Sea ice trends in Canada
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Mark Henry, Environment Accounts and Statistics Division
This article examines trends in the average area covered by sea ice during the summer in Canada's north over a 43 year period (1968 to 2010). Trends of total (all) and multi-year (older than one year) sea ice are reported. Time series data were analysed for nine sea ice regions and three shipping route regions for total ice cover and five sea ice regions and two shipping route regions for multi-year ice cover (Maps 1 and 2).
Total ice cover represents the area covered by all sea ice and multi-year ice cover is the area covered by older ice which has survived at least one summer's melt. 1 Multi-year ice poses a particular threat to ships and navigation.
The summer season is defined as the period from June 25 to October 15 for those sea ice regions found in the Canadian Arctic domain and from June 19 to November 19 for the regions in the Hudson Bay domain (Textbox "Background and methodology" and Map 1).
An ongoing data collaboration
This article is the fourth of an ongoing series in EnviroStats showcasing data related to Canada's climate and the impacts of climate change. The series focuses on short statistical analyses of climate-related data. To date, the series has included trend analysis on glacier mass balance, temperature and precipitation. Previous articles can be found at www.statcan.gc.ca/bsolc/olc-cel/olc-cel?catno=16-002-X&chropg=1&lang=eng.
The articles in the series are the product of an ongoing collaboration between Statistics Canada, Environment Canada and Natural Resources Canada.
Data featured in the articles will be made available through the Statistics Canada website, both in free CANSIM data tables and through new articles re-examining trends in the data every few years.
Sea ice is considered by the World Meteorological Organization-Global Climate Observing System to be an Essential Climate Variable. 2 Sea ice is also one of several variables used to support the work of the United Nations Framework Convention on Climate Change (UNFCCC) and the Intergovernmental Panel on Climate Change (IPCC). 3
Background and methodology
Time series data used for this article were derived from weekly sea ice charts produced by the Canadian Ice Service (CIS) and disseminated through their Canadian Ice Service Digital Archive. 4 The sea ice charts were produced using a combination of aerial surveys, surface observations, airborne and ship reports, and remotely sensed (satellite) data. 5 The ice charts were then compiled into a time series by the Climate Processes Section of the Climate Research Division at Environment Canada, with minor corrections made to the early years in the data record to improve quality and ensure homogeneity through the time series. 6 , 7
The average area covered by sea ice is expressed in square kilometres and the rate of change is expressed as the absolute change in sea ice coverage per decade and as a percentage relative to the first year of the time series (1968). The rate of change is based on the overall decline in the linear trend.
The nine sea ice regions reflect geographies delineated and used by the CIS and are based upon the type of ice as well as climate, bathymetric, current and ocean temperature conditions. The regions are spread across two domains, the Arctic Domain and the Hudson Bay Domain. In addition, three regions are included that cover northern shipping routes.
The summer season was chosen for the time series as data are more extensive during this time period. Historically, sea ice charts have been generated to support the shipping season, which is mainly focussed on the summer and not the winter. Winter charts have been produced for some areas and periods of time, most notably after 1980, but are less comprehensive in temporal and geographic coverage. 8
The time series data were tested for the presence of serial correlation and for anomalous observations (outliers). A Statistical Analysis Software (SAS) procedure, PROC ARIMA, was used to compute the overall trend. The PROC ARIMA process produces a linear trend and the associated significance level adjusted for any existing serial correlation and anomalous observations. 9 All of the linear trends shown are statistically significant. 10
Results
Sea ice regions
Total (all) sea ice
All regions showed decreases in summer coverage over the study period for total sea ice (Charts 1 and 2, and Table 2). The largest rates of decline were seen in the five southern and eastern regions: Northern Labrador Sea (1,536 km2, or 17%, per decade), Hudson Strait (4,947 km2, or 16%, per decade), Davis Strait (6,581 km2, or 14%, per decade), Hudson Bay (16,605 km2, or 11%, per decade) and Baffin Bay (18,658 km2, or 10%, per decade).
Smaller decreases were seen in the trends for the western and central portions of the north, an area that includes the Foxe Basin, Southern Beaufort Sea, the most northern region of Kane Basin and the Canadian Arctic Archipelago (Table 2).
During the study period, six of the nine regions experienced their maximum average summer ice extent in the 1970s and seven of the nine regions experienced their minimum extents during the last five years of the study period (2005 to 2010).
Multi-year ice
A decrease of 385 km2 (21%) per decade was seen in summer coverage of multi-year sea ice in the Baffin Bay region (Chart 3). This represents the largest decline of all time series in the study but it should be noted that multi-year ice only covers a very small part of this region.
Results did not indicate statistically significant trends for the Canadian Arctic Archipelago, Beaufort Sea, Foxe or Kane Basin regions and multi-year ice is not present in the other four regions.
Shipping route regions
Two of the shipping route regions cover the north and south routes of the Northwest Passage that link the Atlantic and Pacific oceans. The third region is the Arctic Bridge, which covers the Canadian portion of a route linking North American markets to Eurasian markets via the ports of Churchill, Manitoba and Murmansk, Russia.
The presence of sea ice significantly limits navigation through these northern channels. When navigable the Arctic Bridge shipping route can save days over a St. Lawrence Seaway passage. If it were commercially navigable, the Northwest Passage could cut thousands of kilometres off the journey from Europe to Asia via the Panama Canal. The Northwest Passage shipping routes are usually blocked by sea ice during all seasons, though both were navigable in late summer and early fall of 2007.
Total (all) ice
Two of the shipping route regions saw decreases in total sea ice cover during summer, with the Arctic Bridge (Canadian portion) declining at a rate of 14,147 km2 (15%) per decade and the southern route of the Northwest Passage declining by 6,986 km2 (6%) per decade (Chart 4 and Table 2).
The results did not indicate a statistically significant trend for the northern region of the Northwest Passage.
A comparison of total ice cover during the summer of 2007, a year of light ice conditions when routes were navigable in late summer and early fall, to the average for the study period indicates that the Arctic Bridge region and the southern region of the Northwest Passage had 47% and 41% less summer coverage in that year. Total sea ice cover in the northern region of the Northwest Passage was also lower than average (-24%) in 2007.
Multi-year ice
Results did not indicate statistically significant trends for the northern or southern regions of the Northwest Passage and multi-year ice is not present in the Arctic Bridge region (Chart 5).
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