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Polar Ice and Salinity
Name: Pam
Status: Student
Grade: 9-12
Location: N/A
Country: Australia
Date: May 2007
Question:
If salt melts ice, and the ocean is salt water, would
raising the salinity of oceans cause or contribute to the melting of
polar ice caps?
Replies:
Pam,
Salt helps to melt ice by lowering the melting temperature of the ice. The
egree of lowering of the melting point, dT, depends on the molality of the
solution. Molality is calculated by finding the moles of solute (equal to the
mass of solute divided by the molecular weight of the solute) divided by the
kilograms of solvent.
From this we see that 1 mole of solute (58 grams of NaCl) would need to be
dissolved for every 1 kg of water in order to lower the freezing point of ice
by 1 degree (from 0degC to -1degC). Currently, on average, about 25g of salt
is dissolved for every kilogram of water. We would need to double that in order
to allow for the 1degC change in melting temperature of the ice.
Considering that it is very cold in regions where there are icebergs, we would
need a lot more salt to be added to ocean water in order to allow the ice to
actually melt at the temperatures of the region.
So, in short, while adding salt does lower the temperature of melting of ice,
quite a bit of salt is needed (especially considering the amount of water in
the ocean) in order to affect the polar ice caps.
Greg (Roberto Gregorius)
Yes "in principle". However the issue is more complex than just "increasing
the salinity". First, it takes MUCH salt (primarily NaCl) to raise the salinity.
Second, the ocean is a dynamic system, and increasing the salt content increases
the density of the water so that it tends to "sink", but this process introduces
turbulence, which further complicates the whole process. Third, related to this
are ocean currents that move both deep water and surface water to and away from
the polar ice caps. These currents are driven by processes that are not isolated
from the rest of the global waters. The Gulf Stream is one example of this
global mixing. Fourth, it is not exactly accurate to say that "salt melts ice".
What is happening is that salts (and other dissolved substances) lower the
freezing point of water. So the solid left is "pure" water. It is only at a
much lower temperature that (water + salt) solidifies. This of course does not
cause salt water from being dispersed in the ice cap by physically being
incorporated by winds and tides. So your question, while pertinent, is a very
special case of a number of process that are occurring.
Vince Calder
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Update: June 2012
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