Damocles delivers its secrets

© iceberg 853

In 2008 Jean-Claude Gascard, a mainstay of the Damocles programme, returned to Lorient with the tara after drifting 507 days across the arctic ocean. At the time he was adamant that the schooner, ‘a kind of SpaceShip placed in polar orbit’, was bringing home to the 48 participating Laboratories ‘a war chest’.

The Tara Arctic programme alone was an immense success. Since then, the assessment has gained in precision and certitude: over the time that scientists have been measuring the arctic ice-cap, it has shrunk by as much as the surface area of the countries of France, Germany and Spain put together. How? Why? Jean-CLaude Gascard gets to the heart of the matter. Explanation.

The fourth International Polar Year (2007–2008) was a unique opportunity to rediscover the Arctic Ocean thanks to the support of modern technology, remarkable infrastructures and powerful logistical solutions. It was a year of extreme and exceptional events such as the unprecedented and unexpected shrinking of the ice-cap in September 2007, and the profound changes to the opposing pressure patterns in the Arctic atmosphere known as Arctic oscillation (AO).

After a positive phase in the period 2007–2008, Arctic oscillation became very negative in 2010. If we are to understand the probable causes of the changes that affect the atmosphere, the ice floes and the Arctic Ocean, we must first identify the chain of events that connects these three key components of the Arctic climate and the positive or negative interactions and counter-reactions that led to the 2010 situation. Arctic oscillation alternates between positive and negative phases. Positive phases tend to be cyclonic and result in lower atmospheric pressure at sea level, higher surface air temperatures and weaker ice formation. On the other hand, a negative phase tends to be anticyclonic and results in high atmospheric pressure at sea level, low surface temperatures and greater ice formation (greater surface area and thickness of ice). The severe negative phase of AO in 2010 resulted in excessively low tem- peratures, more than 10°C below normal, in Europe, Russia, and North America.

Changes in the Arctic were initially noticed not in the surface area of the sea-ice but its thickness. These observations date from the early 1990s. From an average thickness of more than 3 metres in the 1970s, the Arctic ice-floe measured less than 2 metres in the middle of the 1990s. American nuclear submarines cruising under the Arctic ice-cap were the first to raise the alarm. This shrinkage has continued in recent years and it is estimated that the current average thickness of the Arctic ice-floe is half what it was thirty years ago. In fact the old perennial ice which was more than 3 metres thick has gradually been replaced by year-old ice which is less than 2 metres thick.

An increasing interval between breaking up and reforming
Paradoxically, it was a reduction in the surface area of the sea-ice that attracted the attention of Arctic observers in the first decade of this millennium. In the summer of 2007 the ice-cap shrank spectacularly and by September of the same year, the surface area of the Arctic floe had been reduced to four million square metres or half what it had been thirty years before. If we consider that both the thickness and the surface area of the ice has diminshed by 50 %, that means the ice- cap has lost 75 % of its mass or volume. A huge reduction! Some think that by the end of the summer the icecap’s loss of mass or volume will be close to 60 %. With DAMOCLES, the European scientific programme which included the Tara Expedition, we studied the progressive breaking-up of the ice in the spring and the reformation of the ice-floe in the autumn. We observed that the breaking-up of the ice occurred one or two days earlier per year over the last decade, and reformation in the autumn experienced one or two days later. This growing interval between breaking-up and reforming (which is currently about one month compared to observations over the last decade or so) is an important way of measuring the gradual increase in the icecap’s thawing period-which explains the ever-more pronounced minimum values observed each September.

Another equally spectacular phenomenon that we managed to identify concerned the speed with which the sea-ice moved: it has practically doubled over the last century. The schooner Tara drifted with the Transpolar Drift Stream for 507 days between September 2006 and January 2008, from the Laptev Sea to the Fram Strait; and yet it took the Fram, a Norwegian ship carrying the explorer Fridtjof Nansen, more than one thousand days, or three years, to drift along exactly the same route a century earlier. The Rus- sian station NP35 travelled in ten months, from October 2007 to July 2008, the same distance that the Fram covered in 2 years between 1894 and 1896. During the Tara’s 507-day transpolar drift, she was trapped between the old ice ahead of her and the young ice in her wake.

The peaceful Arctic Ocean, but for how long?
Sea-ice reflects more than 80 % of the sun’s incident radiation. Once it is covered with snow this percentage, known as “albedo�? [editor’s note: the proportion of light reflected from a surface], can increase to 90 %. On the other hand, an ocean free from ice absorbs 80 % of the sun’s incident radiation and turns it into heat. This huge contrast between the albedo of ice and that of seawater goes a long way to explaining why global warming tends to be amplified in polar regions. We were therefore studying (in great detail) the behaviour of the ocean’s surface at a time when sea-ice and the Arctic atmosphere were undergoing profound changes. The principal conclusions are surprising. Without calling into question the phenomenon of positive feedback linked to the very low albedo of the ocean compared to that of ice, it seems the subsurface layers of the ocean and the principal structures that feature in the vertical stratification of the Arctic Ocean are remarkably stable. We observed that relatively warm water with low salt content from the Pacific Ocean, which enters the Arctic through the Bering Strait, may be speeding up the significant thawing of the sea-ice in the Canada basin and the Chukchi Sea.

On the other hand the waters coming up from the Atlantic Ocean, which are relatively warm and saltier, circulate at greater depths than those of the Pacific and, as a result, seem to have a minimal thawing effect on the sea-ice. Thanks to the Tara Arctic Expedition’s contribution to the DAMOCLES project, we have identified a localized layer in the thermoclime-at approximately 100 metres above the Atlantic water mass and situated at about 300 metres below sea level in the Eurasian Basin and 400 metres deep in the Canada Basin-where a dual convective process has developed. This produces a very unusual structure consisting of horizontal layers a few metres thick. This structure demonstrates the remarkable tranquility of the Arctic Ocean and how unlike other oceans it is, they being exposed to turbulence caused by the wind. But how long will this structure last? The Tara Arctic Expedition produced a wealth of information that has since been published in top-level articles in some of the great international scientific journals. More than twelve DAMOCLES articles are related directly to the Tara Arctic Expedition and six of these were recently published in international journals. Another twen- ty scientific publications related to the expedition are expected to be published in 2012.

Jean-Claude Gascard, coordinator of the European scientific programme Damocles