Saturday, 24 September 2016

Decline in Arctic sea ice extent looks set to result in dramatic fall in albedo

The Threat Of Arctic Albedo Change

23 September, 2016

Arctic sea ice extent in 2016 was the lowest since satellite measurements started, when extent is averaged over the period from March 20 to September 22, as illustrated by the image below.

As the added trend also illustrates, this decline in Arctic sea ice extent looks set to further accelerate and result in a dramatic fall in albedo. The trend points at zero sea ice over this entire period in less than two decades from now.

Zero sea ice on a single day looks set to occur much earlier; a similar trend points at minimum sea ice extent reaching zero in about a decade from now, as illustrated by the image below.

Above image also shows average sea ice extent data for the period January 1 to September 22, i.e. the year to date (blue line). The added trend points at zero being reached in 2037. The data show that Arctic sea ice extent also was the lowest since satellite measurements started, when extent is averaged over the period from January 1 to September 22.

Finally, the image also shows data for the average sea ice extent over the entire year. Data for 2016 are not available yet, but it does look like 2016 will also be have the lowest sea ice extent when averaged over the entire year.

Anyway, the period between the equinoxes of March 20 and September 22/23 is most important, as the Arctic receives most sunlight during this period. This is illustrated by the image on the right and by he image below, from an earlier post, which further shows that the amount of solar radiation received by the Arctic at the time of the June Solstice is higher than anywhere else on Earth.

Thick sea ice covered with snow can reflect as much as 90% of the incoming solar radiation. After the snow begins to melt, and because shallow melt ponds have an albedo (or reflectivity) 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, while the ocean reflects only 6% of the incoming solar radiation and absorbs the rest.

Professor Peter Wadhams, University of Cambridge, once calculated, a collapse of the sea ice would go hand in 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.

Professor Peter Wadhams on albedo changes in the Arctic

Collapse of the sea ice could occur even faster than this. Rapid loss of sea ice thickness has taken place over the years, as discussed in a recent post. Thin sea ice is more vulnerable to the stronger storms that can be expected to hit the Arctic Ocean during the northern summer more frequently, and they could push huge amounts of ice out of the Arctic Ocean.

The sea ice acts as a heat buffer by absorbing energy in the process of melting. In other words, as long as there is sea ice, it will absorb heat and this will prevent this heat from raising the temperature of the water 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 150º C.
This buffer is now largely gone and further decline of Arctic sea ice means that a lot more heat will be absorbed by the Arctic.

As the water of the Arctic Ocean keeps warming, the risk increases that methane hydrates at the bottom of the Arctic Ocean will destabilize. Increases in temperature due to albedo changes and methane releases in the Arctic will go hand in hand with further feedbacks, in particular increased levels of water vapor in the atmosphere.

Taken together, we are facing the possibility of a 10 degrees Celsius rise in temperature within one decade. The image below, from the 
extinction page, shows that we may be on a trend that is rising even faster than the rapid temperature increases in 2016 may indicate. Indeed, a large part of global warming is currently masked by aerosols and, as we progress with the necessary shift to clean energy, the full wrath of global warming looks set to become manifest.

The situation is dire and calls for comprehensive and effective action, as described in the Climate Plan

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