Pourrait-on revoir des périodes de
Les variations du climat font
intervenir différentes échelles de temps.
On sait quil existe des périodes régulières de glaciation (cycle
de Milankovitch), avec des cycles denviron 125 000 ans et ce depuis
plus dun million dannées. Il ny a donc pas de raison logique
pour que cette périodicité ne continue pas.
A plus court terme, la question est qualitativement différente, car les
modifications peuvent être liées à lactivité de lhomme.
Une chose est sûre, le
climat de la Terre a vu se succéder des périodes glaciaires et
interglaciaires. Les variations des caractéristiques de l'orbite
terrestre autour du soleil modifient la distribution de l'ensoleillement
à la surface du globe et entraînent des changements climatiques
suffisamment importants pour être enregistrés dans les glaces encore
présentes à la surface terrestre.
Estimation des températures
à la surface du globe depuis 250.000 ans
(Source : Dansgaard
et al., Nature, 15 July 1993)
L'analyse de carottes
glaciaires, comme ici au centre du Groenland, permet de reconstituer une
estimation de la température à la surface du globe (on parle de "proxy")
depuis plus de 250.000 ans. L'échelle verticale représente une température,
plus chaude vers le haut, variant peut-être de plusieurs degrés des périodes
glaciaires à la période actuelle. Depuis 10.000 ans, fin du dernier
age glaciaire, les variations de température sont relativement faibles
comparées aux 250.000 ans précédents. Il semblerait donc que le
climat de la Terre n'ait jamais été aussi stable que depuis le dernier
maximum glaciaire !
Les raisons :
Cela est du à
: les précipitations ont augmentées de 20% depuis 1970
d'humidité (El Nino)
solaire a un lien direct avec un age glacière
actuelle du tapis roulant serait au bord de l'instabilité
petite augmentation des apports d'eau douce à l'est de l'Atlantique
Nord suffirait pour bloquer la circulation en tapis roulant.
Si le réchauffement global entraîné par le renforcement de l'effet de
serre conduit à une augmentation des précipitation sur la mer de Norvège,
ou à des fontes de glace aux marges du Groenland, risquons-nous un
refroidissement en Europe ?
Méditerranée, bénéficiant d'un fort ensoleillement qui stimule l'évaporation,
est, on le sait, plus salée que l'Atlantique. Ses eaux salées et
denses se déversent dans l'Atlantique au seuil de Gibraltar, cette
perte étant compensée par un contre-courant de surface faisant entrer
des eaux atlantiques dans la Méditerranée. Si la salinité des eaux méditerranéennes
augmente, suite à la réduction par le barrage d'Assouan des apports
d'eau douce du Nil et à une intensification éventuelle de l'évaporation,
faut-il craindre qu'une sortie intensifiée d'eau méditerranéenne vers
l'Atlantique perturbe le système du tapis roulant ? Un géophysicien américain
prône la construction d'un barrage à Gibraltar afin d'empêcher un
retour prématuré des glaces. Faut-il le prendre au sérieux ?
: "L'incertitude des climats" par Robert KANDEL
catastrophes d'une rapidité et d'une violence inouies
variations brusques apparaissent, dues à des accumulations de poussières
et de minéraux, signes de coup de froids violents.
: - 115 000 ans chute de la T° de 14° pendant 70 ans
: - 131 000 ans chute de la T° de 10° pendant 750 ans
La calotte polaire descendait jusqu'à Londres...)
cause de ces changement se trouve-t-elle dans les océans ?
relevés fin 2001
La tendance récente au réchauffement
Une difficulté majeure pour décrire les variations du climat,
c'est de mettre en évidence des indicateurs simples et représentatifs
: la température moyenne de l'air à la surface du globe par exemple.
Pourtant, il y a peu de chances que l'évolution de la température de
l'air en un endroit donné suive les mêmes variations.
