TRENDS IN SEA ICE IN THE CANADIAN ARCTIC

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1 Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice Dunedin, New Zealand, 2nd 6th December 22 International Association of Hydraulic Engineering and Research TRENDS IN SEA ICE IN THE CANADIAN ARCTIC John C. Falkingham 1, Richard Chagnon 1 and Steve McCourt 1 ABSTRACT Observational evidence indicates that there has been a significant reduction of sea ice in the Arctic Ocean over the past few decades lending support to predictions of global climate models that there will be less sea ice in the northern hemisphere in the future. To add to the global body of knowledge about the state of sea ice, the Canadian Ice Service has digitized its weekly sea ice charts spanning the Canadian Arctic over the period to. Preliminary results from the analysis of this database were presented in indicating a general reduction in the extent of the sea ice in the Canadian Arctic during. This paper updates the previous analysis and strengthens the conclusions that the coverage of sea ice in the Canadian Arctic in the summer months has decreased by about 15 % in the Arctic areas, roughly north of 6N over the period. In the Canadian sub-arctic areas of Hudson Bay and the Labrador Sea, the coverage of sea ice during the summer has decreased by 4 % and 72 % respectively over the period. INTRODUCTION Numerical simulations of future climate by different Global Climate Models (GCMs), including that of the Meteorological Service of Canada, agree that climate warming will occur first and most intensely in Arctic and sub-arctic regions (e.g. IPCC, Flato ). Observational evidence indicates that the Arctic has been warming and that this warming is unprecedented over the past 4 years (Serreze et al., 2). The extent of Arctic sea ice (i.e. the area of ocean covered by ice), as observed mainly by satellite, has decreased at a rate of about 3 % per decade since the 197 s (Parkinson et al., ). Of potentially greater significance, estimates based on submarine measurements indicate that summertime Arctic sea ice thickness has diminished by 4 % over the past 4 years (Rothrock et al., ). If the recent rate of thinning were to be sustained, the Arctic Ocean could be essentially ice-free in summer as early as 25. The Canadian Arctic is not well resolved by Global Climate Models and is likely driven by different environmental forcing mechanisms than the Arctic Ocean. The Northwest Passage offers a tempting shortcut between the Atlantic and Pacific Oceans that has only been made inaccessible by the ever present sea ice. The sea ice itself provides a stable surface for the people and animals of the Arctic to travel on for much of the year, 1 Canadian Ice Service, Meteorological Service of Canada, Ottawa K1A H3

2 effectively increasing their land area by some 5 %. If a future climate were to allow a marked decrease in sea ice, it would bring unprecedented social, economic and environmental change to this fragile region. Previous work by the authors (Falkingham et al., ) showed some evidence for a trend of decreasing sea ice in the Canadian Arctic. The Total Accumulated Coverage (TAC) was found to decrease by 3 8 % per decade over the period, although there was less than 9 % confidence that these declines were statistically significant. A parallel was drawn with similar declines in sea ice extent reported by others (e.g. Parkinson et al., ) over the northern hemisphere. THE CANADIAN ARCTIC Historically, the Canadian Arctic waters are covered by an essentially solid ice pack throughout the winter. This ice starts to break up in July permitting a 3 5 month shipping season before freeze-up begins again in October. Some areas, such as Hudson Bay and the coastal zone of the Beaufort Sea, generally become completely ice free in August September. Other areas, such as Viscount Melville Sound and M Clure Strait remain covered with close pack ice throughout most years. In virtually all areas, marine navigation demands the utmost caution because of pack ice drifting under the influence of winds and currents. Multi-year ice, which has survived at least one summer melt and has re-frozen to become very dense and hard, is present in varying amounts throughout most of the area presenting a particularly dangerous hazard for even ice-strengthened ships. Icebreaker assistance is often essential. 5, 45, 4, 35, 3, 25, 2, 15, 1, 5, Figure 1: Total Ice Coverage in the Eastern Arctic Region on September 1 of each year Sea ice conditions in the Canadian Arctic are characterized by substantial inter-annual variability. Since when reliable records began, we have observed the sea ice coverage to double or halve within one or two years (e.g. Figure 1). For example, in the Eastern Canadian Arctic on September 1, there was nearly twice as much ice during,, and as there was only one or two years later. Clearly anyone planning to travel to the Arctic at any particular time is faced with a wide range of possible conditions. Under any future scenario of generally decreasing sea ice in the Arctic, it is important to remember that this significant variability will remain. We could expect to see a regular occurrence of light ice years interspersed with heavy ice years. It is this variability that increases the risk of Arctic operations. THE CANADIAN ICE SERVICE DIGITAL DATABASE The Canadian Ice Service, a division of the Meteorological Service of Canada, Environment Canada, has developed a digital database of sea ice information from its regional ice charts. The database is documented fully in Crocker and Carrieres (2a) which is summarized here.

