Joe Romm / Climate Progress: New research finds that “unusual changes in atmospheric jet stream circulation caused the exceptional surface melt of the Greenland Ice Sheet (GrIS) in summer 2012.”

Prof. Jennifer Francis tells me these changes are consistent with those caused by warming-driven “Arctic Amplification.” And that means GrIS may melt faster than climate models have projected.

Back in May, a study found that by 2025, there is a “50-50 chance” of this unprecedented ice melt happening annually simply based on the continued rapid warming of GrIS.

This new study, “Atmospheric and oceanic climate forcing of the exceptional Greenland ice sheet surface melt in summer 2012,” suggests this kind of melt may become commonplace even sooner.

As the news release explains, an international team used a computer model and satellite data “to confirm a record surface melting of the GrIS for at least the last 50 years – when on 11 July 2012, more than 90 percent of the ice-sheet surface melted. This far exceeded the previous surface melt extent record of 52 percent in 2010.” Weather station data “showed that several new high Greenland temperature records were set in summer 2012.”

The research “clearly demonstrates that the record surface melting of the GrIS was mainly caused by highly unusual atmospheric circulation and jet stream changes, which were also responsible for last summer’s unusually wet weather in England.”

What were these changes? Professor Edward Hanna from the University of Sheffield’s Department of Geography explains:

“The GrIS is a highly sensitive indicator of regional and global climate change, and has been undergoing rapid warming and mass loss during the last 5-20 years. Much attention has been given to the NASA announcement of record surface melting of the GrIS in mid-July 2012. This event was unprecedented in the satellite record of observations dating back to the 1970s and probably unlikely to have occurred previously for well over a century.

“Our research found that a ‘heat dome’ of warm southerly winds over the ice sheet led to widespread surface melting. These jet stream changes over Greenland do not seem to be well captured in the latest Intergovernmental Panel on Climate Change (IPCC) computer model predictions of climate change, and this may indicate a deficiency in these models. According to our current understanding, the unusual atmospheric circulation and consequent warm conditions of summer 2012 do not appear to be climatically representative of future ‘average’ summers predicted later this century.

“Taken together, our present results strongly suggest that the main forcing of the extreme GrIS surface melt in July 2012 was atmospheric, linked with changes in the summer North Atlantic Oscillation (NAO), Greenland Blocking Index (GBI, a high pressure system centred over Greenland) and polar jet stream which favoured southerly warm air advection along the western coast.

“The next five-10 years will reveal whether or not 2012 was a rare event resulting from the natural variability of the NAO or part of an emerging pattern of new extreme high melt years. Because such atmospheric, and resulting GrIS surface climate, changes are not well projected by the current generation of global climate models, it is currently very hard to predict future changes in Greenland climate. Yet it is crucial to understand such changes much better if we are to have any hope of reliably predicting future changes in GrIS mass balance, which is likely to be a dominant contributor to global sea-level change over the next 100-1000 years.”

These changes in the jet stream and blocking patterns sounded quite similar to the findings of Francis, NOAA, and others. While the news release says, “The analysis shows that ocean temperatures and Arctic sea-ice cover were relatively unimportant factors in causing the extra Greenland melt,” I asked Dr. Francis if in fact these findings were consistent with her work.

She replied:

Hanna’s 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 behavior, I think it’s likely that Arctic Amplification (AA) IS playing an important role.

Sea ice loss is only one factor driving AA. In spring and summer, the AA appears to be caused mainly by 2 other factors: 1) the decline of snow cover on high-latitude land areas creates an albedo feedback similar to sea ice but is instead involves the earlier drying and heating of the soil under the snow, which promotes an earlier warm season on the continents and contributes to enhanced Arctic warming, and 2) increasing water vapor transport into the Arctic.

As you know, water vapor is a powerful greenhouse gas, it releases heat into the atmosphere when it condenses into clouds, and extra water vapor promotes additional clouds, which are also effective trappers of heat below them. The water vapor effect may be the most important for the summer ridging over Greenland, as it causes warming through a deeper layer of the atmosphere than the snow/ice effects (see Alexeev et al, Climate Dynamics, 2005; Porter et al, JGR, 2012), and thus has a more direct impact on weakening the poleward temperature gradient and consequently the jet-stream zonal winds. There was also a recent study by Von Walden and coworkers showing that cirrus clouds also caused additional surface warming over Greenland last summer, and these are just the type of clouds you’d expect to see under an upper-level ridge of high pressure.

If this ridging pattern continues to be as persistent as it has been since 2007 (see Hanna et al, 2012), then it could be that the climate models are underestimating the amount of surface melt from the Greenland ice sheet.

It would seem that warming-driven nonlinear effects are starting to dominate Greenland ice melt. It may be time to redo projections of sea level rise once again.

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