Évolution de la température moyenne de l'air à la surface du
globe depuis 1856
(Source : Climatic Research Unit, University of East Anglia, UK)
Cette figure représente l'évolution de la température moyenne
de l'air à la surface du globe de 1856 à 1999, par rapport à la
moyenne de 1961 à 1990. Elle combine des enregistrements de température
à terre et en mer analysés conjointement par l'Unité de Recherche
Climatique et le Centre Hadley en Grande Bretagne. Un réchauffement du
climat est très net des années 1910 à 1940, et depuis 1975.
L'augmentation de la concentration des gaz à effets de serre dans
l'atmosphère, dû aux activités industrielles, en est la cause la plus
probable. En effet, ce réchauffement des températures au XXeme siècle
succède à une longue période de lent refroidissement du climat
terrestre, à en croire les reconstructions des températures au cours
des 1000 dernières années. Les mécanismes responsables de ces
changements sont certainement : les variations de l'insolation, la fréquence
et l'intensité des éruptions volcaniques, et les changements de la
composition atmosphérique résultants des activités humaines. Les
variations naturelles dominent jusque vers 1850, mais seules les activités
humaines permettent d'expliquer le réchauffement net du XXeme siècle.
Température moyenne dans l'hémisphère nord au cours du dernier
(Source : Mann et al. 1999)
Cette figure montre l'évolution de la température atmosphérique
de surface moyenne sur l'hémisphère nord depuis 1000 ans. Il apparaît
clairement que l'amplitude du récent réchauffement est significative
et succède à plus de 900 ans de lent refroidissement du climat
Quel sera le climat de demain, cela dépendra certainement de
notre capacité à modérer nos émissions de gaz carbonique. À l'aide
de modèles numériques du climat, les centres de prévisions
climatiques essayent d'estimer le réchauffement pour les prochaines décennies
suivant plusieurs scénarios de rejets de gaz carbonique... il est
probable que la Terre continuera à se réchauffer encore quelques temps
Prévisions du réchauffement global par les modèles climatiques
Évolution de la température moyenne à la surface du globe : en
vert, les observations de 1860 à 2000 ; en noir, les résultats d'un
modèle numérique sans modification de la composition atmosphérique en
gaz à effet de serre (c'est la simulation de contrôle pour s'assurer
que le modèle reproduit bien un climat stable) ; en bleu et rouge, les
résultats de deux simulations du modèle numérique en modifiant la
composition atmosphérique en gaz à effet de serre telle qu'elle a été
observée jusqu'à maintenant et telle qu'elle est prévue suivant
Variations géographiques du réchauffement
Les tendances climatiques observées depuis le début du siècle
varient largement d'une région du globe à l'autre, par exemple l'hémisphère
sud semble se réchauffer plus lentement que l'hémisphère nord (les
pays produisant actuellement le plus de gaz carbonique se trouvent
principalement dans l'hémisphère nord et le mélange des propriétés
atmosphériques d'un hémisphère à l'autre prend plusieurs années
Anomalies de température par rapport à 1961-1990
(Source : Climatic Research Unit, University of East Anglia, UK)
Variation de la température par rapport à la
(Source : A. Moberg et G. Demaree, La recherche, juin 1999)
Les fluctuations de la température depuis qu'on la
mesure. En haut, les variations des moyennes annuelles de l'ensemble de
la planète entre 1856 et 1998, au milieu de l'Europe entre 1781 et
1998. Ces deux courbes expriment les écarts de température par rapport
à une période de référence 1961-1990. En bas, la série la plus
longue, reconstituée à partir des mesures mensuelles réalisées
depuis 1659, donne la valeur absolue des températures, l'horizontale
indiquant la référence 1961-1990.
Structure spatiale du réchauffement
(Source : données de P. Jones, Climatic Research Unit, UK)
À partir de la base de données de température
atmosphérique à la surface du globe de P. Jones, il est facile de
calculer les tendances sur les 100 dernières années pour différentes
régions du globe. On voit immédiatement une large disparité des
tendances, tantôt positives (jusqu'à 3°C par siècle au centre des
continents nord-américains et européens), tantôt faiblement négatives,
mais il y a aussi de larges régions du globe où il n'y a pas assez de
mesures pour estimer ce réchauffement.
