A weather pattern that may have become more common in recent years was largely responsible for causing last summer’s record melt in Greenland, according to a new study. The research, published this week in the International Journal of Climatology, found that a giant protective dome of high pressure established itself above Greenland throughout the summer of 2012, leading to above-average temperatures and more abundant sunshine than in a typical summer season.
As was first reported by Climate Central, 98 per cent of Greenland’s ice surface experienced melting between July 8-15, 2012, an astonishing feat considering that this included the frigid high-altitude zones where temperatures occasionally exceed the freezing mark, but for relatively short periods of time.
The new study, along with other recent work, points to the unsettling possibility that the Arctic climate system is evolving in ways that have not been anticipated by sophisticated climate models. Scientists have been using those models to project the future melting of the vast Greenland ice sheet, which with 680,000 cubic miles of ice, is a critical factor in determining the eventual course of sea level rise.
To illustrate the importance of the Greenland ice sheet to sea level rise, consider that if all of the ice were to melt – which won't happen anytime soon – the oceans would rise by more than 20 feet, endangering the world's coastal cities from New York to Shanghai.
According to the study, the summer of 2012 featured the strongest blocking high pressure area on record for that region since 1948. Blocking highs act much like an offensive line keeping rushers away from the quarterback in football, preventing weather systems from moving into or through an area. They can lock in warmer-than-average conditions for weeks or months, and have been linked to deadly extremes such as the Russian heat wave of 2010.
Summertime blocking highs have been increasingly frequent in the vicinity of Greenland since 2007, but long-term trends across the northern hemisphere have not shown a statistically significant increase in blocking patterns, researchers say.
The new study, which was led by Edward Hanna, a researcher at the University of Sheffield in the UK, found that the high pressure area was related to a particular phase in a climate pattern known as the North Atlantic Oscillation (NAO), and this pumped warmer-than-average air northward across the western flank of the ice sheet throughout much of June, July, and August. Numerous surface weather stations throughout Greenland set records for the warmest summer, although the network of temperature sensors on the island is quite sparse and contains discrepancies, according to the study.
Although the study pins the blame on atmospheric phenomena, rather than ocean dynamics, for causing the record Greenland melt, its findings are nevertheless consistent with other recent research that has shown that Arctic sea ice loss may promote the formation of blocking highs in the northern mid-latitudes, like the one that dominated last summer in Greenland.
Jennifer Francis, a professor at Rutgers University whose work has shown that diminishing Arctic sea ice may be altering the jet stream in the Northern Hemisphere, told Climate Central that the new study bolsters the case she and her colleagues have been making.
“His findings seem very consistent with the research I've been involved with, particularly our observations of an increasing tendency for ridging in the north Atlantic during summer. While I agree with him that sea ice loss, per se, may not be the primary driver of this behaviour, I think it's likely that Arctic Amplification IS playing an important role,” she said in an email conversation.
Arctic amplification refers to the tendency for temperature changes in the Arctic to be magnified, leading to more warming. For example, when sea ice melts it exposes darker surface waters, which absorb more of the sun’s incoming energy, causing temperatures to increase further and in turn melt more ice. And so on. This feedback loop helped lead to the lowest recorded Arctic sea ice extent on record in 2012, and has primed the Arctic for further warming to come.
As Hanna explained, the hypothesis that Francis (who was not involved in Hanna's study) and her colleagues have put forward is that by warming the Arctic more rapidly than areas further to the south, there is less of a temperature gradient between northern and southern latitudes, which in turn may tend to weaken the North Atlantic polar jet stream and make it more wobbly and stationary at times. And a wobbly jet stream is more prone to getting stuck, leading to weather patterns that hang around for longer than they otherwise would.
“Greenland tends to lie under a northward-moving branch of this recently wavier jet stream, which draws warmer air masses up the western flank of the ice sheet,” Hanna said in an email message.
The new study also raises important questions about the accuracy of complex computer models of the climate system, since it found that the latest generation of climate models don’t reproduce the increase in the Greenland melt rate that has occurred since 2000, and do not project that similar atmospheric circulation patterns will occur with regularity in the coming years to decades.
That could mean that the models are missing some key piece of information or process taking place in the Arctic climate system, or it may indicate that the past several years, during which time the Greenland ice sheet has sped up its shedding of ice mass raising global sea levels, are an anomaly.
“The next 5-10 years will reveal whether or not 2012 was a one-off, rare event resulting from the natural variability of the NAO or part of an emerging pattern of new extreme high melt years,” the study said.
Jason Box, an expert on the Greenland ice sheet who first called attention to the extensive melt data from last summer, but was not involved in the new study, said that most computer model simulations may be too optimistic about the future of the ice sheet. If that's true, then it’s disturbing news, considering that Greenland is thought to be the largest contributor to sea level rise through 2100, and has the potential to raise sea levels by several feet in worst-case scenarios.
“Here's the important implication: it takes general-circulation models until the end of year 2100 to produce the climate we observed in the past six summers over Greenland, suggesting that if this type of persistent amplified wave behaviour continues – which seems plausible – Greenland will melt 'faster than forecast,'" Box said.
Hanna, for one, said the jet stream changes could become a more typical feature of Arctic summers as global warming continues. “I believe there is a strong possibility, according to meteorological theory, that the anomalous circulation seen last year (and indeed for the past six summers) over Greenland will become more entrenched in future summers. Of course, the real climate system is complex, and there will always be year-to-year variation,” Hanna said.