Ice cover (time-series)

Sub-challenge: Total ice cover in sea (kg) over past 100 years plotted as time series
While sea ice cover is generally measured in kg and the data then calculated as ‘ice mass’ (kg/m^2), the majority of data  collected does not cover actual sea ice mass. Sea ice mass balance is the gain and loss of sea-ice from the Arctic. This is most often calculated by using extent and thickness data alongside data on ice surface and bottom melt, sea level pressure, heat fluxes and surface air temperature and says more about the thermodynamics of sea ice then about the total amount of cover. So although data on sea-ice mass balance is available it mostly shows the growth and melting of ice in centimetres (Cold regions research and engineering lab, 2018).
When looking at Sea Ice cover the majority of the data available is focussed on sea ice extent or sea ice thickness. As weight not only depends on area size and thickness, but also on the density and age of the ice or snow (Richter-Menge et al., 2006), it is difficult to determine the mass weight. Nevertheless, to create a time-series on the sea-ice cover it was most practical to focus on using sea-ice extent and thickness as parameters instead of mass. These parameters are widely available for download from various websites.

Arctic researchers are aware of the need for data sharing. Multiple collaborations like APECS, the Association of Polar Early Career Scientists, are investing in data management and making data from the polar regions publicly available. Through their website multiple other databases where accessed, including https://www.polardata.ca/pdcsearch/. This site presents various studies and datasets from research performed in the Arctic region, however not all are publicly available and none cover the entire arctic or a time span of 100 years.


Sea Ice Extent
Data on extent are widely available through websites like National snow and ice data centre (nsidc.org). All data are available free of charge, but for most data a (cost free) registration is required as websites often use FTP directories to exchange data. Most of the data available began from 1979, the beginning of satellite data (Kattsov et al., 2010).

Each subset of data has its own page describing not only the data in the files, but also the way these were obtained and filed. Explanations of file names, parameters and variables are present on this page. The nsidc website can be searched through this page (http://nsidc.org/data/search/).
For Sea ice extent specifically one of the databases on the northern hemisphere can be  downloaded through an FTP directory. An FTP is a File transport protocol, which enables data transport between a computer and a server. Using at least windows 10, connection to the FTP directory can be made by adding a new network location in the file directory.

The FTP used contains data NOAA data on sea ice extent (NSIDC; NOAA, 2018) in the Northern and southern hemisphere. Using Northern hemisphere data, the following timeline of sea ice extent could be produced. This data is generated by Fetterer, F., K. Knowles, W. Meier, M. Savoie, 2017

Sea Ice extent in Northern Hemisphere
Figure 1: Sea Ice extent in the Northern Hemisphere in January, from 1979 to 2016, in million square km.

Data for figure 2 was downloaded from the Blue ice website. This data opened in a webpage, which could be converted to a text-file and imported into Excel. The temporal resolution is 1979-2007 and contains data on nine different sample sites throughout the Arctic. These nine sites are combined by the data-collectors into a total amount of sea ice extent for the entire Arctic. The data was used for multiple papers on sea ice cover (Cavalieri, D. J., C. L. Parkinson, P. Gloersen, J. C. Comiso, 1999; Cavalieri, D. J., P. Gloersen, C. L. Parkinson, J. C. Comiso, 1997; Gloersen, P., C. L. Parkinson, D. J. Cavalieri, J. C. Comiso, 1999; Gloersen, P., W. J. Campbell, D. J. Cavalieri, J. C. Comiso, C. L. Parkinson, 1992; Parkinson, C. L., D. J. Cavalieri, P. Gloersen, H. J. Zwally, 1999; Parkinson, C. L., 2002; ).  

