History of Arctic Sea Ice, part 2

Since satellite observations began, the extent of sea ice in the arctic has declined dramatically; this year the summer minimum extent was the 3rd-lowest on record. There is also considerable observational evidence that arctic sea ice extent over the last few decades is much lower than it has been for at least a century prior to modern times.

Yet to put such dramatic change in context, one hopes to stretch even further back in time. A team of researchers led by Leonid Polyak of Ohio State University’s Byrd Polar Research Center has culled the available evidence from hundreds of studies of proxy data for sea ice extent. As reported at Science Daily, in a recent paper in Quaternary Science reviews they report their findings: that the present extent of arctic sea ice is at its lowest for at least several thousand years.

The array of proxy evidence is impressive, including plant and pollen remains (both marine and land), marine sediment cores, ice cores, ice-rafted debris, organic compounds (including specific biomarkers) in sediment, coastal conditions, driftwood remains, bones of marine mammals, and historical records, which while highly uneven, in some locations go back much farther than one might expect.

What story does all this evidence tell?

The period considered by Polyak et al. begins about 50 million years ago, just after the Paleocene-Eocene Thermal Maximum (PETM). At that time the arctic was much warmer than today, with summer temperatures estimated as high as 24 deg.C (75 deg.F!). Of course the Arctic was dramatically different, “This warm environment is consistent with forests of enormous Metasequoia that stood at the same time on shores of the Arctic Ocean – such as on Ellesmere Island across low-lying delta floodplains riddled with lakes and swamps.”

But such warm conditions didn’t persist, in fact the consistent trend over the last 50 million years has been cooling. This is testified by the steady progression of changes in land plants, especially in forests, from warmer to cooler to colder-tolerant forest species, and in northernmost areas eventually to none. As early as 47 million years ago there’s evidence of the onset of drifting sea ice and possibly even Arctic glaciers. Overall cooling continued throughout the remainder of the Eocene (up to 34 million years ago) as atmospheric CO2 concentration declined, culminating in an abrupt drop in CO2 and temperature at the Eocene-Oligocene boundary. This triggered the massive glaciation of Antarctica and possibly the first Greenland glaciers. By the early Miocene (about 23 to 16 million years ago), cool-temperature trees populated the high arctic, with much of the Canadian Arctic Archipelago hosting forests not unlike modern-day southern Maritime Canada and New England.

The cooling pattern continued as atmospheric CO2 declined, until by 13 or 14 million years ago sea ice may have persisted year-round, with debris rafted by sea ice over distances which would have taken at least a year to traverse. Shortly afterward pine forests begin to dominate in northern Alaska, and the Antarctic glaciation expands. Yet even in the early-to-middle Pliocene (about 5 to 3 million years ago) conditions were still much warmer than today, with persistent pine, birch, spruce, and larch along the arctic coast. Such flora are inconsistent with extensive perennial sea ice, as are foraminifers and ostracodes in sediment cores from that time.

In the late Pliocene (3 to 2.6 million years ago) arctic cooling wrought profound changes. The tree line retreated from the coast, and ice sheets of continental scale began to grow around the arctic boundary. Yet there are warm intervals throught the Pliocene and even past the beginning of the Pleistocene epoch (2.6 million years ago).

The Pleistocene epoch ushers in an era of cooler temperatures, and of the extreme climate swings commonly (but mistakenly) referred to as “ice ages,” driven by variations of earth’s orbit and axial tilt. The colder episodes are the glacial periods, with much colder temperature and massive ice formations, while the warmer periods between them are interglacials. During glacial periods ice dominates the arctic, in fact there’s evidence (but by no means certainty) that ice as thick as several hundred meters could have covered all or part of the Arctic Ocean. But during warm interglacials sea ice is much reduced; as recently as the last interglacial (about 100 thousand years ago) microfossils indicating seasonally open water are found in the central arctic, and most of the Arctic Ocean may have been ice-free in summer.

The present interglacial has been with us for about eleven and a half thousand years, and marks the commencement of the Holocene epoch. Its warmest period seems to have been the early holocene, around 8.5 to 6 thousand years ago. Orbital factors which warm the arctic peaked about 11 thousand years ago and have steadily declined since then, but warming in many areas was partly suppressed by lingering ice sheets. Yet the story is more complex than just warming and cooling, more or less ice, as many of the changes and their timing are not pan-Arctic but regional.

Along the northeast coast of Greenland, remnants of wave-generated beach ridges show that at summer minimum the sea ice could retreat up the northeast coast as far as 83 deg. N latitude, although ice persisted year-round along the northern stretch of coast. Still, that’s considerably less ice than at present; these days it remains icebound above 74 deg. N. After about 6 thousand years ago, the region of permanent sea ice creeps down the Greenland coast. About 5.5 thousand years ago ice shelves begin to form along Ellesmere Island, indicating that the ocean to the north had very high ice concentration, a condition which spread to the northeast Greenland coast by 3 thousand years ago.

But in other regions, the pattern was different. In the Canadian Arctic Archipelago warming peaked earlier, around 10 to 9 thousand years ago, opening up the northwest passage sufficiently for Bowhead whales to move freely between Atlantic and Pacific oceans. After about 8 thousand years ago the passageway appears to be blocked (or at least difficult), although during regional warmth around 5 thousand years ago it seems to have opened for Atlantic but not Pacific populations to reach the central Canadian Arctic Archipelago. Meanwhile, the North Atlantic and Nordic Seas show a very early Holocene minimum in ice cover, from about 11 to 10 thousand years ago, followed by increased ice, then another minimum around 6 thousand years ago. In one interesting case Polyak et al. note that close to the Alaska Chukchi coast more open-water conditions occurred at the same time as higher-ice conditions further north, illustrating the complexity and inhomogeneity of sea ice changes in the early Holocene.

Over the last few thousand years we can detect centennial variability in arctic climate. It seems to have been more subdued than climate variability in the northern hemisphere as a whole, although events such as the medieval warm period have left signs; a final peak in bowhead whale bones happened just before the year 1000 A.D., first in the Beaufort Sea and later in the eastern Canadian Arctic Archipelago. Biomarkers suggest reduced spring ice from about 800 to 1200 A.D., and it was at the end of this period that the Thule Inuit (hunters of bowhead among other prey) expanded their territory eastward, eventually reaching Greenland and Labrador. The subsequent decline in Bowhead abundance has since caused some hunting territories to be abandoned, and many Inuit to focus on other food sources.

Since the mid-19th century, not only has the arctic warmed more than 1 deg.C compared to the “little ice age,” it has reached its highest temperatures in at least the last two thousand years. Retreating glaciers have exposed material which hasn’t seen the light of day for millenia, while the ice shelves around Ellsemere Island declined by more than 90%, and continue to break up, after having been a stable feature of the arctic for over five thousand years. It is especially after the middle of the 20th century that the decline in sea ice becomes surprisingly rapid, and unlike so many other variations observed in the Holocene, the sea ice decline of the last several decades is not regional, but unquestionably arctic-wide. As Polyak et al. say, “On suborbital time scales, ice distributions varied in the Holocene, but no evidence exists for large, pan-Arctic fluctuations.” Not, at least, until the last several decades:

The current reduction in Arctic ice cover started in the late 19th century, consistent with the rapidly warming climate, and became very pronounced over the last three decades. This ice loss appears to be unmatched over at least the last few thousand years and unexplainable by any of the known natural variabilities.