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WILL IPCC´s PREDICTIONS HOLD TRUE? |
According to satellite measurements, the present level of
sea
level
rise is about 3 mm per year. In hundred years, this will amount to 30
cm. So, when the IPCC predicts a sea level rise of about 30 cm during
the 21st century, they actually say that the present rate of rising
will continue unchanged. And when they have a lower boundary saying 18
cm, this means that they keep the possibility open that the rate of sea
level rise will decrease.This is problematic, because up to now, the
rate of sea level rise is faster than what IPCC is able to explain by
summing up the known contributions.
In the most recent (fourth) IPCC assessment from
2007, the scientists have utilised information that was available since
the third assessment in 2001, but they have not utilised the very
newest information, because they had to go through a time-consuming
review process and did not have the opportunity to include the latest
evidence. For instance, their predictions about the extent of
melting of the polar sea ice during the arctic summer are already
outdated - the extent of melting in 2006 and 2007 has been much larger
than predicted by IPCC.
Another point where IPCC may have underestimated the
rate of melting, is the melting of land ice on Greenland and
Antarctica. In their calculations, they have included the increased ice
flow from Greenland and Antarctica at the rates observed for 1993-2003,
but they have not included the further increases in the ice flow
observed after 2003.
The processes governing the rate of discharge of
land ice via glaciers into the sea are not fully understood.
Information obtained in recent years is that melt water on the surface
of the ice may form socalled `moulins´, that is vertical holes
through which the water runs to the bottom of the glacier, where it
forms a lubricating layer which allows the glacier to slide faster
towards the sea. The effect is similar to what is seen in those
glaciers whose bottom is below sea level, which means that during
periods of high sea water level (tides), the bottom of the glacier is
lifted up, which accelerates the outward movement of the glacier ice.
During recent years, much new information has been
gathered on the movements of certain glaciers on Greenland, which are
outlets for about 10 % of the total inland ice there. In the Jakobshavn
glacier in west Greenland, the velocity of ice movement increased from
1996 to 2000, and increased further from 2000 to 2005. The extent of
melting hit a new record in 2007, and altogether the rate of melting
during the latest decade was almost twice that of the preceding decade,
see this
link. In glaciers in
mid east Greenland (Helheim and Kangerdlugssuaq) the volocity of ice
movement has increased only after 2000. In the latter glacier, the
velocity of ice flow has tripled from 2000 to 2005 (data in an article
by Rignot and Kanagaratnam, see this
link and also this).
Such
sudden
accelerations
of
glacier
flow have not yet happened further
north in Greenland, but may come if temperatures continue to increase.
The point is that IPCC has not foreseen these
events, and not included them in their projections. This means that the
projected net losses of ice from Greenland and Antarctica are already
lower than the actual losses (see this lecture
by
R.
Bindschadler). IPCC has considered the possibility that melt
water may increase ice flow by lubricating the glacier bottoms, but
they do not include this effect in their sea level projections,
and only notice that this effect may probably give an extra increase of
0.1 to 0.2 m over the century.
The point is that you may calculate what will happen
if ice discharge into the sea grows linearly with temperature rise. But
the sudden drastic accelerations in ice flow that have been observed in
Greenland after 2000 cannot be incorporated into the
mathematical models. One paper has calculated that the contribution to
sea level rise from melting Greenland ice has increased from about 0.23
mm per year in 1996 to 0.57 mm per year in 2005 (Rignot
and
Kanagaratman), but after proper reductions due to snow
accumulation in the centre of Greenland, the net contribution to sea
level rise is maybe more correctly estimated at about 0.04 mm in
2000 and 0.23 mm in 2005. Some others arrive at a contribution of 0.28
mm per year for 2003-2005 (Luthcke et al. 2006), and still others reach
estimates of up to 0.7 mm per year for this period (Chen, Wilson &
Tapley 2006; references in Cool it).
Altogether, what happens when the ice on Greenland
starts to melt is so complicated that it cannot be simulated properly
by mathematical models. This means that nobody knows for sure what will
happen. Important facts are 1) that the system does not behave linearly
- we see sudden accelerations in ice discharge. 2) The rate of ice loss
from Greenland (and Antarctica) seems already to be larger than
projected by the linear models of IPCC. The recent
acceleration of ice discharge is more than just a random fluctuation
due to unusual weather for a few years, but we do not know if it will
last - maybe it stops again when the
affected outlet glaciers have retreated from the coast line. However, a
continued acceleration of ice flow
is a definite possibility.
An important question is if the projected
atmospheric levels of CO2 will cause a temperature rise
that is sufficient to destabilise the whole inland ice on Greenland and
parts of Antarctica. Existing computer models show that a partial or
full deglaciation of Greenland may be triggered by even quite modest
future increases in CO2 (see this
link).Another important question is, if
it happens, will we have sufficient time to react? Time passes from the
point when greenhouse gases have increased in the atmosphere till the
full effect on melting of Greenland ice is seen. And from the time when
this is seen, further time passes until the political system wakes up
and reacts. The crucial question is if the effects of greenhouse gas
reductions appear soon enough to revert the unwanted melting effects.
Lomborg is sure that we have time enough to react,
should this be necessary. Others are not so sure.
RISING SEA LEVELS |