The melting of Arctic sea ice during the winter of 2015/16 had received immense media attention. Several causes had been debated and many saw global climate change as the main culprit. But now, researchers at the ETH Zurich found unique weather anomalies to be the main causing factor. Yet, the researchers have not given an all-clear-signal.

Large parts of the Arctic Ocean are covered with sea ice every year. While in former days, the ice stayed all year round, it nowadays disappears in many areas in summer and reforms in winter. Credit: Michael Wenger
Large parts of the Arctic Ocean are covered with sea ice every year. While in former days, the ice stayed all year round, it nowadays disappears in many areas in summer and reforms in winter. Credit: Michael Wenger

In the winter of 2015/16, something happened that had never before been seen on this scale: at the end of December, temperatures rose above zero degrees Celsius for several days in parts of the Arctic. Temperatures of up to eight degrees were registered north of Svalbard. Temperatures this high have not been recorded in the winter half of the year since the beginning of systematic measurements at the end of the 1970s. As a result of this unusual warmth, the sea ice began to melt.

“We heard about this from the media,” says Heini Wernli, Professor of Atmospheric Dynamics at ETH Zurich. The news aroused his scientific curiosity, and a team led by his then doctoral student Hanin Binder investigated the issue. In November 2017, they published their analysis of this exceptional event in the journal Geophysical Research Letters. In it, the researchers show how these unusual temperatures arose: three different air currents met over the North Sea between Scotland and southern Norway, carrying warm air northwards at high speed as though on a “highway”. Two of these currents originally consisted of cold air, but were then warmed due to special conditions and then were led north on the air highway. This highway was formed by a specific pressure system constellation above Scandinavia and Iceland, which in the end led the warmed air masses into the Arctic. Within a week, some parts of the Arctic experienced a reduction of sea ice thickness by 30 cm. “These weather conditions and their effect on the sea ice were really exceptional,” says Hanin Binder. The researchers were not able to identify a direct link to global warming. “We only carried out an analysis of a single event; we didn’t research the long-term climate aspects” emphasises Binder.

Warm air from the Sahara (S), warmed Artic air (A) and compressed air masses from the troposphere (M) had been channeled by two intense, stable pressure systems and were led close to ground into the Arctic. This led to a massive loss of sea ice. Credit: Sandro Bösch, ETHZ
Warm air from the Sahara (S), warmed Artic air (A) and compressed air masses from the troposphere (M) had been channeled by two intense, stable pressure systems and were led close to ground into the Arctic. This led to a massive loss of sea ice. Credit: Sandro Bösch, ETHZ

But how to explain the massive losses in 2007 and 2012 when the Arctic sea ice minimum had reached record lows? This question was assessed by Professor Wernli in collaboration Lukas Papritz from the University of Bergen. According to their research, the severe melting in the aforementioned years was caused by stable high-pressure systems that formed repeatedly throughout the summer months. Under these cloud-free weather conditions, the high level of direct sunlight – the sun shines 24 hours a day at this time of year – particularly intensified the melting of the sea ice. The climate scientists’ investigation demonstrated that in the summers of 2007 and 2012, during which these high-pressure situations occurred particularly frequently, they led to cloud-free conditions every third day. The high level of solar radiation intensified and accelerated the melting of the sea ice. “The level of solar radiation is the main factor in the melting of the ice in summer. Unlike with the winter anomaly, the “injected” air at about 8 kilometre altitude from the south is not warm – with minus 60 degrees it’s ice-cold,” says Wernli. “The air temperature therefore has very little effect on the ice.” Furthermore, the northward transport of warm, humid air masses at the edge of the high-pressure systems reduces (heat) emission, which further intensifies melting. Their analysis has allowed the researchers to understand the meteorological processes leading to significant variations in summertime ice melt for the first time. “Our results underline the fundamental role that weather systems in temperate latitudes play in episodes of particularly intense ice melt in the Arctic,” says the ETH professor.

At the end of the summer in 2012, the Arctic sea ice minimal extension had reached a record low in terms of thickness and area. However, this was not a single event, but has been reported now repeatedly. Credit: NASA Goddard Space Flight Center
At the end of the summer in 2012, the Arctic sea ice minimal extension had reached a record low in terms of thickness and area. However, this was not a single event, but has been reported now repeatedly. Credit: NASA Goddard Space Flight Center

Source: Peter Rüegg, ETH Zürich