Structure saisonnière du réchauffement
(Source : A. Moberg et G. Demaree, La recherche, juin 1999)
A l'échelle de l'Europe, l'évolution des températures de 1891
à 1990 révèle une grande disparité régionale et saisonnière. Les
hivers plus chauds, surtout en Europe de l'Est, expliquent une grande
partie du réchauffement global. La carte des variations estivales
montre que les étés se réchauffent moins que les hivers. En Europe
centrale, ils se sont même refroidis d'environ 0,5°C (d'après Schönwiese
et Rapp, 1997).
Réchauffement de l'océan et niveau de la mer
Le réchauffement des basses couches de l'atmosphère s'accompagne
d'un réchauffement progressif des couches supérieures de l'océan. Ce
réchauffement est beaucoup plus lent car l'océan a une inertie
thermique environ 1000 fois plus importante que l'atmosphère.
Réchauffement de l'océan Atlantique
(Source : S. Levitus, Science, 2000)
Évolution du contenu thermique (10²² Joules, échelle de
gauche) et de l'anomalie de température moyenne équivalente (échelle
de droite) des 300 premiers mètres de l'océan Atlantique depuis 1948.
Associé à ce réchauffement de l'océan correspond une
augmentation de son volume par dilatation thermique. Cet effet contribue
avec la fonte des glaciers alpins à l'augmentation du niveau de la mer
observé, environ 15 à 20 cm au cours du dernier siècle.
Augmentation du niveau de la mer : observations et prévisions
À gauche, le niveau de la mer mesuré depuis un siècle (moyenne
annuelle en bleu, moyenne courante sur 5 ans en rouge). À droite, les
prévisions d'augmentation du niveau de la mer au cours du siècle
prochain pour différents scénarios d'évolutions des gaz à effet de
serre, avec des concentrations d'aérosols variables (lignes continues)
ou fixées à leur niveau de 1990 (pointillés).
(Source : R. Kerr, Science, 2000)
Deux états de l'océan
: l'analyse des anneaux des arbres, dont la croissance enregistre les
variations climatiques, permet de reconstruire de longues séries de
température en de nombreux points du globe. Ces données suggèrent que
l'Atlantique Nord oscille entre une phase froide (en haut) et une phase
chaude (en bas) sur une période de 60 à 80 ans. On suppose
actuellement que ce mode est associé aux variations de la circulation
océanique thermohaline, c'est-à-dire forcée par les flux de chaleur
et d'eau douce à la surface (par opposition aux grands tourbillons océaniques
forcés par le vent).
Évolution des températures
dans l'océan Atlantique
(Source : D. Hansen et H. Bezdek, NOAA, USA)
Anomalies de température
de surface dans l'océan Atlantique Nord (chaudes en rouge, froides en
bleu) de 1949 à 1996. On voit des conditions plutôt froides avant
1951, une longue période chaude de 1951 à 1967, et à nouveau une période
froide de 1968 à 1977. Après 4 années sans anomalie notable, les années
1983-1986 sont modérément fraîches. Des conditions chaudes dominent
depuis 1987. Une certain nombre d'anomalies de grande échelle
(typiquement 2000 km), persistant pendant plusieurs années, sont
identifiables facilement : elles se déplacent généralement en suivant
la circulation des grands tourbillons subtropicaux et subpolaires, à
des vitesses de quelques km par jour. (Notez que de nombreux filtrages
sont nécessaires pour mettre en évidence ces anomalies.)