3 The digital data base contains: (i) for the East Coast of Canada - weekly ice charts from 1968 to ; (ii) for the Western (Canadian) Arctic - weekly ice charts from 1968 to for the summer navigation season (approximately June to October) and monthly ice charts for the entire year from 198 to ; (iii) for the Eastern (Canadian) Arctic - weekly ice charts from 1968 to for the summer navigation season (approximately June to October) and monthly ice charts for the entire year from 198 to ; (iv) for Hudson Bay - weekly ice charts from to for the summer navigation season (approximately June to October) and monthly ice charts from for the entire year from 198 to. The database is updated as quickly as quality control procedures permit. For the years up to, it is derived from 1:4 million scale hard copy charts that have been digitized in a topologically complete polygon database in an Arc/Info Geographic Information System. After, the charts are produced directly in the GIS. Each digital chart covers a sea surface area of million km 2 and contains all of the information contained in the WMO international sea ice code. Software is available to permit the extraction of ice parameters on any arbitrary grid. Additionally, the database contains grid-point data and pre-calculated statistics, including minimum, maximum and median ice frequencies, for three different standard grids. UPDATING PREVIOUS WORK The analyses presented in Falkingham et al. () were based on data between ( for Hudson Bay) and. These analyses have been updated to in Figure 2 which show the annual Total Accumulated Coverage (TAC) for each of three broad areas of the Canadian Arctic. Recall that TAC is calculated by summing the sea ice coverage for each of the weekly ice charts throughout the season, defined as a 17- week time window from June 25 to October 15. Ice Coverage is the areal extent of sea ice multiplied by the average concentration. TAC is used because it is the most stable parameter in the database and is the most robust indicator of long term climate change. It has the advantage of being relatively insensitive to anomalies on individual ice charts and so is a good parameter for inter-annual comparisons. While it has no obvious physical meaning, TAC does take into account the length of time that ice may be present or absent in an area as well as its concentration and, therefore, is an excellent indicator of the severity of a particular season. The higher the TAC, the more ice that was present throughout the season. The updated TAC trends further strengthen the conclusions reached earlier. Over the period, the Total Accumulated Coverage of ice in Hudson Bay has decreased by 4 % (13 % per decade). This decrease is statistically significant at the 95 % confidence level. In the Eastern Arctic region, the TAC over the period has decreased by 15 % (5 % per decade). This decrease is also statistically significant at 95 %. In the Western Arctic region, the decrease in TAC between and is also 5 % per decade but this trend is not statistically significant above 8 %. We also looked at the minimum ice coverage in summer during each of the years in the database (Figure 3). Because all of the ice in Hudson Bay frequently melts, this statistic is not meaningful for that region. However, for both the Eastern and Western Arctic regions, there has been a decrease in the ice coverage at the summer minimum of 8 %

4 per decade over. This trend is statistically significant at 95 % confidence in the Eastern Arctic but only with 9 % confidence in the Western Arctic. 5,, 4,, 3,, 2,, Hudson Bay 1,, 14,, 12,, 1,, 8,, 6,, Baffin Bay 4,, 2,, 12,, 1,, 8,, 6,, 4,, 2,, Beaufort Sea Figure 2: Trends in Total Accumulated Coverage for the Hudson Bay (top), Eastern Arctic (middle) and Western Arctic (bottom) regions of the Canadian Ice Service database REGIONAL VARIATIONS IN ICE COVERAGE TRENDS To better understand the nature of the decreases in sea ice, we further sub-divided the three regions and looked at similar statistics. We postulated that the Western Arctic region might actually encompass several different forcing regimes. The Beaufort Sea portion includes a large area of multi-year ice that is part of the general ice circulation in the Arctic Ocean. The Western Arctic Waterway portion comprises the more protected waters of Amundsen Gulf, Coronation Gulf and Queen Maud Gulf that have little ice circulation. Ice cover in this portion is predominantly first year ice. The Viscount Melville portion contains large quantities of multi-year ice that do not seem to be affected by strong circulation patterns, although there has been little quantitative study of ice movement in this area. The results are shown in Figure 4. As expected, the Western Arctic Waterway portion of

