My first job as an archaeologist in Boston (having moved there from New York) had to do with Deer Island, the northern of two islands that separate Boston’s Inner and Outer Harbors. The actual archaeology was uninteresting but the historical research was fascinating. Among the things I learned is that Boston’s Inner Harbor regularly froze over. It no longer does. When the Continental Army placed the British in Boston under Siege at the beginning of the Revolution, the idea of holding off an assault until the harbor froze over was routine. No one expected the harbor to not freeze over. More recently, the harbor is generally open all winter, and when it does freeze over, it is a significant bother.
As I continued to carry out prehistoric and historic research up and down the New England coast, it became apparent to me that it was normal for bays and harbors now open all year round to ice up most winters, in the 18th century through the 19th century and into the 20th century. In the old manuscripts, no one wondered how wolves and deer got out onto islands to establish colonies. Everyone knew they just walked over the ice. There are lighthouses that were once staffed by keepers (now they are automated) and it is said in many instances that in the old days the keepers could “walk to work” during the winter, across the ice, but in more recent times they needed to row out to the island on which the lighthouse was situated in a dinghy. I’m pretty sure the discomforts and dangers of rowing about in a little wooden craft in the North Atlantic winter contributed to the impetus to mechanize these quaint but vital facilities.
This has never been systematically studied, but it appears that prehistoric Walrus remains along the Gulf of Maine suggest sea ice existed off New Hampshire and farther north (as well as east of there, as the sea level was still rising). All this indicates, indirectly and inconclusively but quite suggestively, that the best estimate of of “Arctic Sea Ice” … an average of several years in the 20th century (typically 1978-2000) as seen in current reconstructions … is an underestimate. If conditions in the northern hemisphere were cold enough to cause islands to link to land via ice on a regular basis as far south as Boston and Nantucket, then unless there were major changes in currents, most likely the arctic sea ice was more extensive earlier in the present millennium than today.
Farther north, we know that Ellesmere Island’s northern bays were encased in year round sea ice which has been breaking up at least since the 1950s, and today is virtually gone. Data from the late 19th century and the 1950s provide further evidence; The extent of Arctic ice shrank slowly but steadily as fossil Carbon was being released, slowly but steadily, into the atmosphere as CO2.
Climate change deniers will point out that this is expected, because of the Little Ice Age. However, that is not correct for two reasons. First, although climate change deniers like to refer to the Little Ice Age whenever they can, they move around when in time it happened as needed to support their arguments. I’m talking here about the mid 18th century through the start of the 20th century. That was not the little ice age.
The second reason this is incorrect is that the Little Ice Age is a fluctuation in climate that happened during the Holocene, as the Medieval Warm period was as well. What I’m saying here is this: We are using a relatively warm baseline, the 1970s through 2000, to estimate Arctic ice and that we should be using a longer time period to include a longer period of natural variation, if possible. What climate change denialists are saying is that any time in the past that it was cold or warm, depending on the argument one is making, disproves global warming. I’m saying we should use all the data, they are saying we should use the data that suits them when it suits them.
But enough of that. Let’s get back to the ice cap and reality. After a century of steady decline in the size of the ice cap, corresponding to an increasing greenhouse effect, both the amount of CO2 in the atmosphere and the decline of the ice has accelerated dramatically.
The people who track the Arctic sea ice have recorded the smallest extent of ice observed since good records have been kept in the last few years. This graphic from the National Snow and Ice Data Center shows the average sea ice extent from 1979 through 2000 (the red line) compared the extent of ice yesterday (October 7th, 2011, the white blob).
This graph shows the current year’s track (the ice is now reforming) and the smallest year ever 2007) in relation to the 1979-2000 average +/- two standard deviations. Most of the last several years have been well below average.
At the larger time scale, the following comparison shows the estimated extent of sea ice thickness (different from extent) in the 1950s compared to the projected thickness 100 years later, based on reasonable assumptions of climate change. The most recent analyses actually suggest that this is optimistic.
And finally, here’s a simulation designed by UCAR showing actual and projected sea ice minimum extent through from the recent past through the year 2049:
Now that the Arctic summer is passing quickly into snowy oblivion, we will keep an eye on what happens in the Southern Hemisphere with the coming warm season. Chunks of the Antarctic Ice Sheets, where the big glaciers intersect the Southern Ocean and large embayments are “permanently” frozen over, have been falling off now and then, but overall Antarctica has not experienced the dramatic reduction in ice-extent as the Arctic for two reasons: 1) They are not the same thing; The Arctic circle is mostly ocean, so the northern “ice cap” is mostly sea ice (though of course there is Greenland) while Antarctica is mostly covered with continental glaciers. 2) The Southern Continent is surrounded by ocean, but the Arctic Circle is a complex interconnected mess of seas and land masses. The seas circulating around the Southern Continent make for a more self contained system. This means that whatever climate prevails there will be harder to change as global condition change. But it will change.
Sources and other information:
- National Snow and Ice Data Center
- Class M Planet: Ice recap Summer 2011
- Real Climate: Speculative polar cartography
- IARC-JAXA Data of sea Ice Extent
- Climate Progress: German Physicists: Historic Low Arctic Ice is a “Consequence of Man-Made Global Warming with Global Consequences”
- Climate Progress: The New Arctic Abnormal: Record Low Sea Ice Volume, Area and Extent
- MinPost (Don Shelby): What Tea Party supporters think about global warming