Stein Sandven, Nansen Environmental and Remote Sensing Center
The recently observed and predicted reduction in sea ice has stimulated interest in arctic areas that were previously considered inaccessible. Sea ice is a major obstacle to accessing the large potential petroleum resources in arctic shelf areas. Operations in the presence of sea ice require specialized vessels and production systems designed to withstand ice pressure.
Sea ice exhibits strong regional and year-to-year variability in extent, thickness and other characteristics. This variability is the most important factor for planning exploration and transport of oil and gas form arctic fields.
Sea ice is formed when seawater freezes in the polar oceans. The amount of ice formed reflects a delicate balance of heat and mass exchange in atmosphere and ocean. Sea ice is an important component of the global climate system, and the recent reduction of it is associated with warmer temperatures in the arctic regions.
The total sea ice area, which has been accurately monitored by satellite data since 1978, shows a significant decreasing trend of about 3% per decade averaged over all months. Predictably, the decrease is lower for the winter months (2% for March) and higher for the summer months (maximum of 7% for September).
In 2005, the area occupied by multi-year ice was 5.5 million sq km (2,123,562 sq mi), compared with around 7.5 million sq km (2,895,766 sq mi) in the early 1980s. The difference of 2 million sq km (772,204 sq mi) corresponds to the size of Greenland.
Map of sea ice extent for November for 1978 and 2005 derived from satellite data. The blue colors indicate concentration of multi-year ice. The yellow line shows the ice edge from 1978 superimposed on the 2005 map.
A look at the difference in coverage for the month of November between 1978 and 2005 reveals that most of the multi-year ice in the Beaufort Sea, Chukchi Sea, East Siberian Sea, and Laptev Sea disappeared.
A stronger reduction of multi-year ice compared to the total ice cover means that a relative larger part of the remaining sea ice is seasonal first-year ice that freezes in winter and melts away in summer.
Reduced areas of multi-year ice, which is thicker and harder than first-year ice, will facilitate operations in the coastal areas where most of the arctic oil and gas fields are located.
In addition to area, the most important ice parameters are thickness and drift. The thickness of sea ice is a consequence of past growth, melt, and deformation, which explains why multi-year ice is normally thicker than first-year ice. Thickness is closely connected to ice strength, another key factor for ice operations and navigation.
Available data, however, is relatively sparse because there has been no systematic monitoring of ice thickness from satellites as there has been for ice area.
Three-monthly mean ice area in the Barents Sea from 1978 to 2006 based on passive microwave satellite data.
The most useful data have been obtained from submarine surveys starting in the 1950s. These surveys, covering the central deep Arctic basin, indicate that the average sea ice thickness in the central arctic has decreased by more than 1 m (3 ft) over the last four decades, from about 3 m (10 ft) to less than 2 m (7 ft) currently.
Model-based simulations support this conclusion, suggesting an arctic-wide reduction of 0.6 to 0.9 m (2-3 ft) over the same period. Ice thickness measurements of the seasonal ice cover around the periphery of the Arctic basin do not indicate any statistically significant change in thickness in recent decades.
In addition to the mean ice thickness, the dynamic behavior of sea ice can generate much higher thickness locally in areas of ice convergence. When ice floes converge, they are crushed and stacked on top of each other. In cases when the wind drives sea ice against a coast, ridges can be tens of meters thick. Over the entire arctic region, ridges are thought to account for nearly half of the total ice volume. Ridges constitute a major impediment to transportation and operations in sea ice areas. However, there are no indications that ridging in coastal areas has declined over recent decades.
Sea ice conditions can differ from one region to another. Sea ice in the Barents Sea, for example, is influenced by wind and warm air masses from the North Atlantic and by oceanographic conditions, notably the warm Atlantic water.
The Barents Sea shows significant inter-annual variability as well as a decreasing sea ice trend from 1978 to 2006. For the whole of the arctic, the ice cover decrease is most pronounced during the summer months, with the Barents Sea showing the greatest decrease in winter and spring.
The mean ice area in the Barents Sea for April-June was above 1 million sq km (386,102 sq mi) in 1979, but fell to less than 300,000 sq km (115,831 sq mi) in 2006. The decrease was similar for the January-March period, mostly occurring during the last three winters (2004-2006), at a time of high ocean temperatures and extreme air temperatures that were 5° to 6° C above normal.
The Shtokman gas field in the southwestern part of the Barents Sea, has been ice free during the last four winters. In previous years, ice built up at this location during the winter months in 13 out of 26 years. The Prirazlomnoye oil field near the coast in the Pechora Sea, however, has had ice almost every winter up to now. This field is located in shallow coastal waters where sea ice normally forms locally every winter.
Another threat for offshore operations in the Barents Sea is drifting icebergs. The mass of icebergs is much larger than for sea ice because the vertical dimension of icebergs can be 100 m (328 ft) or more. And the characteristic mass of icebergs in the Barents Sea is several hundred thousand metric tons.
The drift speed of icebergs can be 20-30 km (12.43-18.65 mi) per day. In recent years, the distribution and drift of icebergs in different parts of the arctic have exhibited abnormal behavior. Observers have found many icebergs in areas where the probability of occurrence is low. The increased melting and calving of arctic glaciers is believed to be the main reason.
The probability of icebergs approaching a platform or subsea pipeline is a severe risk factor for arctic operations. In the Barents Sea, the majority of the icebergs are small, typically 20 m (65.62 ft) bergy bits and growlers, while tabular icebergs, glacier bergs, and other types at scales of 100 m (328 ft) or more represent a minority. The probability of icebergs is highest in the northeastern Barents Sea, but recently icebergs have drifted southward to the Shtokman field.
Satellites can observe the largest icebergs, typically 100 m (328 ft) or more in horizontal extent, and under specific wind and sea ice conditions. But there are no regular iceberg monitoring or forecasting systems in the Barents Sea and other arctic exploration areas. Operators interested in exploration and production in these areas will need to establish their own ice management systems that include icebergs.
The future
The biggest concern most people have is knowing what sea ice changes can be expected in the arctic over the next decades. It is likely that the observed trend of reduced ice extent will continue and that summer ice extent could disappear some time between the middle and the end of the 21st century. This means that the multi-year ice will be absent, leaving the Arctic Ocean with only first-year ice.
These conditions will be advantageous to icebreakers and platforms designed to operate in ice because first-year ice is generally thinner and less hard than multi-year ice.
Thinning of the ice cover also will continue, but the rate and geographical distribution are difficult to estimate. Melt seasons will be longer, and reduction of the ice areas in the summer season will lead to greater access to the petroleum resources on the arctic shelves and a longer navigation season for shipping around the Arctic basin.
Winter ice will remain in most of the shelf areas, and ridging processes will continue to create extreme ice thickness in specific locations. The climate model predictions for the high northern latitudes show large discrepancies, suggesting that there is considerable uncertainty in estimating future ice conditions in the arctic. The trend of reduced ice cover is a consistent and robust result of the climate predictions, but the expected variability in ice conditions between regions and from year to year are unknown.
It is very likely that some years will be colder and have more severe ice conditions in specific regions. On the other hand, the reduction of sea ice could also proceed faster than predicted by the models.
Other effects of climate change will be more wind and precipitation in arctic regions. The effect of more wind and waves in ice-covered areas will be increased ridging and stamukhas in near coastal regions. Waves will break up the ice in the ice edge region, causing dangerous situations for vessels and installations.
Certainly, sea ice and icebergs will still be major obstacles to arctic exploration in the next decades even if the sea ice cover shows a decreasing trend.
