This is a great question because it strikes
right at the heart of what controls the climate
on earth. The most direct effect of melting of
continental ice caps is that sea level rises.
For example, during an inter-glacial period when
the earth is ice free, sea level is hundreds of
feet higher!!!! The oceans FLOOD THE LAND, and
since most of the continental area is at
elevations (today) less than a few hundred feet,
this means that serious amounts of land get
flooded when the ice caps disappear.
But there are other effects that are more
subtle but perhaps even more important. A
significant fraction of solar light that falls
upon the cross section of the earth is REFLECTED
DIRECTLY back out into space. This fraction is
around 30 percent. This is important because the
more reflected away the less energy is available
for heating, and the lower the average earth
temperature becomes; now here is where it gets
interesting. Ice has a high reflectivity...snow
too!!! So, if the planet was COVERED in ice,
then its reflectivity would increase and the
temp of the Earth’s atmosphere would be LOWER,
since less energy is available (it has been sent
out to space!)!
On the other hand, if the Earth heats up due
to greenhouse warming, then ice melts and the
reflectivity goes down, which makes the Earth
warmer, which causes more ice to melt, which
makes the Earth warmer STILL, which melts even
MORE ICE, which makes it warmer, and warmer!!!!!
THIS IS A POSITIVE FEEDBACK LOOP! Eventually
the system comes to equilibrium and the Earth
will have a constant mean temperature; but that
value will be HIGHER (warmer) than when we had a
lot of ice to reflect all that sunlight.
In this sense, we can say that melting of the
ice caps decreases the reflectivity of the Earth
and hence more solar energy is available. This
leads to a higher average temperature of the