Page 133 - Sjæklen2004

and the ocean dynamics. Today the models have reached a
state where they are able to simulate quite accurately the
present climate and its internal variability. Furthermore,
when forcing by the observed increased greenhouse gas
concentrations, solar and volcanic variations is taken into
account, the models have been successful in simulating the
observed climatic evolution during the past 150 years.
It can be concluded on the basis of climate model simu-
lations and theoretical considerations that the global war-
ming in the first half of the 20th century was most likely a
combination of increased greenhouse effect, increasing
solar activity, and internal climatic variations. It is very
likely that most of the more recent warming during the last
30 years or so is due to increased greenhouse gas concen-
trations.
Simulations with climate models have shown that the
global climate will warm some 2-5˚C during the coming
100 years. These simulations are based on so-called scena-
rios for the future evolution of greenhouse gas concentrati-
ons and other forcings.
In Denmark and the Wadden Sea area, it is likely that
temperatures by the end of the 21st century will be 3-4
degrees higher than today. Furthermore, the simulations
show that summer precipitation will decrease slightly,
while autumn and winter precipitation will increase. The
nature of precipitation will also change: when it rains it will
generally rain more, but in summer the length of the dry
spell will increase considerably. Several climate model
simulations have indicated that storm activity in the North
Sea region may increase somewhat in the warmer climate.
This increase is related to a general change in atmospheric
pressure resulting in more westerly winds in winter in
north-western Europe. This is somewhat analogous, but not
identical, to a general increase in the NAO index. If the
magnitude of the strongest winds in the North Sea increa-
ses by up to 5-10% as suggested by regional climate model
simulations, one immediate consequence is that the Wadden
Sea Region may suffer higher storm surges.
There are three main types of uncertainties in the esti-
mates of future global warming. The first is related to
uncertainties in the emissions of greenhouse gases and
small particles, and to the lifespans of these gases in the
atmosphere. The second, which is as large or larger, is
related to uncertainty in the sensitivity of climate, i.e. how
much the climate will warm as a result of the increased
greenhouse gas concentrations. The third is related to natu-
ral climate variations: we know that internal climatic varia-
tions were very large during the last ice age, although they
have been small for the last 10,000 years. If such variations
- possibly triggered by the global warming itself - are ini-
tiated, they may amplify or reduce the global warming. The
same can be said of possible large but unpredictable future
changes in the solar forcing of climate. It seems, however,
unlikely that future solar forcings will be as large as future
anthropogenic forcings.
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