(1) research reports in the peer-reviewed scientific literature
(2) popular articles for the lay public. Publications in the first category are typically too specialized for the lay reader. Publications in the second category often simplify because they have to in order to reach the audience.
This makes "in-between" publications rare and valuable. This is the case for an article in the latest (October 2013) issue of Physics Today, entitled the Arctic shifts to a new normal. The authors, Martin Jeffries, James Overland and Don Perovich, are research scientists with academic appointments. For brevity, the authors are referred to collectively as JOP.
JOP document the decline of sea ice. From September 1980 to September 2012, the minimum extent of sea ice has declined by 55%. The six most recent years (2007-2012) have seen the lowest ice extents (i.e., area) since the satellite record began in 1979. This dramatic decline has been driven by an increase of CO2 at Barrow, Alaska, of only 15% from 1980 to 2012.
The sea ice is also thinner and newer: In September 1980, 62% was multi-year ice, which survives one or more more summer melt seasons, and only 38% was first-year sea ice. By September 2012, 58% of the sea ice was less than a year old.
The minimum extent of sea ice (in September) is declining at -13% per decade, relative to the 1979-2000 average. The maximum extent of sea ice (typically in March) is declining more slowly, at -2.6% per decade. In March 2013, only 30% of the ice cover consists of ice more than a year old.
The retreat of sea ice leads to a decrease in the albedo, because ocean water absorbs more sunlight and reflects less, compared with white ice. This causes "polar amplification", meaning that the increase in the Arctic temperature is higher than the increase in the global average temperature.
JOR state that "polar amplification" is driven by modest external forcing from mid-latitudes, combined with multiple positive feedbacks within the Arctic system itself. For details, JOR refer to a 2011 article by Mark Serreze and Roger Barry and a 2012 article by Julienne Stroeve et al.
Mean annual temperature in the Arctic is now 1.5 C higher than the 1971-2000 average. The sea-surface temperature in August is now as much as 3 C higher than the 1982-2006 average. Upper ocean heat content has increased by as much as 25% in the Canada Basin's Beaufort Gyre, compared to the heat content in the 1970's.
There is much more in the JOR article, including changes in ocean currents, plankton growth, and changes in the migration of Arctic mammals (e.g., walruses) and other sea life.
The changes in the Arctic are happening much more rapidly than expected several decades ago. In 1979, President Jimmy Carter appointed a high-level study group to investigate the possibility of global warming. The study group was headed by Jules Charney, physicist at MIT, who framed the problem thus: suppose that the carbon dioxide in the atmosphere were instantly doubled, but nothing else changed. What would be the resulting temperature rise? (This way of posing the question is still used by the IPCC today).
At the time, Charney (and everybody else) assumed that polar ice sheets would respond so slowly that their change could be neglected on a time scale of decades. It was thought that ice sheets would change mainly on millennial (thousand year) time scales. (Hansen, p. 41)
Even today's computer models do not predict the rapid and profound changes that are observed. Computer models (used for the AR5 report) predict that ice-free summers will not happen until about 2060, i.e., 47 years from now. The extrapolation of today's trends predicts that this will happen in about another decade.
Among the arguments advanced by the deniers is that computer models are not perfect. The deniers then go on to claim that the threat posed by global warming has been overestimated by imperfect computer models. In fact, the threat has been underestimated because computer models underestimate the rate of change in the Arctic, the region of the earth where the rate of change is the most pronounced.
Hat's off to Physics Today for publishing the excellent JOR article!
M. C. Serreze, R. G. Barry, Global Planet. Change 77, 85 (2011)
J. C. Stroeve et al., Climatic Change 110, 1005 (2012)
James Hansen, Storms of My Grandchildren (Bloomsbury, 2009).