On thin ice
The extent of sea ice in the Arctic has never been so low
On September 16, the sea ice covering the Arctic Ocean reached its minimum extent for the year, setting a record low of 3.41 million square kilometres. According to the United States’ National Snow and Ice Data Centre, this was 18 per cent less ice than the previous record, set in 2007, and around half the average minimum measured by satellite-borne instruments during the years 1979–2000.
Abundant ship reports filed in the years before satellite imagery enable a good record of the sea ice to be compiled back to 1900. These show that minimum ice areas at the end of each melt season were much greater than now—typically three times greater in the first half of the 20th century.
Research published in Nature last November pushed the record of arctic sea ice back further. Using data from ice cores, tree rings, lake sediments, ocean cores and documentary sources from around the Arctic, researchers calculated that the current low levels of ice are without precedent in the past 1450 years.
Going back much further, evidence for low levels of summer ice in the Arctic has been found in the north of Greenland, where a series of wave-generated beaches have been dated to between 6000 and 8500 years ago. Known as the Holocene maximum, this was a time when summer sunlight was more intense in the Arctic than it is now. Back then, the sun was closest to the Earth during the northern hemisphere
summer, whereas now it is closest in January, during the southern hemisphere summer.
The ice is also getting thinner. US submarines sailing under the ice back in the 1950s found large areas more than five metres thick.
Russian scientists have taken observations on the ice since the 1930s. Among other things, they have measured the slow undulations that ripple through the ice caused by ocean swells impacting the ice-edge. The speed that these undulations propagate across the ice has been getting faster—a sign that it has been getting thinner.
Satellite observations show that most of the multi-year ice has now gone, and the vast majority
of the remaining ice is one year old and only a metre or so thick. Estimates of ice volume at the end of the melt
season show a steady decline over the past decade. This year had only about one-fifth the ice volume of the average minimum between 1979 and 2000.
Thin ice takes less heat to melt, which helps to explain why the record was shattered this year, even though the weather was cooler and cloudier than during the previous record melt of 2007.
Sea ice reflects most of the incoming sunlight back to space, but open water absorbs most of the sunlight and heats up. Surface temperatures of large parts of the Arctic Ocean have increased by 4ºC or more. Warm water melts the remaining ice faster, creating a feedback cycle that increases the rate of melt over subsequent years.
The current decline is much faster than computer models had indicated, leading Peter Wadhams, professor of ocean physics at Cambridge University in England, to predict that the Arctic Ocean could be largely ice-free in summer within just three years.
Replacing sea ice with open water has a major impact on weather systems. The warmer water transfers more heat to the air than the cold ice does, both by conduction and also by increased evaporation, which happens more readily from water than solid ice. Once in the air, the extra water gas gets caught
up in storms, where it condenses, releasing the heat used to evaporate it. This heat then intintensifiese storm.
An example of this occurred in early August when an intense low developed over the Arctic. The central pressure dropped to 963 hPa, making it one of the deepest lows to form in the region. The gale-force winds around the low broke up more of the weakened sea ice and hastened its melting by mixing warmer sub-surface water with the
cooler surface water. A study by scientists at the University of Alaska found that both the number and intensity of Arctic storms increased over the second half of the 20th century, particularly in summer.
Increasing evaporation also increases rainfall. The Alaskan village of Kivalina had 210mm of rain in the month of August, half its annual average, causing the nearby Wulik River to reach its highest flood crest since records began in 1985.
The retreat of the sea ice has also led to increased coastal erosion as large waves generated over ice-free ocean now pound shingle banks that have thawed.
The consequences of the vanishing ice extend far beyond the Arctic. Increasing the air temperature over the Arctic
reduces the North Poleto-equator temperature difference, and this weakens the strength of the circumpolar westerly
wind flow. Also, the warmer air causes ridges of high pressure to extend further into the Arctic from mid-latitudes.
Both of these effects combine to slow the progression of the large-scale ‘Rossby Waves’ that control individual weather systems. This makes it more likely that one country will get stuck with a long period of rain—as England
was this summer, with one of the wettest Junes on record—while other countries, even on the same continent, endure drought.
Paradoxically, the warming Arctic has also been linked to the outbreaks of heavy snow occurring in early winter in the northern hemisphere during the past few years. (The increased energy intensifies the storms,
while increased water gas in the atmosphere increases snowfall. The enhanced ridges enable the air flowing out of the Arctic to penetrate further into mid-latitudes, while the slower movement of the ridges allows the heavy snowfall to last longer.)
The warming Arctic is also releasing methane into the atmosphere from thawing permafrost, further enhancing greenhouse warming of the Earth.
This year, for the fifth year in a row and the fifth year in recorded history, navigation was possible through the Northwest Passage along the coast of Canada and also the Northeast Passage along the coast of Russia. It’s a boon for shipping companies and prospectors, who are able to sail into seas previously blocked by ice. However, Earth’s climate is now in unknown territory as well, and that may not be such good news for the rest of us.