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Daniel Bailey

Greenland ice sheet albedo feedback: Thermodynamics and atmospheric drivers J. Box, X. Fettweis, J. Stroeve, M. Tedesco, D. Hall, and K. Steffen

The Cryosphere Discuss., 6, 593–634, 2012
www.the-cryosphere-discuss.net/6/593/2012/
doi:10.5194/tcd-6-593-2012

Abstract

Greenland ice sheet mass loss has accelerated in the past decade responding to combined glacier discharge and surface melt water runoff increases. During summer, absorbed solar energy, modulated at the surface primarily by albedo, is the dominant factor governing surface melt variability in the ablation area.

Using satellite observations of albedo and melt extent with calibrated regional climate model output, we determine the spatial dependence and quantitative impact of the ice sheet albedo feedback in twelve summer periods beginning in 2000. We find that while the albedo feedback is negative over 70% of the ice sheet, concentrated in the accumulation area above 1500m, positive feedback prevailing over the ablation area accounts for more than half of the overall increase in melting. Over the ablation area, year 2010 and 2011 absorbed solar energy was more than twice as large as in years 2000–2004.

Anomalous anticyclonic circulation, associated with a persistent summer North Atlantic Oscillation extreme since 2007 enabled three amplifying mechanisms to maximize the albedo feedback: (1) increased warm (south) air advection along the western ice sheet increased surface sensible heating that in turn enhanced snow grain metamorphic rates, further reducing albedo; (2) increased surface downward solar irradiance, leading to more surface heating and further albedo reduction; and (3) reduced snowfall rates sustained low albedo, maximizing surface solar heating, progressively lowering albedo over multiple years. The summer net radiation for the high elevation accumulation area approached positive values during this period.


Money Quotes:
"In the 12 years beginning in 2000, the reduced albedo combined with a significant increase in downward solar irradiance yielded an accumulation area net radiation increase from −0.9 to −0.2Wm−2. Another similar decade may be sufficient to shift the average summer accumulation area radiation budget from negative to positive, resulting in an abrupt ice sheet melt area increase."
And
"Future work should therefore be concerned with understanding potential tipping points in ice sheet melt regime as the average radiation budget shifts from negative (cooling) to positive (heating), as it seems the threshold of this has just been reached."
And
"Further warming would only hasten the amplification of melting that the albedo feedback permits."

Neven

That's a great paper, Mr Yooper. Thanks.

Andrew Xnn

So, this paper is stating that in the past 12 years, despite warming temps and melting ice, there has been a net cooling of the ice sheet from more fresh snow in some areas.

However, with a little bit more continued warming, the albedo will shift so that higher elevation dry areas begin to feed back positively and result in net warming over the entire sheet.

In other words, the sheet has not reached a state of maximum melting, since it's only the lower elevation area that have positive feedbacks.

Daniel Bailey

But wait, it gets worse:

Multistability and critical thresholds of the Greenland ice sheet
A. Robinson, R. Calov & A. Ganopolski

Nature Climate Change (2012)
doi:10.1038/nclimate1449

Abstract

"Recent studies have focused on the short-term contribution of the Greenland ice sheet to sea-level rise, yet little is known about its long-term stability. The present best estimate of the threshold in global temperature rise leading to complete melting of the ice sheet is 3.1 °C (1.9–5.1 °C, 95% confidence interval) above the preindustrial climate1, determined as the temperature for which the modeled surface mass balance of the present-day ice sheet turns negative.

Here, using a fully coupled model, we show that this criterion systematically overestimates the temperature threshold and that the Greenland ice sheet is more sensitive to long-term climate change than previously thought. We estimate that the warming threshold leading to a monostable, essentially ice-free state is in the range of 0.8–3.2 °C, with a best estimate of 1.6 °C.

By testing the ice sheet’s ability to regrow after partial mass loss, we find that at least one intermediate equilibrium state is possible, though for sufficiently high initial temperature anomalies, total loss of the ice sheet becomes irreversible. Crossing the threshold alone does not imply rapid melting (for temperatures near the threshold, complete melting takes tens of millennia). However, the timescale of melt depends strongly on the magnitude and duration of the temperature overshoot above this critical threshold."

