"Spring snow cover extent reductions in the 2008-2012 period exceeding climate model projections," by C. Derksen & R. Brown, GRL 39 (2012); doi:10.1029/2012GL053387

Geophysical Research Letters, 39 (2012) L19504; doi:10.1029/2012GL053387

Spring snow cover extent reductions in the 2008-2012 period exceeding climate model projections

Chris Derksen and Ross Brown

Abstract

Analysis of Northern Hemisphere spring terrestrial snow cover extent (SCE) from the NOAA snow chart Climate Data Record (CDR) for the April to June period (when snow cover is mainly located over the Arctic) has revealed statistically significant reductions in May and June SCE. Successive records for the lowest June SCE have been set each year for Eurasia since 2008, and in 4 of the past 5 years for North America. The rate of loss of June snow cover extent since 1979 (-21.5% decade^-1) is greater than the loss of September sea ice extent (-10.8% decade^-1) over the same period. Analysis of Coupled Model Intercomparison Project Phase 5 (CMIP5) model output shows the marked reductions in June SCE observed since 2005 fall below the zone of model consensus defined by +/-1 standard deviation from the multi-model ensemble mean.

Received 30 July 2012; accepted 9 September 2012. 

Citation: Derksen, C. and R. Brown (2012), Spring snow cover extent reductions in the 2008-2012 period exceeding climate model projections, Geophys. Res. Lett., 39 (2012) L19504; doi:10.1029/2012GL053387.

http://www.agu.org/pubs/crossref/pip/2012GL053387.shtml

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WaPo: Snow cover and Arctic sea ice extent plummet suddenly as globe bakes

by Jason Samenow, Capitol Weather Gang, The Washington Post, July 18, 2013

Temperature difference from average during June around the globe (NASA)

NOAA and NASA both ranked June 2013 among the top five warmest (NOAA fifth warmest, NASA second warmest) Junes on record globally (dating back to the late 1800s).  But, more remarkable, was the incredible snow melt that preceded the toasty month and the sudden loss of Arctic sea ice that followed.

NH snow cover extent difference from average April to May (NOAA)

The amazing decline in Northern Hemisphere snow cover during May is a story few have told, but is certainly worth noting.  In April, hefty Northern Hemisphere snow cover ranked 9th highest on record (dating back to 1967), but then turned scant, plummeting to third lowest on record during May. Half of the existing snow melted away.

“This is likely one of the most rapid shifts in near opposite extremes on record, if not the largest from April to May,” said climatologist David Robinson, who runs Rutgers University Global Snow Lab.

The snow extent shrunk from 12.4 million square miles to 6.2 million square miles in a month’s time. By June, just 2.3 million square miles of snow remained in the Northern Hemisphere (a decline of 63 percent from May), third lowest on record.

“In recent years it hasn’t seemed that unusual to have average or even above average winter snow extent rapidly diminish to below average values come spring,” Robinson said.

Northern Hemisphere spring snow cover is in the midst of long-term free fall, similar to its relative, summer Arctic sea ice extent.

You may recall, late last summer the Arctic sea ice extent dropped to its lowest level on record, 49 percent below the 1979-2000 average.

Temperature difference from normal over the high latitude Northern Hemisphere over first 10 days of July (National Snow and Ice Data Center)

It’s not clear if 2013 levels will match 2012′s astonishing record low, but – with temperatures over the Arctic Ocean 1-3 degrees above average – the 2013 melt season has picked up in earnest during July.

“During the first two weeks of July, ice extent declined at a rate of 132,000 square kilometers (51,000 square miles) per day. This was 61% faster than the average rate of decline over the period 1981 to 2010 of 82,000 square kilometers (32,000 square miles) per day,” the National Snow and Ice Data Center writes on its website.

Despite this rapid ice loss, the current mid-July 2013 sea ice extent is greater than 2012 at the same time by about 208,000 square miles NSIDC says.

2013 sea ice extent (blue line) compared to average (black line) and record (dashed green line). (National Snow and Ice Data Center)

Will 2013 close the gap? That depends on the weather, as the Arctic Sea Ice blog explains:

….the amount of easy-to-melt ice is starting to run out. Even though this year’s ice pack consists of a record amount of first-year ice [which melts most readily], the weather still plays an important role. . . .

. . . there’s no telling what could happen if the weather is very conducive to melting/compacting/transport for a week or two….

http://www.washingtonpost.com/blogs/capital-weather-gang/wp/2013/07/18/snow-and-arctic-ice-extent-plummet-suddenly-as-globe-bakes/

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Joe Romm: Exceptional 2012 Greenland Ice Melt Caused by Jet Stream Changes that May Be Driven by Global Warming

by Joe Romm, Climate Progress, June 17, 2013

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.

Extent of surface melt over Greenland’s ice sheet on July 8, 2012 (left) and July 12, 2012. In just a few days, the melting had dramatically accelerated and an estimated 97% of the ice sheet's surface had thawed. Credit: NASA.

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% of the ice-sheet surface melted. This far exceeded the previous surface melt extent record of 52% 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 5–10 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 100–1,000 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 non-linear effects are starting to dominate Greenland ice melt. It may be time to redo projections of sea level rise once again.

Related Posts

• Science Stunner: Greenland Ice Melt Up Nearly Five-Fold Since Mid-1990s
• JPL bombshell: Polar ice sheet mass loss is speeding up, on pace for 1 foot sea level rise by 2050
• Greenland Ice Sheet Melt Nearing Critical ‘Tipping Point’
• Greenland Ice Sheet “Could Undergo a Self-Amplifying Cycle of Melting and Warming … Difficult to Halt,” Scientists Find

http://thinkprogress.org/climate/2013/06/17/2169321/exceptional-2012-greenland-ice-melt-caused-by-jet-stream-changes-that-may-be-driven-by-global-warming/

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