Actualité scientifique
Scientific news
Actualidad científica
N° 440
September 2013
Ocean and climate: the new theory
(© IRD / C. Costantini) Ngor in Senegal.
Is this the end of a scientific paradigm on the effects of climate change? It would seem
that global warming is not intensifying the rise of cold deep water, a phenomenon known
as ‘upwelling’, characteristic of certain coastal zones. For over twenty years, specialists
have believed that climate change is reinforcing the trade winds at the origin of these
phenomena, thereby cooling the surface water. A new study, led by a team from the IRD
and its partners, off the coast of West Africa, has shown that this is not the case for the
ecosystem of the ‘Canary Current’. On the contrary, it reveals that the coastal waters
from Morocco to Senegal have been getting warmer for the past 40 years.
Good to know
Four major upwelling ecosystems are found on the Western edge of the four great continents: the Canary Current for North and West Africa,
the Benguela Current for South Africa, the Humboldt Current for Latin America, and the California Current for North America. They represent
around 5 % of the surface of the oceans and provide 20 % of global fish catches, mainly sardines and anchovies.
Glossary
Upwelling : Cold oceanic deep water, rich in nutritive substances, rises to the surface due to the action of the trade winds combined with the
rotation of the Earth.
Trade winds : The trade winds are winds in the inter-tropical regions blowing from East to West.
Plankton : Plankton refers to all, generally microscopic, vegetal organisms (phytoplankton) and aquatic animals (zooplankton), which are swept
along in the ocean’s currents.
CONTACTS
Ecosystems at the heart of concerns
Assessing the effect of climate change on
upwelling ecosystems is essential to be able
to predict the future of marine resources. The
zones concerned by this upwelling of cold deep
water, which is very rich in nutrients, provide
up to 20 % of global production of fish. Since
the 1990s, the theory adopted by the majority
of the scientific community affirmed that these
phenomena were intensifying. The rising temperatures of the air masses above the continents
were expected to quicken the trade winds, which
would in turn increase the upwellings, thereby
cooling the surface water. But this theory has
been contradicted by the recent work of researchers from the IRD and its partners.
The reaction of the coastal ecosystems to cli-
Coastal waters are getting considerably warmer
In their new study, led off the coast of North and
West Africa, the scientists reviewed the wind
measurements taken over the past 40 years and
the data of the meteorological models along the
Spanish and West African coastline, and discovered that they do not show an acceleration
of the wind on a regional scale that would be
likely to significantly cool the coastal waters. In
fact, quite the opposite is true, since the satellite
images and in situ measurements of the surface
water temperature show a distinct upward trend
in the temperature for the entire zone, at a rate of
1°C per century. These new findings contradict
the hypothesis that the upwelling of the Canary
Current is intensifying.
all these questions.
depends greatly on local specificities – other
upwelling systems, such as that of the Califor-
Gaëlle Courcoux
Information and Culture
nia Current, clearly show a trend of intensification and cooling of the water in recent decades.
Department
At the level of the ecosystem itself, the effects
Tel: +33 (0)4 91 99 94 90
of the warming of the surface waters can be
antagonistic: it can for example encourage the
growth of fish larvae, but also increase the temperature gradient between the surface water and
the deeper water and thereby modify the food
Fax: +33 (0)4 91 99 92 28
fichesactu@ird.fr
www.ird.fr/la-mediatheque
chain, etc. Researchers will now have to address
Partners
The Spanish National Research Council (CSIC)
and the University of Hawaii in the USA.
Media Contact
References
E.D. BARTON, D.B. FIELD, CLAUDE ROY. Canary
current upwelling: More or less? Progress in
Cristelle DUOS
Tel: +33 (0)4 91 99 94 87
Oceanography, 2013, 116, 167–178.
http://dx.doi.org/10.1016/j.pocean.2013.07.007
presse@ird.fr
Contacts
Claude Roy, researcher at the IRD
Tél. : +33 (0)2 98 22 45 00
claude.roy@ird.fr
Indigo, IRD Photo Library
Laboratoire de physique des océans - (LPO)
Daina Rechner
(UMR IRD / CNRS / Ifremer / université de
Bretagne occidentale)
Des Barton, researcher at the Instituto de
Tel: +33 (0)4 91 99 94 81
indigo@ird.fr
Investigaciones Marinas (CSIC)
Tél: +34 (0)986 231930 Ext 368
e.d.barton@iim.csic.es
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© IRD/DIC, Septembre 2013 - Conception et réalisation graphique : L. CORSINI
A reinterpretation of the paleoclimatic data
Until now, the study of this ecosystem focused
primarily on paleoclimatic reconstructions based
on samples of marine sediments. According to
the geochemical analysis of these samples, planktonic organisms have evolved in an increasingly
cold environment over the last few decades. This
led scientists to conclude that the temperature
of the surface water was dropping. But in view of
the new findings, the oceanographers have put
forward another explanation: the thermal signal
deduced from the paleoclimatic data is due to a
progressive migration of plankton towards the
depths because, on the contrary, the surface
water is getting warmer!
mate change remains complex, because it
Coordination