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Figure 2.3 – Monitoring snow cover with the Normalized Difference Snow Index (NDSI)
(Left) RGB image acquired by Terra MODIS on May 5, 2001 in Alaska (Right) NDSI,
notice how the index erroneously indicates snow cover over the ocean
2.2.3 Cryosphere and climate change
The cryosphere designates the Earth system where water can be found in solid form. It
is composed of snow, lake and sea ice, glaciers, ice caps and frozen ground and represents
during the winter seasons, 17 % of the Earth surface. For instance, the Greenland and
Antarctic Ice sheets together are equivalent to 70 m of sea level rise. Given its mass and
latent heat capacity, the cryosphere plays a role in climate change almost as important as
the oceans. First, the high surface reflectivity of snow covered regions have a significant
impact on the Earth albedoand even small variations in snow or ice fractional cover can lead
to short-term climate changes. In addition, the quantity of fresh water stored on cryosphere
can interfer with ocean currents. The variations in sea level, temperature and salinity of
ocean waters due to seasonal snowmelt can modify the interactions between oceans and the
atmosphere. Furthermore, global warming in the polar region induces permafrost melting
which in turn releases large quantities of methane stored in frozen soils. Understanding
the interactions between cryosphere and other parts of the Earth system, requires short
and long-term quantitative measurements provided by satellite instruments.
2.2.4 Recent exemples
The year 2010 was subject to few unfortunate hazards. The political and economical
implications of these events have emphasized the crucial role played by satellite sensors in
the management of such disasters.