5 the region shows the strongest decrease in TAC totalling 36 % over the 32 year period (11 % per decade). The trend is statistically significant at the 95 % confidence level. In the Viscount Melville portion of the region, we have observed a total decrease in the TAC of 1 % (3 % per decade) and in the Beaufort Sea portion, the decrease has been 12 % (4 % per decade). The trends for the latter two sub-regions are significant at 9 % and 8 % confidence only. 5, 45, 4, 35, 3, 25, 2, 15, 1, 5, 5, 45, 4, 35, We believe that the Western Arctic Waterway portion of the region is strongly driven by the local thermodynamic regime and is therefore reflective of the rising surface temperatures. The ice in this area does not circulate into or out of the area on any large scale. It mostly comprises a first year ice cover that melts in situ each summer and re-forms in winter. 3, 25, 2, 15, 1, 5, Figure 3: Minimum Ice Coverage in the Eastern Arctic (top) and Western Arctic regions The Beaufort Sea portion of the region, in contrast, has a large flux of Arctic Ocean multi-year ice entering from the north and exiting to westward. The Arctic Ocean tends to advect ice into this area under any of the observed circulation patterns, although the magnitude of the advection may be stronger or weaker. The decline in TAC observed may be explained by a greater area of first year ice melting in summer between the coast and the multi-year pack, a conclusion supported by the trend in minimum ice cover. The Viscount Melville portion of the region has a more complex ice regime that is not well understood. Arctic Ocean ice has been observed to enter the area through M Clure Strait but ice also exits the area through M Clure Strait. It is believed that there is very little ice circulation in Viscount Melville Sound and M Clintock Channel while the eastern part of the sub-region experience a slow drift to eastward. Further work is required to explain the observed decreases in TAC in this area. In the Eastern Arctic region, we looked at only two sub-regions - Baffin Bay and Lancaster Sound. As might be expected, because it dominates the ocean area of the region, the decrease in Total Accumulated Coverage in Baffin Bay reflects that of the entire Eastern Arctic. However, in Lancaster Sound, which is important to navigators contemplating the Northwest Passage, there is no evidence of a decrease in total accumlated ice coverage over the period the TAC trend is virtually flat (Figure 5). It had been speculated that one result of generally decreasing sea ice in the Canadian Arctic might actually be an increase in the amount of multi-year ice invading the archipelago from the Arctic Ocean. In our earlier work (Falkingham et al ), we could find no significant evidence of this over the large Eastern and Western Arctic

6 1,8, 1,6, 1,4, 1,2, 1,, 8, 6, 4, 2, Beaufort Sea 4,5, 4,, 3,5, 3,, 2,5, 2,, 1,5, 1,, 5, Beaufort Sea 1,, 9,, 8,, 7,, 6,, () 5,, 4,, 3,, 2,, 1,, Beaufort Sea Figure 4: Total Accumulated Coverage for the Western Arctic Waterway (top), Viscount Melville (middle) and Beaufort (bottom) sub-regions. 5, 45, 4, Baffin Bay 35, 3, 25, 2, 15, 1, 5, Figure 5: Total Accumulated Coverage for the Lancaster Sound sub-region.

