304
affect the tropical climate on timescales of several years and fluctuations
in the thermohaline circulation which are common in the North Atlantic
on even longer timescales. Bryan (1986) has shown, by using an idealized
model of the Atlantic Ocean and a given forcing of the circulation with fresh
water fluxes, that the ocean can develop at least two completely different
stable sets of circulation depending on the initial state. One of these states
is characterized by the present vigorous North-Atlantic circulation with a
strong northward transport of heat, the other state has practically no circulation at all and hardly any heat transport. Whether major changes in
the thermo-haline circulation will take place on timescales of centuries and
shorter is hardly likely, although a major melting of Arctic ice or increase
in freshwater due to atmospheric processes such as increased precipitation
cannot be ruled out. Other examples of long term fluctuations are the
draughts in the Sahel region. When such draughts are established, they
have a tendency of becoming very long-lasting due to the strong feedback
between local evapotranspiration and convective precipitation, convective
precipitation being the dominating precipitation mechanism in this part
of the world. At higher latitudes such feedbacks can enhance and prolong
a dry summer, but are generally broken during the winter when the large
scale synoptic circulation can transport moisture over long distances. In
conclusion, the variation in climate during the last century or so can by
and large be explained by natural internal variations.
4
Modelling climate change
The complexity of climate processes and the many interactions and feedbacks in the climate system require a consistent quantitative approach.
Series of different models have been developed over the years from simple
conceptual ones to full scale 3-dimensional models. For an in-depth evaluation of climate models the reader is referred to available textbooks such
as Trenberth (1992). The models which best hold the promise of being
able to deal with the complexity of the climate system are the comprehensive 3-dimensional global coupled models, incorporating the interaction
between the atmosphere, oceans, cryosphere and the land surfaces. These
are the only models which can reproduce realistically the characteristics
of climate variability in time and space and the many feedback processes
of the climate system. Dynamical processes in the atmosphere and ocean
must be properly resolved, and physical processes and exchange mecha-
affect the tropical climate on timescales of several years and fluctuations
in the thermohaline circulation which are common in the North Atlantic
on even longer timescales. Bryan (1986) has shown, by using an idealized
model of the Atlantic Ocean and a given forcing of the circulation with fresh
water fluxes, that the ocean can develop at least two completely different
stable sets of circulation depending on the initial state. One of these states
is characterized by the present vigorous North-Atlantic circulation with a
strong northward transport of heat, the other state has practically no circulation at all and hardly any heat transport. Whether major changes in
the thermo-haline circulation will take place on timescales of centuries and
shorter is hardly likely, although a major melting of Arctic ice or increase
in freshwater due to atmospheric processes such as increased precipitation
cannot be ruled out. Other examples of long term fluctuations are the
draughts in the Sahel region. When such draughts are established, they
have a tendency of becoming very long-lasting due to the strong feedback
between local evapotranspiration and convective precipitation, convective
precipitation being the dominating precipitation mechanism in this part
of the world. At higher latitudes such feedbacks can enhance and prolong
a dry summer, but are generally broken during the winter when the large
scale synoptic circulation can transport moisture over long distances. In
conclusion, the variation in climate during the last century or so can by
and large be explained by natural internal variations.
4
Modelling climate change
The complexity of climate processes and the many interactions and feedbacks in the climate system require a consistent quantitative approach.
Series of different models have been developed over the years from simple
conceptual ones to full scale 3-dimensional models. For an in-depth evaluation of climate models the reader is referred to available textbooks such
as Trenberth (1992). The models which best hold the promise of being
able to deal with the complexity of the climate system are the comprehensive 3-dimensional global coupled models, incorporating the interaction
between the atmosphere, oceans, cryosphere and the land surfaces. These
are the only models which can reproduce realistically the characteristics
of climate variability in time and space and the many feedback processes
of the climate system. Dynamical processes in the atmosphere and ocean
must be properly resolved, and physical processes and exchange mecha-