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Researchers François-Marie
Bréon and Sylvia Generoso ("Laboratoire des sciences du climat
et de l'environnement," Laboratory of Climate and Environment Sciences,
a joint CEA-CNRS research unit)(1) and Didier Tanré
("Laboratoire d'optique atmosphérique," Atmospheric Optics
Laboratory, USTL/CNRS)(2) have observed that aerosols
— particles in suspension in the atmosphere — act on the characteristics
of clouds. This phenomenon had already been brought to light by local
measurements. It has now been verified on a global scale, by means of
the POLDER (polarization and directionality of the Earth's reflectances)
instrument(3), developed with the CNES (French Space
Research Center). The results obtained by the researchers from the CEA
(French atomic energy authority) and the CNRS are published in the February
1, 2002 issue of Science.
Through research into the impact
of human activity on the climate, we are well acquainted with the greenhouse
effect, which is due essentially to carbon dioxide, but also to some other
gases, such as methane and CFCs(4) . However, considerable
uncertainty remains as to the effect of aerosols. Today, it is known that
they have two types of action on the radiative balance of the Earth.
A direct radiative effect, which is relatively well known, and which
depends both on the quantity of the aerosols, and also on their optical
properties. Aerosol particles diffuse sunlight and thus send a portion
of the radiation back into space before it can contribute to warming the
Earth.
An indirect radiative effect, which acts via clouds, and which is difficult
to quantify. Aerosols serve as "cloud condensation nuclei" for
cloud formation: the droplets of water that make up clouds form around
these particles, which are always present in the atmosphere, at various
concentrations. In an atmosphere that is heavily loaded with aerosols,
the same quantity of water can be distributed over a larger number of
droplets, which are then smaller in size. A cloud formed by a larger number
of smaller droplets reflects a higher proportion of the solar radiation
back into space. In addition, it is reasonable to think that such small
droplets give rise less easily to precipitation, thereby increasing the
life span of the cloud. The researchers from the LSCE and LOA have shown
that this effect on droplet size is significant over the entire Earth.
The effect is very significant for the cleanest atmospheres — only
a small quantity of aerosols is necessary to reduce droplet size considerably—
and it then saturates quickly.
Furthermore, the results of the study show that it is essentially human
activity (aerosols of anthropogenic origin, coming from farming, industry,
household activity, and transport) that generates the observed effect
on clouds.
These results were obtained from measurements taken by the POLDER sensor
on board the Japanese Advanced Earth-Observing Satellite (ADEOS) in 1996-1997.
This sensor was designed by French researchers from the CNRS and the University
of Lille and manufactured by the CNES ("Centre national d'études
spatiales," the French Space Research Center). It will also be part
of the payload of ADEOS2 which is scheduled to be launched at the end
of 2002.
(1)
The "Laboratoire des Sciences du Climat et de l’Environnement"
is a joint CEA-CNRS research unit. The aim of the laboratory is to observe,
understand, and model the processes that interact with climate, such as
the carbon cycle or the aerosol cycle.
(2) The "Laboratoire d'Optique Atmosphérique"
(LOA) is a joint research unit (Université des sciences et technologies
de Lille, University of Science and Technology of Lille, USTL, and CNRS).
The work conducted at LOA is part of overall climate research. It is aimed
at observing, understanding, and modeling the processes of interaction
between clouds, aerosols, and radiation.
(3) http://polder-mission.cnes.fr
(4)
Chlorofluorocarbons: these gases are entirely man-made, and they are responsible
for the increase in the famous hole in the ozone layer. Banned by the
Montreal Protocol, they have been replaced with other gases, which themselves
give rise to an additional greenhouse effect.
CEA researcher
contact:
François-Marie BRÉON
Laboratoire des sciences du climat et de l'environnement
Tel: +33 1 69 08 94 55
e-mail: fmbreon@cea.fr
CNRS researcher contact:
Didier TANRÉ
Laboratoire d’optique atmosphérique
Tel: +33 3 20 33 70 33
e-mail: Didier.Tanre@univ-lille1.fr
CEA press contact:
Alexandra BENDER
Tel: +33 1 40 56 17 16
e-mail: alexandra.bender@cea.fr
CNRS press contact:
Magali SARAZIN
Tel: +33 1 44 96 46 06
e-mail: magali.sarazin@cnrs-dir.fr
INSU communications contact:
Hélène DOCO
Tel: +33 1 44 96 43 74
e-mail: helene.doco@cnrs-bellevue.fr
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