Monthly extent of sea ice
Figure 2. Monthly extent of sea ice in the arctic. From source: https://blueice.gsfc.nasa.gov/seaice_datasets.html (Extent Monthly)

Sea-Ice Thickness
The dataset with the biggest temporal resolution on sea ice thickness is available from Nsidc, this data is gathered using upward looking sonar and has a temporal range from 1960 until 2005. It includes data from the Royal Navy and U.S. military submarines. Although governmental submarines, the data collected is not classified and is therefore available with date and location (National snow and ice data center).
This data can be downloaded through an FTP directory and is provided in .tgz files, which can be opened using software like Winzip.
The data however is organised per year, with each year having different submarine tracks. This means that although the majority of the Arctic is covered it is difficult to make an accurate time series as no two measurement points are the same. Points can be divided into areas which would allow year by year comparison, but this is a time-consuming exercise. This data has been used to present the decline of sea ice thickness over time (Kwok & Rothrock, 2009; Rothrock, Yu, & Maykut, 1999).

Zwally, Yi, Kwok, & Zhao (2008) also provide data on sea ice thickness through the Nsidc website (requires a free Earthdata account). However, the data is logged in a way that makes it very time consuming to use and ranges only five years, from 2003 to 2008. The data in this database is logged in irregular time periods, and within every time period data is logged to a specific date. On each date a sample run was done using boats. However, these runs do not generate the same sample points for each run. This means that although data is freely available it is inconsistent and required much manual processing.

Sturm (2009) provides data on sea ice thickness, but only in a two week time period on a single location. Spatially this data only provides insight into the Barrow, Alaska area.

Data on Sea ice thickness is also available via the NSF Arctic Data Centre (Hajo Eicken, Stefan Hendricks, Mette Kaufman, 2009). The temporal coverage is only single flights, done between October 2010 and March 2011. Flights only occur in one location north-east of Barrow Point, Alaska.

Data use, availability and gaps
Major gaps in this challenge include information on ice cover mass; nodata on sea-ice cover mass in kg/m3 was found during this study.  
There was a multitude of data found on sea ice extent and thickness. Some data was easy to download and use. Other data, like that from Zwally et al., (2008) need to be integrated into usable spreadsheets, which is time-consuming. Making graphs from the available data can also be very time consuming due to the way data is stored. Most datasets are categorised in some way, and while there may be up to 40 years of data, the data logging locations vary. Quite often data is logged per month or year, this means that creating a time line first starts with integrating all those measured months (or years) into one spreadsheet. Various data sets also log data categorised by location. This seems logical, but also required manual combining of datasets to gain insight in the entire Arctic.
The availability of long-term data sets is also a major limitation in this challenge, with the earliest datasets starting in the 1960s.

On the other hand, thickness is measured in meters or centimetres for all datasets, and extent is measured in Km2. Data found was presented in Excel, txt, of CSV files, all of which can be easily adjusted for use in Excel.

Conclusion and lessons learned
In conclusion it can be said no data is available on sea ice mass. However, time-series can be made using sea ice extent and sea ice thickness. Most of the datasets found have a starting point in the 1960s, when sonar became available, or 1978 when satellite became available. It is near impossible to create a time-series spanning 100 years.

A lot of data concerning sea ice extent is freely available and being regularly updated. Furthermore, scientists are increasingly seeing the need to publicly share data. Getting a total overview of data available on sea ice extent throughout the Arctic requires combining datasets, only then will true gaps in knowledge become obvious. Keeping track of sea ice extent will eventually lead to a 100 year time-series.

Sea ice thickness is a different matter, although data is available, none of it is directly usable. All data needs to be combined, converted or processed before time-series can be made. However, many papers have used time-series created using the available data, suggesting that differently organised data-sets do exist.

It is recommended to rephrase the challenge set as there is the question of sea ice cover in kg/m3. Is it necessary to have a time-line of sea ice in kg over time? Or would time lines including sea ice extent (km^2) or thickness (m) also be sufficient? If sea ice extent and sea ice thickness are desired there is a wide range of research being done and data available.

It is recommended to think about the way data is going to be logged, depending on the nature of the data and its use (images or time series). Consider logging data into one spreadsheet or multiple smaller files. Depending on the question at hand contact with scientists throughout the Arctic region and possibly creating a primary data base might be enough to gain full insight into the sea ice extent. No need is seen to generate new research as so much is already being done at this moment. Bringing researchers and their data together however might bring new insights to light.


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