From Columbia University and the “any weather event is now proof of global warming” department comes this reach of a paper trying to claim that a northern jet stream excursion is a signature of “Arctic amplification”. I don’t put much stock in it especially when Nature had an editorial a few years ago that said:
Better models are needed before exceptional events can be reliably linked to global warming.
And in this case, they are citing a single event to claim “Arctic amplification” has set in. I’m sorry, I just don’t buy it. We are told by climate alarmists that any cold event, such as a cold winter, or a string of record low temperatures don’t mean anything in the context of climate, that it is the trends of events that matter, not the individual events. Now, we are asked to believe that one event, a cut-off high pressure system in summer of 2015 is the signature of an Arctic-scale climate change. Forecasters are well aware of this sort of blocking high.
Even the IPCC AR4 suggests that the high latitudes in the Northern Hemisphere were likely as warm, or warmer, than present in the past:
Plus, they make the claim:
Jet stream reached northern latitudes never before recorded
We don’t have reliable weather maps that can accurately portray the position of the jet stream over Greenland prior to World War II, when the phenomenon was observed, as this reference in Wikipedia notes:
Many sources credit real understanding of the nature of jet streams to regular and repeated flight-path traversals during World War II. Flyers consistently noticed westerly tailwinds in excess of 100 mph (160 km/h) in flights, for example, from the US to the UK. Similarly in 1944 a team of American meteorologists in Guam, including Reid Bryson, had enough observations to forecast very high west winds that would slow bombers going to Japan.
It wasn’t until around 1950, that it began to be tracked regularly, so the “never before recorded” claim really doesn’t mean much in the scope of weather patterns for Greenland when you only have about 60 years of data.
The recent conditions in Greenland likely do not represent an aberrant fluctuation in the region’s climate or the beginning of “Arctic amplification”. Instead, events like the summer of 2015 lie within the known limits of natural variations that just haven’t been seen in the 60+ years of jet stream position data we are aware of.
Also, lead author Tedesco has made some baseless claims in the past regarding Greenland: Ridiculous claim by Marco Tedesco: ‘Darkening of the Greenland Ice Sheet is projected to continue as a consequence of continued climate warming’ where he completely ignored the contribution of soot to the changing albedo of the Greenland ice sheet.
So, I don’t think the claim about Arctic amplification they are making is all that credible; the claim from a single event is a big stretch.
Here’s the PR via Eurekalert:
Greenland’s 2015 melt records consistent with ‘Arctic amplification’
Jet stream reached northern latitudes never before recorded
Following record-high temperatures and melting records that affected northwest Greenland in summer 2015, a new study provides the first evidence linking melting in Greenland to the anticipated effects of a phenomenon known as Arctic amplification.
Arctic amplification, in the simplest terms, is the faster warming of the Arctic compared to the rest of the Northern Hemisphere as sea ice disappears. It is fueled by a feedback loop: rising global temperatures are melting Arctic sea ice, leaving dark open water that absorbs more solar radiation, and that warms the Arctic even more. Arctic amplification is well documented, but its effects on the atmosphere are more widely debated. One hypothesis suggests that the shrinking temperature difference between the Arctic and the mid-latitudes will lead to a slowing of the jet stream, which circles the northern latitudes and normally keeps frigid polar air sharply separated from warmer air in the south. Slower winds could create wilder swings of the jet stream, allowing warm, moist air to penetrate farther north.
The new study, published this week in Nature Communications, shows that those anticipated effects occurred over northern Greenland during the summer of 2015, including a northern swing of the jet stream that reached latitudes never before recorded in Greenland at that time of year.
This animation shows changes in the polar jet stream from June 1, 2015 to July 31, 2015. The jet stream is approximated by crosses. The northerly shift of the jet stream may be linked to a warming arctic, and record melt of the Greenland ice sheet in 2015. CREDIT Marco Tedesco/Lamont-Doherty Earth Observatory
“How much and where Greenland melts can change depending on how things change elsewhere on earth,” said lead author Marco Tedesco, a research professor at Columbia University’s Lamont-Doherty Earth Observatory and adjunct scientist at NASA Goddard Institute for Space Studies. “If loss of sea ice is driving changes in the jet stream, the jet stream is changing Greenland, and this, in turn, has an impact on the Arctic system as well as the climate. It’s a system, it is strongly interconnected and we have to approach it as such.”