(Source : R. Sutton
et M. Allen, Nature, 1997)
Peut-on prédire les
températures de surface dans l'océan Atlantique Nord ? Une analyse
statistique suggère une propagation des anomalies de température de
surface océanique à travers l'Atlantique Nord de la côte américaine
au nord de l'Angleterre, en une dizaine d'années. Les couleurs
correspondent au champ de température de surface moyen en hiver de 1945
à 1989 (échelle en degrés Celsius). Les contours délimitent les régions
où les correlations entre les variations basse-fréquence de la température
hivernale locale et la température de la région 80-60°W 31,5-38,5°N
(près de Cap Hatteras) sont les meilleures pour différents délais
("lag") de 0 à 9 ans (les contours correspondent à une corrélation
de 0,8 pour des lags de 0 à 8 ans, et de 0,75 pour 9 ans).
transport océanique relié à l'oscillation nord-atlantique (NAO)
(Source : R. Curry et M. McCartney, WHOI, Woods Hole, USA)
Cette figure représente
les variations lentes de l'indice NAO d'hiver (en rouge pour les indices
positifs et en bleu pour les indices négatifs), les variations de la
température des eaux profondes formées en mer du Labrador (en vert,
sur l'échelle de droite), et les variations du transport vers l'est du
Gulf Stream et de la dérive nord-atlantique (en noir, sur l'échelle de
gauche), estimées à partir des différences d'anomalies d'énergie
potentielle entre la mer du Labrador et les Bermudes (un analogue océanique
de l'indice NAO atmosphérique).
des températures avant 1970 (indice NAO négatif) et leur
refroidissement postérieur (indice NAO positif) se reflète aussi dans
la température de surface des eaux subpolaires. Les changements de
transport océanique semble suivre les changements de l'indice NAO de 4
à 5 ans : le transport diminue en même temps que les eaux de la mer du
Labrador se réchauffent et que l'indice NAO diminue pendant les décennies
1950 et 1960. Le transport océanique augmente à nouveau avec le
refroidissement des eaux du Labrador (et plus généralement des eaux
subpolaires) lorsque l'indice NAO augmente à nouveau au cours des décennies
1970, 1980 et 1990. Ces changements de température de 0,8°C pour les
eaux subpolaires, et les changements de circulation de plus de 30%,
montrent que l'océan participe activement dans ces oscillations
nord-atlantique atmosphériques et océaniques." R. Curry et M.
McCartney, WHOI, Woods Hole, USA.
La circulation océanique
dans l'Atlantique Nord
(Source : M.
McCartney, WHOI, Woods Hole, USA)
Pour en savoir plus
Crowley, T. J., 2000 :
Causes of climate change over the last 1000 years. Science, 289,
Dansgaard, W., et al., 1993 : Evidence for general instability of past
climate from a 250-kyr ice-core record. Nature, 364, 218-220.
Hansen, D. V., et H. F. Bezdek, 1996 : On the nature of decadal
anomalies in North Atlantic sea surface temperature. Journal of
Geophysical Research,101, 8749-8758.
Kerr, R. A., 2000 : A North Atlantic climate pacemaker for the
centuries. Science, 288, 1984-1986.
Levitus, S., J. I. Antonov, T. P. Boyer, and C. Stephens, 2000 : Warming
of the world ocean. Science, 287, 2225-2229.
Mann, M. E., R. S. Bradley, et M. K. Hughes, 1999 : Northern hemisphere
temperatures during the past millenium: inferences, uncertainties, and
limitations. Geophysical Research Letters, 26, 759-762.
Moberg, A., et G. Demaree, 1999 : Ce que nous apprennent les thermomètres.
La recherche, 321, juin 1999.
Schönwiese, C.-D., et J. Rapp, 1997 : Climate trend atlas of Europe
based on observations 1891-1990. Kluwer Academic Publishers, 228 pp.
Sutton, R. T., et M. R. Allen, 1997 : Decadal predictability of North
Atlantic sea surface temperature and climate. Nature, 388, 563-567.
Levitus, S., J. I. Antonov, T. P. Boyer, et C. Stephens, 2000 : Warming
of the world ocean. Science, 287, 2225-2229.
Research Unit Information sheets
of weather, climate and climate change
sur le front des glaciers
déplaise aux théoriciens du réchauffement de la planète, les
glaciers auraient repris leur croissance.
par Frédéric Lewino
En glaciologue perd
rarement son sang-froid, mais celui-ci est vraiment très énervé.
