Ice cover (map)
Sub-challenge: Average extent of ice coverage over past 5 years, past 10 years, past 50 years, past 100 years, plotted on maps.
For this sub-challenge enough data was available to complete it, data used came from U.S. NOAA. It was chosen to add a ‘grid’ to the maps for optimal viewing. However, one must keep in mind that historical Arctic data more than a few decades ago is very different than modern data due to the changes in sampling methods (e.g. invention of the CTD, and initiation of polar orbiting satellites) and sample locations from individual expeditions to full polar coverage. Archival data is increasingly being merged into digital forms so that historical conditions can be reconstructed. Older data are becoming more available as more reanalysis data sets become available, e.g. Walsh et al (2016) we recently published. Quantities such as "ice concentration" (numerical model friendly), rather than "ice coverage" (shipping terminology), are available due to their use in climate scale numerical simulations, so the key records related to these parameters are the first to be put into synthesized digital forms. As more historical data becomes available in digital forms, we expect reanalyses of more types of ice information to become available.
The data set we found that contains the oldest historical data is from the NOAA / National Environmental Satellite, Data and Information Service (NESDIS) / National Snow and Ice Data Center (NSIDC) at the University of Colorado. The Gridded Monthly Sea Ice Extent and Concentration, 1850 Onwards, version 1. These data are newly released by Walsh, Chapman et al. (2016). Sea ice concentration is a proxy for coverage, e.g. 15%-20% ice concentration is considered a reasonable definition of "covered", and, if combined with an estimate of ice density and thickness information, one could estimate coverage (kg m2/year). Ice coverage is an important consideration for determining when an ice class vessel is required. Error in ice concentration estimates is generally on the order of 5%, but can be greater (nsidc.org). The older data are a monthly gridded data based on ship observations, naval oceanographer records, and analysis by different national ice services (see Table 1). Satellite passive microwave data began to collect full Arctic data in 1979. Full data sets were first published in 1991 (with historical data back to the 1990s). These new data are intended to be used as boundary conditions for model applications such as reanalyses (e.g. model runs to simulate historical conditions or climatologies). Although these data show the decrease in area, they do not show how significantly sea ice thickness has declined over the last six decades. Kwok and Rothrock (2009) use declassified submarine data to show the average sea ice thickness at the end of the melt season has decreased by 1.6 m (53%) over 40 years in the area of interest, with negative trends in all regions.
Table 1: Sea Ice Data Sources used by NSIDC. Table from page 5 of Walsh et al (2016). Note that numbers 6,7,14,15 and 19 are not used as they are placeholders for the processing software.
|1||DMI yearbook narrative|
|4||Kelly ice extent grids|
|5||Walsh and Johnson|
|8||Satellite passive microwave|
|13||Danish Meteorological Institute|
|16||Whaling log books open water|
|17||Whaling log books partial sea ice|
|18||Whaling log books sea ice covered|
|20||Analog filling of spatial gaps|
|21||Analog filling of temporal gap.|
Figure 1: Map of Sea Ice Concentration 5 years ago (from 2016). Data from Walsh, Chapman et al. (2016). Consider 15% concentration as the lower boundary for ice coverage. March is shown, as that is the month of maximum sea ice extent.
Figure 2: Map of Sea Ice Concentration 10 years ago (from 2016). Data from Walsh, Chapman et al. (2016). Consider 15% concentration as the lower boundary for ice coverage. March is shown, as that is the month of maximum sea ice extent.
Figure 3: Map of Sea Ice Concentration 50 years ago (from 2016). Data from Walsh, Chapman et al. (2016). Consider 15% concentration as the lower boundary for ice coverage. March is shown, as that is the month of maximum sea ice extent.
Figure 4: Map of Sea Ice Concentration 100 years ago (from 2016). Data from Walsh, Chapman et al. (2016). Consider 15% concentration as the lower boundary for ice coverage. March is shown, as that is the month of maximum sea ice extent.
- Kwok, R., D.A. Rothrock (2009) "Decline in Arctic sea ice thickness from submarine and ICESat records: 1958-2008" Geophysical Research Letters 36 L15501, doi:10.1029/2009GL039035.
- Walsh, J. E., W. L. Chapman and F. Fetterer (2016). Gridded Monthly Sea Ice Extent and Concentration, 1850 Onward, Version 1. https://nsidc.org/data/g10010.