The Greenland ice sheet, earth’s second largest after Antarctica, holds enough ice that, if it were to melt entirely, it would raise average global sea level by about seven meters. Understanding the drivers of melting is critical to understanding how quickly and by how much sea level will rise in the future and how Greenland’s freshwater runoff will affect ocean circulation and ecology.
Northwest Greenland’s summer of melt started in June 2015, when a high-pressure ridge squeezed off from the jet stream, the study shows. It moved westward over Greenland until it sat over the Arctic Ocean and affected weather across the island through mid-July.
That high-pressure system, called a cut-off high, brought clear skies and warmed northern Greenland, helping set records for surface temperature and meltwater runoff in the northwest, the study shows. With less summer snow falling and melting underway, northern Greenland’s albedo, or reflectivity, also decreased. A less-reflective surface absorbs more solar energy, which feeds more melting, as Tedesco illustrated in a study earlier this year on the darkening of Greenland.
Northern Greenland also set an unusual July record for wind: the winds blew east to west on average, rather the usual west to east; only two other years on record show easterly winds on average in July, both slower. At the same time, the jet stream’s northernmost ridge swung farther north than ever recorded for that month, passing 76 degrees North latitude, nearly 2 degrees farther north than the previous July record, set in 2009, the authors write.
The same atmospheric pattern had a different impact on southern Greenland, where new melting records have been set over the past decade. The south saw more snow during summer of 2015 and less melting than previous years.
The authors stop short of confirming Arctic amplification as the cause of the warming, but they say the results fit the anticipated effects of Arctic amplification described by Jennifer Francis of Rutgers University and Stephan Vavrus of the University of Wisconsin in a 2012 paper.
Recent studies exploring the potential effects of Arctic amplification have showed that high-pressure blocks connected to northward swings of the jet stream have become more common near Greenland. Edward Hanna of the University of Sheffield, a co-author of the new Nature Communications paper, released a study in May using the Greenland Blocking Index to measure the strength of stationary high-pressure systems over the past 165 years and found that seven of the top 11 systems had occurred since 2007.
“The significant increase in Greenland high-pressure blocking that has occurred in the last 20 to 30 years is clearly related to recent record warming over the region, as well as jet-stream changes,” Hanna said. “This makes it more likely than not that within the next five to 10 years we will witness further record Greenland melt events like in 2012 and 2015.”
“The Arctic is full of climate surprises, and Greenland is a key player,” said James Overland, an oceanographer and climate scientist at the National Oceanic and Atmospheric Administration who was not involved in the new study. “Climate models suggest a 4 degree Celsius Arctic temperature increase by mid-century, but such jet stream related surprises acting on Greenland as reported by Tedesco et al. can accelerate Arctic climate change.”
Whether the patterns seen in 2015 will continue in the future remains to be seen. This spring, Arctic sea ice set another record low for its maximum extent for the year. “Greenland also experienced early season melt in early April of this year comparable to April 2012. Record setting melt occurred later that summer, but it is too early to tell whether the same will hold true in 2016,” said co-author Thomas Mote of the University of Georgia.
“The conditions we saw in the past aren’t necessarily the conditions of the future,” Tedesco said. “If humans change the forcing, we are going into uncharted territory.”
The other co-authors of the newpaper are Xavier Fettweis of University of Liege; Jeyavinoth Jeyaratnam, James Booth, and Rajashree Datta of City College of New York; and Kate Briggs of University of Leeds. The study was supported by funding from NASA’s Interdisciplinary Data Science Program, NASA’s Cryosphere Program and the National Science Foundation.
The paper, “Arctic cut-off high drives the poleward shift of a new Greenland melting record,” is available from the author.