7 regions. In focusing on the Lancaster Sound sub-area, we have found that the accumulated seasonal coverage of multiyear ice increased substantially (76 %) over the period (Figure 6). Note that this represents an increase from approximately one-tenth concentration of multi-year ice to two-tenths concentration on average throughout the season. While there is only 8 % confidence that this trend is statistically significant, it is an indication that there may indeed be more old ice from the Arctic Ocean working its way through the archipelago into the Northwest Passage. Taken together with the result on total accumulated coverage, it would indicate that multi-year ice is taking up space vacated by first year ice. LABRADOR SEA As an adjunct to this study of Canadian Arctic areas, we also examined the CIS database in the Davis Strait Labrador Sea area. Some Global Climate Models predict a local cooling centre in this general area, even in 6, global warming scenarios. Baffin 5, Bay As can be seen from Figure 7, the somewhat 4, surprising result over the 3, period is that 2, the Total Accumulated 1, Coverage for the summer season has decreased by a very substantial 72 % (24 % per decade). This Figure 7: Total Accumulated Coverage in the Labrador trend is statistically Sea sub-region significant at the 98 % confidence level. In some recent years, almost all of the ice in the sub-region has melted completely before June 25 and ice has not formed nor been advected into the sub-region before October 15, resulting in a near-zero TAC. In examining the ice coverage on June 25 for each of the years, this same decreasing trend is apparent. At the start of the period in, the average ice coverage is about 7, km 2 and decreases to about 15, km 2 by. It appears that there is either less ice forming during the winter or the ice is melting more quickly during the spring so that the summer season is beginning with less and less ice on average subject to large interannual fluctuations as noted earlier CONCLUSIONS We have presented updated results from an analysis of sea ice distribution based on the Canadian Ice Service digital database spanning the past three decades. This analysis strengthens conclusions reached in an earlier paper that the amount of sea ice in the Canadian Arctic in the summer months has decreased significantly over this period. In the Arctic areas, roughly north of 6N, the coverage of sea ice has decreased by about 12, 1, 8, 6, 4, 2, Figure 6: Total Accumulated Coverage of Multi-Year Ice in the Lancaster Sound sub-region

8 15 % over the period. Within this average there is considerable regional variation. In the Canadian sub-arctic areas of Hudson Bay and the Labrador Sea, the coverage of sea ice during the summer has decreased by 4 % and 72 % respectively over the period. ACKNOWLEDGEMENT Thanks to Wayne Lumsden, Director of the Canadian Ice Service, for his critical review of the paper and for presenting these results at the IAHR conference. Many thanks to Greg Flato of the Canadian Centre for Climate Modelling and Analysis, Meteorological Service of Canada for his helpful insights and advice. Thanks to Tom Carrieres and Greg Crocker for their explanations of the CIS digital database and its limitations and to Bea Alt for her unbounded enthusiasm and stimulating thoughts. Finally, we would like to thank all of the professional ice forecasters and analysts at the Canadian Ice Service for their innumerable contributions to this paper. It would not be possible without their collective knowledge freely shared. REFERENCES Crocker, G. and Carrieres, T. Documentation for the Canadian Ice Service Digital Sea Ice Database. Ballicater Consulting Ltd. Contract Report -2 (2). Falkingham, J.C., Chagnon, R. and McCourt, S. Sea Ice in the Canadian Arctic in the 21 st Century; In Proceedings of the 16th International Conference on Port and Ocean Engineering under Arctic Conditions., POAC '1, Ottawa, Ontario, Canada (). Flato, G.M. and Boer, G.J. Warming asymmetry in climate change simulations. Geophysical Research Letters 28: (). Intergovernmental Panel on Climate Change (IPCC). Climate Change : Impacts, Adaptation & Vulnerability Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). James J. McCarthy, Osvaldo F. Canziani, Neil A. Leary, David J. Dokken and Kasey S. White, eds., Cambridge University Press (). Parkinson, C.L., Cavalieri, D.J., Gloersen, P., Zwally, H.J. and Comiso, J.C. Arctic sea ice extents, areas and trends,. Journal of Geophysical Research 14: 2,837 2,856 (). Rothrock, D.A., Yu, Y. and Maykut, G.A. Thinning of the Arctic Sea Ice Cover. Geophysical Research Letters 26: (). Serreze, M., Walsh, J., Chapin, F., Osterkamp, T., Dyurgerov, M., Romanovsky, V., Oechel, W., Morrison, J., Zhang, T. and Barry, R.G, Observational evidence of recent changes in the northern high-latitude environment. Climate Change 46: (2).