Robert Vivian a décidé de partir en guerre contre ces prophètes de
malheur qui accusent l'effet de serre d'entraîner, déjà, une fonte
généralisée des glaciers sur Terre. « C'est complètement faux,
seuls un certain nombre de petits glaciers résiduels, situés en marge
des zones de glaciation, reculent. » Au nom de tous ses confrères
glaciologues, cet ancien patron du laboratoire de la montagne alpine du
CNRS à Grenoble s'en prend à tous ces experts en chambre qui ne vont
pas toujours sur le terrain : « Ils s'intéressent le plus souvent aux
langues des glaciers, facilement accessibles, et méconnaissent
l'essentiel du glacier, qui, lui, est en altitude. »
Et d'affirmer que la
seconde moitié du XXe siècle a été marquée par une stabilisation,
voire une avancée, des glaciers plutôt que par un recul ! « Certes,
depuis 1990, un très léger recul a pu être observé dans les Alpes,
mais depuis 1998 on note une reprise sur le front des Bossons, un
glacier du massif du Mont- Blanc dont le comportement sert de sonnette
d'alarme des changements climatiques », explique-t-il, avant de se
demander, avec une pointe d'humour, s'il ne faudra pas bientôt parler
d'un retour de l'ère... glaciaire.
cette page à surtout ne pas louper !
l'Internet : http://virtedit.online.fr/article
Le 14 avril
Auteurs : Eduardo Costa <firstname.lastname@example.org>
& Thierry Huck <email@example.com>
Contenu relu par Bruno Blanke et Alain Colin de Verdière,
Laboratoire de Physique des Océans
(Unité Mixte de Recherche CNRS IFREMER
POSSIBLE "ICE AGE" IN NEAR
Written September 6, 2002
by Joe D'Aleo
Chief WSI/INTELLICAST Meteorologist
Wood's Hole Oceanographic Institute
scientists have theorized that global warming induced by natural
human factors could actually bring
colder temperatures to some highly populated areas like Eastern
North America and Western Europe.
In the North Atlantic, an increasing
amount of fresh water, perhaps coming from melting ice in the
Arctic, has been accumulating and lowering the salinity of the
ocean for the past 30 years. Fresh water in the ocean can upset
the ocean currents that are the key to our planets climate
In February, WHOI oceanographers
presented new evidence that this northern freshwater buildup may
be approaching the threshold where it could alter currents in a
way that would cause an abrupt drop in average winter temperatures
of about 5 degrees Fahrenheit over much of the United States and
10 degrees in the Northeast. This change could happen within a
decade and persist for hundreds of years. The dramatic and abrupt
cooling would be especially noticed throughout the North Atlantic
region of the United States and Europe where some 60 percent of
the worlds economy is based. Read more below!
A perspective on potential
climate changes presented by Dr. Robert B. Gagosian,
President and Director of Woods Hole Oceanographic
Over the past two decades, we have heard about greenhouse
gases and the idea that our planet is gradually warming.
Id like to throw a curveball into that thinkingspecifically
the gradually warming part.
This new thinking is little known and scarcely appreciated
by policymakers and world and business leadersand even by
the wider community of natural and social scientists. But
evidence from several sources has amassed and coalesced over
the past 10 to 15 years. It points to a completely differentalmost
Global warming could actually lead to a big chill in some
parts of the world. If the atmosphere continues to warm, it
could soon trigger a dramatic and abrupt cooling throughout
the North Atlantic regionwhere, not incidentally, some 60
percent of the worlds economy is based.
When I say dramatic, I mean: Average winter
temperatures could drop by 5 degrees Fahrenheit over much of
the United States, and by 10 degrees in the northeastern
United States and in Europe. Thats enough to send
mountain glaciers advancing down from the Alps. To freeze
rivers and harbors and bind North Atlantic shipping lanes in
ice. To disrupt the operation of ground and air
transportation. To cause energy needs to soar exponentially.
