CONTRIBUTION OF A. BERGER IN CLIMATE RESEARCH
André Berger works at the frontier between many disciplines in geosciences. His training
in mathematics and in meteorology and his permanent contacts with physics and geology, led
him to become a cited pioneer of the interdisciplinary study of the Earth system dynamics and
past climate history.
1. From celestial mechanics to climate modelling
A. Berger is particularly known for his research on the astronomical theory of paleoclimates, the so-called Milankovitch theory. It is in the late 60s that he approached this
problem along three directions : the accuracy of the astronomical solution of the planetary
system, the calculation of the insolation available at the top of the atmosphere and the
modelling of climatic variations and its validation using paleoclimate records. He remains one
of the few scientists to be able to master the full multidisciplinary aspect of such a global
approach.
2. A new solution for long-term variations of the astronomical parameters
The solution that he completed in 1973 has rapidly been internationally recognized as
providing the best approximation to the long-term variations of the Earth's orbital parameters
for paleoclimatic studies during the last one million years. It is significant that the quality of
this solution makes its use worldwide since 30 years.
3. Analytical expansion and the theoretical spectra of astronomical parameters
This theoretical work led him to build original analytical developments both of the
planetary solution for the eccentricity, obliquity and climatic precession (beat between the
astronomical precession and the perihelion motion) and of the insolation. These analytical
formula allowed and continue to allow all paleoclimatologists to compute easily the numerical
values they need for their research. These theoretical expansions provided for the first time in
the 70s all frequencies which characterize the long-term behaviour of the astronomical
parameters and their origin, an important work for better understanding the spectra of
geological records and of paleoclimate modelling results. In particular, they have shown the
splitting of the 21,000-year precession period into 23,000 and 19,000 years. It is precisely the
existence of these two frequency components, found in the geological data by Hays, Imbrie
and Shackleton in 1975, which has constituted the first most delicate and impressive of all
tests for the astronomical theory.
4. An astronomical clock
The astronomical frequencies predicted in his 1977 paper were and continue to be very
often found in paleoclimate data. This and the accuracy of his solution help to calibrate the
time scale of geological records and to investigate the causes of past climatic changes at a
frequency range going from one cycle over a few thousands of years to one cycle over a few
millions of years. It is such astronomical solution which has led N. Shackleton, D. Peltier and
A. Berger in the early 90s to age the currently adopted dates for the paleomagnetic polarity
reversals over the last few millions of years, this new time scale having been confirmed since
then by independent datation using radiometric techniques.
5. Origin and instability of the astronomical 100-kyr periods
He has also contributed to show the astronomical origin of the 100,000-year periods and
their instability over the last millions of years and the frequency and amplitude modulations
of the astronomical parameters.
6. Geological calibration of the astronomical solutions
Inversely, paleoclimate data over the last tens of millions of years are now used to improve
the computation of the orbital elements of the Earth and of its rotation, an approach which
was already recommended by André Berger, André Deprit and Pierre Bretagnon in the early
80s at a symposium held in Lamont Doherty Geological Observatory, N.Y.
7. Pre-Quaternary astronomical frequencies
The astronomical theory is valid for the Quaternary, but also for more remote times of the
past (tens to hundreds of millions of years). This is why André Berger and his colleagues have
calculated the impact of the very long-term variation of the elements of the Earth-Moon
system and of the Earth's rotation on the frequencies, amplitudes and phases of the
astronomical parameters. The shortening of the periods that was predicted in the early 1990’s
has been found in paleoclimate data from the Cretaceous, in particular.
8. Daily insolation
In addition to improving the accuracy of the orbital elements, he has also extended the
Milankovitch theory. He was the first to introduce and compute the long-term variations of
daily, monthly and seasonal insolations which he has provided the users with easy manageable formula. He showed that contrary to the caloric season insolations of Milankovitch,
daily insolation varies with a much larger amplitude and that the spectral structure of his
insolation parameters is very rich in the number of frequencies.
9. One of the very first EMICs
Aware of the importance of a rigourous and adequate modeling for better understanding
climatic variations, André Berger has created in the 80s a multi-disciplinary modeling group.
Its coupled model of the atmosphere, the ocean, the sea ice, the ice sheets and the carbon
cycle has allowed to make the first simulation, ever succeeded, of the transient response of the
climate system to the astronomical forcing over the last glacial-interglacial cycles based on
basic laws of physics and in agreement with geological records. It has also simulated the most
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important characteristics of the whole Quaternary climatic variations. This type of models –
presently called Earth System Models of Intermediate Complexity – is now recognized to be a
key link between the general circulation models and the more simple models for
understanding the long-term climatic variations.
10. Our exceptionally long interglacial
Because of the atypical almost circular shape that the Earth’s orbit is approaching now,
André Berger claims that our interglacial should be much longer than the others (contrary to
what was generally thought). This is particularly critical because the atmospheric CO2
concentration is presently exceptionally high as compared to the past. These results lead him
to forebode, probably for the first time, that the impact of man’s activities on climate might
interfere with the climatic evolution also at the geological time scale. The potential that André
Berger’s work offers for analysing the range of such possible future climates is
acknowledged, in particular in assessments of the climatic stability at the repository sites for
nuclear wastes produced in atomic power plants, of the possible earlier origin of the human
impacts on climate and of the birth of the Anthropocene.
11. Feedbacks in paleoclimate models
Finally, this model, being a compromise between scientific complexity and practical
manageability, allows the members of the Louvain-la-Neuve university team to test the
sensitivity of the climate system. In particular, they emphasize the significant amplification of
water vapor in the response of the climate system to both the astronomical and greenhouse gas
forcings during the last glacial-interglacial cycles. They also underline the fundamental role of
the other feedbacks like those associated to albedo and temperature, and land surface cover
and snow field albedo. Moreover, their recent experiments show that the low frequency
changes in climate induced by the astronomical forcing is a prerequisite for the high
frequencies to be generated by the non-linearities related to land cover, sea ice and ocean
dynamics.
12. Modeling climatic variations of the Holocene
All these efforts meet very well the primary objective of André Berger which is to
understand the behaviour of the climate system at the geological time scales. But the
experience gained at the astronomical time scale has helped him and his team to build new
EMICS for studying the transient response of the climate system to the progressive increase
of the greenhouse gases concentration and sulphate in the atmosphere and to volcanic and
solar activities over the more recent times.
13. Other researches and books
André Berger has also worked on modeling air pollution and on data analysis. He has
published reviews on nuclear winter, on desertification, on global climatic impacts related to
man's activities, on climate variations and on modeling climate. He has also organised
symposia and edited books which became classical references not only on climate variations
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but also on their relationships with Society. In 1992, he has published “Le Climat de la Terre,
un passé pour quel avenir”, a clear and original synthesis of the scientific problems related to
natural and man-made climatic variations, written in responsible terms as far as the impacts
on Society are concerned.
14. Training, diffusion of knowledge and expertise
André Berger contributes also significantly to the training of research scientists, to the
stimulation of a multi- and inter-disciplinary approach of the Earth system problems, to
international collaboration, to strengthening the relationship between geophysics,
geochemistry and geosciences in general, to the diffusion of environmental science through
the public at large and to using scientific expertise in many committees.
By a correct appreciation of the numerous factors of the Earth system, André Berger
contributes significantly to answering one of the major concerns of the present civilization,
and takes a distinguished position among the large groups of climatologists, oceanographers,
bio-ecologists, geologists and geo-scientists engaged with him in this research.
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