Wednesday, July 9, 2014

Arctic Warming due to Snow and Ice Demise


Sea Ice

Loss of Arctic sea ice means that a lot more heat will be absorbed by the Arctic Ocean.



Sea ice reflects 50% to 70% of the incoming energy, while thick sea ice covered with snow reflects as much as 90% of the incoming solar radiation.

After the snow begins to melt, and because shallow melt ponds have an albedo of approximately 0.2 to 0.4, the surface albedo drops to about 0.75. As melt ponds grow and deepen, the surface albedo can drop to 0.15. The ocean reflects only 6% of the incoming solar radiation and absorbs the rest.
Sea ice typically reaches its lowest volume halfway in September. Given the shape the ice is in, 2014 will likely be one of the lowest minima on record. In fact, there is a chance that sea ice will disappear altogether in September 2014. As illustrated by above image, by Wipneus, an exponential curve based on annual minima from 1979 points at zero ice volume end 2016, with the lower limit of the 95% confidence interval pointing at zero ice end of 2014.


Latent Heat

Sea ice melting occurs due to heat from above, i.e. absorbed sunlight. Once the sea ice is gone, this heat will no longer go into melting and transforming ice into water. Instead, all absorbed sunlight will go into warming up the Arctic Ocean and the sediments under the seafloor.

In addition, sea ice is also melting due to heat from below, absorbed ocean heat. Much of this heat is carried by the Gulf Stream into the Arctic Ocean. Once the sea ice is gone, all this heat will go into warming up the Arctic Ocean and the sediments under the seafloor.

Loss of sea ice will allow more ocean heat to radiate out into the atmosphere, but at the same time more clouds will also deflect more heat back to the Arctic Ocean. In addition, more clouds will also trap more of the heat that was previously radiated out into space, i.e. while the sea ice was still there. More clouds will also form over land, with similar effects there.

In short, the sea ice acts as a buffer that absorbs heat. As long as there is sea ice, it will absorb heat and make sure there is no temperature rise in the Arctic. Once the sea ice is gone, this latent heat must go elsewhere.

As the sea ice heats up, 2.06 J/g of heat goes into every degree Celsius that the temperature of the ice rises. While the ice is melting, all energy (at 334J/g) goes into changing ice into water and the temperature remains at 0°C (273.15K, 32°F).

Once all ice has turned into water, all subsequent heat goes into heating up the water, at 4.18 J/g for every degree Celsius that the temperature of water rises.

The amount of energy absorbed by melting ice is as much as it takes to heat an equivalent mass of water from zero to 80°C. The energy required to melt a volume of ice can raise the temperature of the same volume of rock by 150ยบ C.


Snow and Ice Cover on Land

Snow and ice cover on land takes up an even larger area than sea ice. The chart below with data up to 2012 shows how dramatic the decline of snow cover on land in the Northern Hemisphere was (until 2012 and without Greenland) for the month June.


The size of the June snow and ice cover is vitally important, as insolation in the Arctic is at its highest at the June Solstice. During the months June and July, insolation in the Arctic is higher than anywhere else on Earth, as shown on the image below, by Pidwirny (2006).


While Greenland remains extensively covered with snow and ice, the reflectivity of its cover can decline rapidly, as illustrated by an earlier post from the meltfactor blog. This rapid decline occurs not only due to exposure of darker soil, but also due to formation of melt ponds and because melting snow reflects less light. Furthermore, huge amounts of dust, soot and organic compounds originating from human activities get deposited on Greenland, reducing its reflectivity. Organic compounds in meltwater pools can furthermore lead to rapid growth of algae at times of high insolation.


Total Warming

 The Arctic Ocean covers 2.8% of Earth's surface. Earth has a total surface area of 510,072,000 square km (196,888,000 square miles), as the table below shows, by Michael Pidwirny, or about 510 million square km.

Surface
Percent of Earth’s Total Surface Area
Area Square Kilometers
Area Square Miles
Earth’s Surface Area Covered by Land
29.2%
148,940,000
57,491,000
Earth’s Surface Area Covered by Water
70.8%
361,132,000
139,397,000
Pacific Ocean
30.5%
155,557,000
60,045,000
Atlantic Ocean
20.8%
76,762,000
29,630,000
Indian Ocean
14.4%
68,556,000
26,463,000
Southern Ocean
4.0%
20,327,000
7,846,000
Arctic Ocean
2.8%
14,056,000
5,426,000

Sea ice extent was below 4 million square km throughout September 2012, as the image below shows. This year, sea ice may well decrease to as little as 4 million square km, or collapse altogether, as discussed above. 


As said, sea ice extent was well under 4 million square km throughout September 2012, and this compares with an extent of under 8 million square km in 1980. In other words, the difference in sea ice extent between those two years is some 4 million square km. The albedo change associated with this difference will be even more dramatic, given the (slushy and this lower albedo) state of the ice in 2012.

How much radiative forcing would this represent, i.e. a retreat from an extent of 8 million square km to 4 million square km, and than another such change, i.e. a collapse from 4 million square km to zero?

Professor Peter Wadhams, University of Cambridge, once calculated that if a sea ice area of 4 million square km, with a summer albedo of about 0.60 (surface covered with melt pools) collapses and disappears altogether, the entire area is replaced by open water which has an albedo of about 0.10. This will thus reduce the albedo of a fraction 4/510 of the earth's surface by an amount 0.50. The average albedo of Earth at present is about 0.29. So, the disappearance of summer ice will reduce the global average albedo by 0.0039, which is about 1.35% relative to its present value.

A drop of as little as 1% in Earth’s albedo corresponds with a warming roughly equal to the effect of doubling the amount of carbon dioxide in the atmosphere, which would cause Earth to retain an additional 3.4 watts of energy for every square meter of surface area (NASA, 2005; Flanner et al., 2011). Based on those figures, a global drop in albedo of 0.0039 is equivalent to a 1.3 W/sq m increase in radiative forcing globally. 

A collapse of the sea ice would go hand on hand with dramatic loss of snow and ice cover on land in the Arctic. The albedo change resulting from the snowline retreat on land is similarly large as the retreat of sea ice, so the combined impact could be well over 2 W/sq m. To put this in context, albedo changes in the Arctic alone could more than double the net radiative forcing resulting from the emissions caused by all people of the world, estimated by the IPCC to be 1.6 W/sq m in 2007 and 2.29 W/sq m in 2013. 


References: 

- March 2014 Arctic Sea Ice Volume 2nd lowest on Record


Post by Sam Carana.

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