The Earth's climate is changing. This is a
conclusion of the Third Assessment Report
published by the International Panel on Climate
Change. The increasing concentrations of
greenhouse gases and sulphate aerosols alter the
energy path through the atmosphere. In the
future, a likely consequence of the induced
global warming is an increased melting of the
Greenland ice sheet. This could lead to abrupt
climatic modifications associated with the
collapse of the thermohaline circulation in the
North Atlantic.
To investigate this issue, a coupled model of
the climate system has been developed. Most
components of this system are taken into account
by this model. LMD5, an atmospheric general
circulation model originating from the
Laboratoire de Météorologie Dynamique in Paris,
simulates the atmosphere. CLIO2, an oceanic
general circulation model set up at the Institut
d'Astronomie et de Géophysique G. Lemaître at the
UCL, accounts for the ocean and the sea ice.
Finally, GISM is a Greenland ice-sheet model
developed at the Vrije Universiteit Brussel (VUB).
The atmospheric and oceanic components had
already been coupled. A new
version has been elaborated during this thesis
to enable long-term realistic simulations. To
restrain the initial drift, adjustements have
been made in atmospheric, oceanic and sea-ice
parameters in collaboration with the research
teams that set up the models. Other modifications
have been performed in the framework of climate
change experiments to separately handle greenhouse
gases and sulphate aerosols. Finally, LMD5-CLIO2
has been coupled to GISM in collaboration with
VUB researchers.
Validation of LMD5-CLIO2 implies a 150-year long
control simulation under constant 1970 forcings.
The validation is twofold: on the one hand, the
LMD5-CLIO2 results have been contrasted with
observational estimates and on the other hand,
these have been compared to other coupled models
results. This leads to the conclusions that
LMD5-CLIO2 simulates relatively well the present-
day climate and that it performs as well as many
other coupled models.
Two climate change experiments have been carried
out using the IPCC SRES B2 scenario. The first
one deals with the impact of greenhouse gases
and sulphate aerosols on the 21st century climate.
The globally averaged warming reaches 2.4°C at
the end of the 21st century compared to 1970. The
hydrological cycle is amplified: precipitation
increases by 3.3 % over the same period. This
corroborates the projections of other coupled
models. Consequently, it improves our confidence
in the LMD5-CLIO2 simulations. The second climate
change experiment performed with LMD5-CLIO2-GISM
investigates the impact of the Greenland ice-sheet
freshwater flux on the North Atlantic
thermohaline circulation. To our knowledge, it is
the first time that such a complex coupled model
is used to analyse this issue. In this simulation,
deep convection in the North Atlantic ocean
shuts down at the end of the 21st century, which
is an unusual result. The strong and abrupt
reduction of the intensity of the thermohaline
circulation implies significant mainly local
modifications of sea ice and of the atmosphere.
Cautions should be exercised to this projection.
Firstly, LMD5-CLIO2 has deficiencies which
probably influence the results. Secondly,
modifications of the initial conditions could
lead to another projection: such spectacular
event not occurring or differences as to when it
happens. Consequently, other ensemble simulations
should been performed. Thirdly, the current
simulation is too short to know if the reduction
is temporary or not. Longer experiments are
therefore required.
| Identifer | oai:union.ndltd.org:BICfB/oai:ucl.ac.be:ETDUCL:BelnUcetd-08162003-120351 |
| Date | 08 September 2003 |
| Creators | Poncin, Chantal |
| Publisher | Universite catholique de Louvain |
| Source Sets | Bibliothèque interuniversitaire de la Communauté française de Belgique |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://edoc.bib.ucl.ac.be:81/ETD-db/collection/available/BelnUcetd-08162003-120351/ |
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