To understand and predict past and future anomalies in the transported sea ice volume, it is 
important to investigate the history of a sea ice parcel between its formation and its export, 
a.g., out of the Arctic Ocean. The origin of a sea ice parcel can be tracked with backward 
trajectories, which requires knowledge of sea ice drift (Pfirman et al. 1997; Krumpen et al. 2016). 
Along these trajectories back in time, the sea ice likely changed in response to several local 
processes: thermodynamic and dynamic thickness changes (growth, melt, and deformation), 
and changes to the snow cover (accumulation, melt, and metamorphism). A comprehensive 
quantification of the changes an ice parcel underwent along its trajectory therefore requires in 
addition information about the ice and snow surface temperature and surface albedo. 
To summarize, sea ice is a complex environment characterized by a large number of geophysi- 
cal variables. These enter many processes and interactions with the rest of the climate system. 
After careful considerations—using notably proxy variables—we 
select a core set of seven geophysical variables that are critical 
fo monitor: sea ice concentration, sea ice thickness, snow depth, 
albedo and its surface partition,“ surface temperature, sea ice age, 
and sea ice drift (Table 1). These are individually and collectively 
key indicators of climate change, with contrasted signals across 
‘he two hemispheres and regions within. 
? By surface partition we refer to the subgridscale 
distribution of the albedo of different surface 
types, such as snow covered or bare ice, melting 
ice, different forms of melt ponds, and different 
forms of young and thin ice. 
The GCOS sea ice ECV in 2021 and its challenges 
In the current Implementation Plan (IP-2016; GCOS 2016), the sea ice ECV is the only ECV 
concerned with all aspects of the sea ice environment. This ECV holds four variables (a.k.a. 
ECV products, see appendix A): sea ice concentration, edge/extent, thickness, and drift. 
Jompared to those discussed in the previous section, it is clear that some critical variables are 
:;oday missing from GCOS monitoring plans. However, before considering if more ECV products 
should be added to the sea ice ECV, we must discuss if the current single-ECV structure serves 
ts purpose well. We argue that this is not the case. 
A first challenge with the current single-ECV model impacts one of GCOS’s core activities: 
:o regularly assess the status of the global observing system to uncover where progress was 
nade and where more efforts are needed. This process is implemented through the intertwined 
cycles of Implementation Plans and Status Reports roughly every 5 years. The sea ice ECV is 
Table 1. Overview of names, short descriptions, main determining processes, and areas of relevance and impact of the core set 
of seven sea ice variables. 
Sea ice concentration (SIC) Fraction of known ocean Ice formation and melt, dynamic processes, Sea ice area and extent, sea ice mass, 
area covered by sea ice SID, SIT net short- and longwave flux 
Sea ice thickness (SIT) Vertical extent of the Thermodynamic growth and melt, dynamic Sea ice mass, ISA. IST. SID 
sea ice processes, SID, SND 
5now depth (SND) Vertical extent of the snow Snow precipitation and redistribution, ice SIT and sea ice mass, ISA, IST 
on top of the sea ice surface accumulation ability, metamorphism 
and melt, SIC, SIT, IST, ISA, AGE 
Sea ice and snow surfaces’ Sea ice growth, melt and aging, snowfall, Net shortwave surface radiation balance, 
ability to reflect solar metamorphism and melt, SND, SIT, AGE sea ice mass, area and extent 
shortwave radiation 
ice surface temperature {IST} Ice or snow surface Sea ice growth, melt and aging, snowfall, Net longwave surface radiation balance, 
temperature metamorphism and melt, SND, SIT, AGE physics of sea ice processes, sea ice 
Mass, area and extent 
zea ice ade (AGE) Lifetime of the sea ice since Thermodynamic processes, drift and dynamic Sea ice mass, ice-type fraction and 
its formation processes, SIT, SND, SID distribution 
ea ice drift (SID) Lateral movement of the Surface wind stress, bottom ocean current SIT distribution, SIC, AGE, sea ice 
sea ice (transport and stress, sea ice surface and bottom surface topography, sea ice bottom 
deformation) topography, SIC, SIT topography 
ame and acer 
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