rising temperatures and dwindling sea ice.
It is a classic prediction of climate theory that's playing out more vividly than ever before.
The study's authors, scientists from NOAA and the Cooperative Institute for Research in Environmental Sciences, used a climate model that could generate heat anomalies based on the greenhouse gas levels found in the late 1800s. They found that the kind of extreme heat that happened in 2016 wasn't possible in the 1800s model.
The presence of greenhouse gas emissions contributed to the spikes, and the loss of sea ice exacerbated the warming.
"While sea ice melts due to Arctic warming, ice loss itself can cause additional warming near the surface and be responsible for most of the Arctic amplification (a phenomena used to characterize the strong Arctic warming compared to lower latitudes)," lead author Lantao Sun said in an email. "The physics behind this is that sea ice acts as a barrier for the heat transport from the ocean to the atmosphere."
Arctic Heat Spikes Again
Similar to 2016, the winter of 2017-2018 has seen extreme Arctic heat events—including one that's happening now.
Over the last few days, temperatures have been as much as 30 degrees Farenheit above normal (about 17 degrees Celsius above normal), and the sea ice has taken a corresponding dip. "The sea ice has taken a huge nosedive in the last couple days," said Julienne Stroeve, a senior sea ice scientist with the National Snow and Ice Data Center (NSIDC).
In April, air temperatures rose throughout many parts of the Arctic, as the sea ice extent dropped, tying the month of April 2016 for lowest extent on record.
The Danger in Sea Ice Loss
Stroeve has also been studying what lessons can be gleaned from the 2016 winter. In a paper that has been accepted but not yet published in The Cryosphere, Stroeve writes about a new chapter in sea ice history.
In the past, winter heat waves didn't have much of an impact on sea ice. That's because even though the temperature would rise higher than normal, it was still comfortably below freezing most of the time. That was important, because the winter is when the sea ice rebounds after the summer melt. Not anymore.
"The warming we're starting to see in winter is enough now to impact ice growth in the winter," she said. As temperatures spiked throughout the winter of 2016-2017, Stroeve found that the growth in ice thickness over the Arctic Ocean was reduced by 5 inches.
Stroeve hasn't yet analyzed the winter of 2017-2018, but she expects the losses to have continued. "This year definitely had the least amount of freezing degree days,"—an equation that looks at the number of days below freezing and how far below freezing they were. "It was probably the warmest winter we've seen," she said.
The temperature measurements at Cape Morris Jesup, the northernmost point of mainland Greenland, tells that story. In the month of February, since 1981, it has been rare for hourly air temperatures to spike above freezing. In 1997 it happened once. In 2011 it happened five times, and in 2017 it happened seven times. This year, it happened 59 times.
"As Arctic sea ice loss continues, we will see stronger warming in the Arctic," Sun said. By the middle of the century, Sun and his co-authors found that when these heat waves happen, the size of the anomaly will likely be double what was seen in 2016. "In other words, the future Arctic surface temperature (e.g. how much warming we will get) is closely tie to how much sea ice melts," he said.