The Bizarre Weather Science Behind Greenland’s Record Melting
Credit to Author: Maddie Stone| Date: Wed, 07 Aug 2019 12:33:27 +0000
Over the last week, Greenland shed more than 60 billion tons of ice as a heat wave enveloped the Arctic island, triggering melting on a scale not seen since 2012. From July 31 to August 1, the ice sheet lost enough weight to cover the state of Florida in nearly 5 inches of water. At the height of the melt event, rates of ice loss approached levels scientists had not expected to see until the late 21st century.
But thanks to a double whammy of natural atmospheric circulation patterns and human-caused climate change, a growing body of research indicates that more mega melts like this could be in Greenland’s future.
On one level, the connection between Greenland’s latest melt-out and climate change is fairly straightforward. Air temperatures over the Arctic are rising at about twice the globally-averaged rate, causing accelerating losses of ice on land and at sea. Rising temperatures are also helping to supercharge heat waves, like the one that roasted Europe in late July before migrating over Greenland. Detailed attribution studies will tell us exactly how much of a role global warming played in that record-breaking event, but a rapid analysis suggests human carbon emissions did help amplify the heat.
In between short-term weather events and long-term climate trends there’s another force to consider: natural circulation patterns in the atmosphere, which can work with or against global warming to counteract or compound Arctic ice melt. And one key atmospheric pattern, the so-called North Atlantic Oscillation (NAO), is currently feeling pretty collaborative. In conjunction with rising temperatures, it seems to have helped prime Earth’s northern ice sheet for historic levels of melting this summer.
You can think of the NAO as a vast seesaw of atmospheric pressure that wobbles between positive and negative states. When the NAO is positive, the two ends of the seesaw—a low pressure center over Greenland, and a high pressure zone that swings between Bermuda and the Azores—are lower and higher relative to one another. For Greenland, that setup brings more cloudy days, more snowfall, and cooler weather.
When the NAO swings negative, the seesaw contracts. The pressure gradient becomes less extreme, and Greenland tends to see sunnier weather and less snowfall. The negative phase of the NAO is also associated with an anti-cyclonic circulation pattern in the atmosphere that helps draw up warm air from the south.
“Both of these kinds of events are extremely exceptional within the longer-term climate record.”
The NAO wobbles frequently and somewhat erratically, but since the start of this year’s melt season it’s been in a persistently negative state. This has led to what Xavier Fettweis, a climatologist at the University of Liège, described as “abnormal persistence” of anticyclonic conditions over Greenland—exactly the sort of conditions in which he’d expect to see major melting.
“An anticyclone over Greenland favours advection of warm air masses along its west side but also sunny and dry conditions over the ice sheet,” Fettweis told Motherboard in an email. Those dry and sunny conditions enhance what Fettweis described as the “melt albedo positive feedback,” wherein the melting of recent snowfall exposes darker glacier ice that absorbs more of the Sun’s energy, triggering even more melting.
We’ve seen conditions like this before. In fact, a study published in PNAS earlier this year found since the early 2000s, summers of major ice losses have tended to coincide with a negative NAO. In the summer of 2012, which saw the most extensive surface melting on record for Greenland, the NAO was deeply negative. Much like this year, a high pressure blocking system parked itself over the island, delivering exceptionally warm weather. The following year, when the NAO flipped positive, ice losses nearly stopped, before ramping up anew in 2015 as the NAO veered negative once more.
“In the last ten to twelve years, the NAO seems to have become the kingmaker of melting,” Michael Bevis, a geophysicist at Ohio State University and lead author on the aforementioned PNAS paper, told Motherboard. “It can either allow it to happen through a double whammy [with global warming], or withdraw its support and shut it down.”
As Bevis notes, it seems to be the 1-2 punch of global warming and favorable NAO conditions that’s helping drive major melt events today. He likened it to the way that El Niño, a natural cycle that tilts ocean temperatures in the tropical Pacific a bit warmer, can now trigger more severe coral bleaching events due to the background warming caused by climate change.
Martin Stendel, a climate scientist at the Danish Meteorological Institute, agreed with this line of reasoning. Negative NAO conditions are nothing new for Greenland, he said, “but it is only now [in recent years] it becomes important, because the ‘background’ temperature is rising.”
Some researchers wouldn’t ascribe quite so much influence on the NAO. Kevin Trenberth, an atmospheric scientist at the National Center for Atmospheric Research, said that while the circulation pattern plays a role when it comes to Arctic ice losses, typically the NAO drives opposing weather patterns over Europe and Greenland. This summer, we’ve seen heat waves roast both parts of the world.
“So while the NAO plays some role it is much more,” he said, adding that the natural variability in Arctic weather is “huge” and connected to multiple different cycles.
Indeed the exact role of the NAO when it comes to Arctic ice melt, and how that role may change in the future, remain active areas of research. Fettweis suggested that the NAO might even be veering more negatively as a consequence of human-caused climate change, a trend that could accelerate Arctic ice losses quite significantly over the 21st century, although it remains highly speculative.
“In general, it’s very clear the NAO is important in controlling the amount of melting we’re getting,” glaciologist Ruth Mottram of the Danish Meteorological Institute told Motherboard. “The question of whether we’re getting more extremes is more contentious.”
Weather aside, the long-term trend is clear: Greenland is losing its ice. The rate of loss has increased sixfold since the 1980s, per a paper published earlier this year. And recent summertime thaws have a big role to play. Sarah Das, a glaciologist at Woods Hole Oceanographic Institution who’s used ice core records to reconstruct Greenland’s recent melt history, described the 2012 melt event that enveloped nearly the entire ice sheet’s surface as “unprecedented” in the last few centuries, perhaps within the last several thousand years. This summer, she said, “would be up there with [2012] if not eclipsing it.”
“Both of these kinds of events are extremely exceptional within the longer-term climate record,” Das told Motherboard.
Greenland’s latest melting spike started to taper over the weekend, with the ice sheet “only” losing 8.5 billion tons of ice on Saturday and 7.6 billion tons on Sunday. Temperatures have eased off and should remain closer to normal this week.
But in an age of warming, “normal” is a moving target for the Arctic. And it’s likely just a matter of time before the mercury surges once more.
This article originally appeared on VICE US.