Neven

Yeah, I saw this one pop up on news sites.

We estimate that the warming threshold leading to a monostable, essentially ice-free state is in the range of 0.8–3.2 °C, with a best estimate of 1.6 °C

We already have that 0.8°C! Great! ;-)

Thanks, Daniel.

Mike

Neven, The question I asked myself was, where is the warming threshold measured?

Aaron Lewis

In the old days, the air over Greenland was dry, so everyone focused on albedo. That was then, this is now. What eats ice now is water vapor condensing out of air flowing over the ice. This occurs at low altitudes and over a small % of the ice sheet. Nobody seems to care.

In the melt season, there is water vapor in the air beside the Greenland ice sheet, and every gram of water vapor that condenses, melts 7.5 grams of ice, resulting in 8.5 grams of runoff. That runoff then falls through moulins, releasing its potential heat as it falls. Then, the weight of the ice collapses the moulin resulting in internal work being done on the ice around the volume of the moulin. Thus, we end up with a core of very weak ice in what is essentially a "buttress" structure, where extensive butresses provide extensive confinement for the compressed ice at the base of the central structure. When the ice gets too weak (warm) to support its own weight, or there are defects in the support/confinement structure, there is a progressive structural collapse.
In (http://www.boemre.gov/tarprojects/040/040BL.PDF) note the importance of confinement in the compressive strength of relatively warm ice. Moulins disrupt this confinement. Big ice does not melt from the top down, it decays from the sides - like a glacier calving into a fjord. The ice loss becomes more a function of structural collapse at the edges, than of melt across the surface.

It is not that hard. It is undergrad physics. You can find the math in an EIT manual. Nobody published it, because nobody likes the results. It happens much faster than melt from the top down.

Global temperatures do not matter. Big ice makes its own weather. When the Arctic sea ice was in place, the weather effects of Greenland were less apparent. Without the sea ice to diffuse the effects, the weather making of Greenland will be easier to see.

The calculation grid size of GCM means that they miss significant heat transport by air currents and ocean currents. The grid size means that the GCM do not see Greenland making its own weather.

I was labeled an "Alarmist" by many, including some of the authors of the papers above, because in 2002, I applied industrial process control statistics to estimate that the Arctic Sea Ice system would go "out of control" and we would see a dramatic decline of the Arctic Sea Ice within a decade. However, it was not an accepted methodology in climate science. I am still using tools that are outside of the accepted methodology of climate science.

Artful Dodger

Sharp et.al (2011) "Extreme melt on Canada’s Arctic ice caps in the 21st century"

Canada’s Queen Elizabeth Islands contain ∼14% of Earth’s glacier and ice cap area. Snow accumulation on these glaciers is low and varies little from year to year. Changes in their surface mass balance are driven largely by changes in summer air temperatures, surface melting and runoff. Relative to 2000–2004, strong summer warming since 2005 (1.1 to 1.6°C at 700 hPa) has increased summer mean ice surface temperatures and melt season length on the major ice caps in this region by 0.8 to 2.2°C and 4.7 to 11.9 d respectively. 30–48% of the total mass lost from 4 monitored glaciers since 1963 has occurred since 2005. The mean rate of mass loss from these 4 glaciers between 2005 and 2009 (−493 kg m−2 a−1) was nearly 5 times greater than the 1963– 2004 average. In 2007 and 2008, it was 7 times greater (−698 kg m−2 a−1). These changes are associated with a summer atmospheric circulation configuration that favors strong heat advection into the Queen Elizabeth Islands from the northwest Atlantic, where sea surface temperatures have been anomalously high

Citation: Sharp, M., D. O. Burgess, J. G. Cogley, M. Ecclestone, C. Labine, and G. J. Wolken (2011), Extreme melt on Canada’s Arctic ice caps in the 21st century, Geophys. Res. Lett., 38, L11501,
doi:10.1029/2011GL047381.

Notice that the near total collapse of the QEI icesheets occurred with a mean warming of only 0.8 C, and ~40% of the melt occurred in just 6 years. Clearly, the analysis show by Aaron Lewis above applies to Greenland, as supported by these results. Climate sensitivity is high, and abrupt changes are possible.

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