June 29, 2013 — andyextance

The almost complete melting of Greenland’s ice sheet surface on July 11-12 2012 was caused by climate processes not projected by models. That’s according to an international team led by Edward Hanna from the University of Sheffield, UK that has looked at what might have driven the melt. “The models used to predict future climate change are clearly deficient in simulating some of the recent jet stream changes that we have shown to be responsible for enhanced warming and ice melt over Greenland,” Edward told me. And the world needs to pay attention when Greenland defrosts, as the water it produces is a major part of sea level rise.

Having long studied the Greenland ice sheet, or GrIS, Edward was perhaps one of the people least amazed by 2012’s events. “Last year’s record melt was a bit of a surprise, but perhaps not so startling in retrospect, given strong recent warming of the ice sheet area since the early-mid 1990s,” he said. With other scientists he has also found a clear change since 2007 in early summer Arctic wind patterns relative to previous decades that has led to warm Greenland summers. In particular, jet stream patterns of winds weaving north and south in drunken circles around the pole have changed to drive warm winds over Greenland. That study also linked these changes to cool, wet summers in the UK since 2007, whose unusual wetness in 2012 is seemingly the other face of the GrIS melt coin.

Call the GrIS team

The UK Met Office explains the jet stream, which plays an important role in the country’s weather, with Edward Hanna linking recent cool wet summers to jet stream changes. Credit: Met Office.

With such a close interest in the region, it was a natural step for Edward to investigate the record melt. But unpicking the complex factors that might cause such an event needs more than just one person’s expertise. So Edward teamed up with scientists from Switzerland, Denmark, Belgium, Chile and the US, as well as the UK, who had the right skills for the challenge. “It’s a truly multidisciplinary team working at the interface between climatology, glaciology and oceanography,” Edward said. “This multi-faceted perspective enabled us to gain significant insights into the processes responsible for the record melt conditions.”

Together, in an International Journal of Climatology paper published earlier this month, the scientists first examined the record status of last year’s GrIS melt. Using a method developed by Thomas Mote from the University of Georgia they worked out melting from satellite measurements. That showed almost 99% of the surface melted at different points in mid-July, compared with a previous high of 52% in 2010, in records dating back to 1960. And using computer models, the team found meltwater and runoff from the GrIS in 2012 was more than double average levels.

Edward and his teammates then called on a wide variety of measurements to study the changes in the ocean and atmosphere behind the melt. The jet stream pattern change that Edward had previously seen continued, pushing relatively warm southerly winds over the west of the ice sheet. That formed a ‘heat dome’ over Greenland, with measuements showing several new high Greenland temperature records set in summer 2012.

Wait and sea?

By contrast temperatures beneath the ocean surface remained near to average, leaving the changing wind patterns as the prime suspect in the melt. But temperatures in 2012 weren’t that much higher than previous years – so why the huge jump in melt area? Edward’s team blame this on the collected effects of melting in the warm years since 2007. Once the snow left on the sheet from the winter before had melted, it uncovered less reflective areas, including bare ice revealed in previous summers. That ice absorbs more of the sun’s energy, worsening the effect of the high temperatures.

When Edward’s team compared their GrIS runoff against some of the best established and most trusted climate models, they found the models weren’t predicting such levels until 2040. The difference comes because none of the model projections to 2100 foresee the jet stream changes. However the scientists emphasise that it’s not yet clear if this particular effect is a human-driven change, or a natural cycle.

As well as stressing the potential significance for sea level rises of models missing these processes, Edward’s team now hopes to make clearer links to climate change in other regions. “We will be very interested to see whether last year’s unusual jet stream and Greenland melt conditions repeat themselves again this summer,” he said. “Is this the beginning of a sustained long-term change in Greenland regional climate that was not foreseen a few years ago?”

Journal references:

Edward Hanna, Xavier Fettweis, Sebastian H. Mernild, John Cappelen, Mads H. Ribergaard, Christopher A. Shuman, Konrad Steffen, Len Wood, Thomas L. Mote (2013). Atmospheric and oceanic climate forcing of the exceptional Greenland ice sheet surface melt in summer 2012 International Journal of Climatology DOI: 10.1002/joc.3743

James E. Overland, Jennifer A. Francis, Edward Hanna, and Muyin Wang (2012). The recent shift in early summer Arctic atmospheric circulation Geophysical Research Letters DOI: 10.1029/2012GL053268