Sea ice covers the surface of the Arctic Ocean. It forms by the seasonal freezing of sea water at the surface. In the Arctic, sea ice reaches its minimum extent in late summer. In the next few weeks, we’ll hear reports of this year’s expected seasonal decline in sea ice to the minimum. I also expect to hear that the extent of sea ice is smaller than in prior years.
While there is year-to-year variation, the extent of sea ice has been declining in a fairly linear trend since global monitoring began in the late 1970s. It has also gotten thinner, as measured by submarines of the U.S. Navy. Changes in the Earth’s climate—global warming—are widely thought to be responsible for this decline, which may lead to a complete loss of ice during the summer within the next dozen years. It is likely that the Arctic has not been completely ice-free in summer during the past 2 million years. See http://blogs.nicholas.duke.edu/citizenscientist/the-ice-that-floats-in-our-global-cocktail/
Merging the records of sea ice decline and carbon dioxide emissions, Dirk Notz and Julienne Stroeve report that each ton of CO2 added to the atmosphere from fossil fuel combustion leads to the loss of about 3 square meters (27 square feet) of sea ice. At current rates of emission, the average U.S. citizen is responsible for 16 tons of CO2 added to the atmosphere each year and nearly 50 square meters of sea-ice loss. Each of us has an indirect, but enormous, impact on the survival of polar bears, which need sea ice for their hunting grounds.
Loss of sea-ice exacerbates the rate of global warming. Normally ice is highly reflective of incoming sunlight. Ocean water reflects less sunlight, and thus absorbs more energy, which heats it. Warmer waters in the Arctic lead to the loss of more ice, in what environmental scientists call a “positive feedback loop.” And, meteorologists believe that an ice-free Arctic, with warmer ocean waters, will have dramatic impacts on the position of the Jet Stream, other wind patterns, and the climate we experience in the temperate zone.
The Arctic and Antarctica differ in fundamental ways—there is no land under the North Pole, whereas Antarctica is a continent which is surrounded by the Circumpolar Current in the Southern Ocean. When we hear of large losses of ice from Antarctica, we must remember that this ice is being lost from the continental ice packs—not sea ice—in this region. See http://blogs.nicholas.duke.edu/citizenscientist/melting-at-the-bottom-of-the-earth/
A long record of the extent of Antarctic sea ice, derived from whaling ships, suggests that it has been declining for decades. But, surprisingly, the recent extent of sea ice around Antarctica has not shown the dramatic losses that we see in the Arctic. Rather the sea ice around Antarctica has shown fluctuations around a fairly constant mean value. This year, however, a big decline in sea ice around Antarctica during its summer (early 2017), is worthy of careful monitoring. We certainly don’t need to begin a positive feedback loop exacerbating global warming at both ends of the Earth.
Cotte, C. and C. Guinet. 2007. Historial whaling records reveal major regional retreat of Antarctic sea ice. Deep Sea Research 54: 243-252.
Kurtz, N.t. and T. Markus. 2012. Satellite observations of Antarctic sea ice thickness and volume. Journal of Geophysical Research 117: doi: 10.1029/2012JC008141
Kwok, R. and D.A. Rothrock. 2009. Decline in Arctic sea ice thickness from submarine and ICESat records: 1958-2008. Geophysical Research Letters 36: doi: 10.1029/2009GL039035
McLaren, A.S., J.E. Walsh, R.H. Bourke, R.L. Weaver, and W. Wittmann. 1992. Variability in sea-ice thickness over the North Pole from 1977 to 1990. Nature 358: 224-226.
Notz, D. and J. Stroeve. 2016. Observed Arctic sea-ice loss directly follows anthropogenic CO2 emission. Science 354: 747-750.
Stuecker, M.F., C.M. Bitz, and K.C. Armour. 2017. Conditions leading to the unprecedented low Antarctic sea ice extent during the 2016 austral spring season. Geophysical Research Letters doi: 10.1002/2017GL07469
Turner, J. and J. Comiso. 2017. Solve Antarctica’s sea-ice puzzle. Nature 547: 275-277.
Vancoppenolle, M., T. Fichefet, and C.M. Bitz. 2006. Modeling the salinity profile of undeformed Arctic sea ice. Geophysical Research Letters 33: doi: 10.1029/2006GL0238342