To force wholesale changes in agricultural practices and
fisheries. To change the way we feed our populations. In
short, the world, and the world economy, would be
And when I say abrupt, I mean: These changes could
happen within a decade, and they could persist for hundreds
of years. You could see the changes in your lifetime, and
your grandchildrens grandchildren will still be
And when I say soon, I mean: In just the past year, we
have seen ominous signs that we may be headed toward a
potentially dangerous threshold. If we cross it, Earths
climate could switch gears and jump very rapidlynot
gradually into a completely different mode of operation.
is not something new under the Sun. It has happened
throughout Earths history, and it could happen again.
schematic of the ocean circulation system, often
called the Great Ocean Conveyor, that transports
heat throughout the world oceans. Red arrows
indicate warm surface currents. Blue arrows indicate
deep cold currents. (Animation by Jack Cook, WHOI)
The key to these climate shifts is that Earths climate is
created and maintained by a dynamic system of moving,
interacting parts. Earths climate system has two main
components. The first one you are all familiar with by
watching your local TV meteorologist or The Weather Channel.
It is the atmosphere, which circulates heat and moisture
around the globe. But, in fact, the atmosphere redistributes
only about half of the energy that the earth receives from
The other half is transported around our planet by a
circulation system that is equally important, but far less
understoodthe ocean. The ocean isnt a stagnant bathtub.
It circulates heat around the planet like the heating and
cooling system in your house. The atmosphere and oceans are
equal partners in creating Earths climate. The atmosphere
is a rabbit. It moves fast. Rapid changes in atmospheric
circulation cause storms, cold spells, or heat waves that
play out over several days.
The ocean, on the other hand, is a turtle. It may take years
or decades or even millennia for similar disturbances
to circulate through the ocean. But the ocean is a big
turtle. It stores about 1,000 times more heat than the
atmosphere. So changes in ocean circulation can set the
stage for large-scale, long-term climate changes.
One example that you may be familiar with is El Niño. Every
few years, oceanic conditions shift, and surface water
temperatures in the eastern tropical Pacific get warmer. The
atmosphere above the ocean shifts, too. El Niño rearranges
worldwide wind and rainfall patterns, causing destructive
droughts, floods, storms, and forest fires. Not to downplay
El Niño in the least, because it causes grave human
suffering and billions of dollars in damagebut El Niño
is relatively short-lived. It lasts only a year or two.
The climate changes Im concerned about last longer and
involve the ocean circulation system that spans the entire
globewhich we often call the Great Ocean Conveyor.
The Great Ocean Conveyor is the oceans major
heat-circulating system. The ocean keeps our planet from
overheating by transporting heat north and south, from the
equator to the poles, in currents traveling near the ocean
surface. In the Atlantic, the Conveyor removes heat from the
Southern Hemisphere and releases it to the Northern
most famous and most important of these currents is the Gulf
Stream. The vast Gulf Stream transports the equivalent
volume of 75 Amazon Rivers. It carries heat absorbed in the
tropics, and moves up the East Coast of the United States,
then northeastward toward Europe.
Great Ocean Conveyor is propelled by the sinking of
cold, salty (and therefore denser) waters in the
North Atlantic Ocean (blue arrows). That creates a
void that pulls warm, salty Gulf Stream waters
northward (red arrows). The Gulf Stream gives up its
heat to the atmosphere above the North Atlantic
Ocean, and prevailing winds (large red arrows) carry
the heat eastward to warm Europe. (Animation by Jack
When the Gulf Streams warm, salty waters reach colder
latitudes, they give up their heat to the atmosphere. The
atmosphere in the North Atlantic region warms by as much as
10 degrees Fahrenheit. Prevailing winds carry the heat
eastward into Europe. Thats one reason London, Englandwhich
is located at the same latitude as Calgary and
Edmontonhas warmer winters than New Yorkwhich is
hundreds of miles farther south.
When the Gulf Streams waters reach the Labrador,
Greenland and other northern seas, and lose their heat to
the atmosphere, they become colderand hence denser. The
waters are also relatively salty. Salty water is denser than
fresher water, so the whole salty mass begins to sink to
When this sinking mass of cold water reaches the abyss, it
then flows at deep levels of the ocean, from the North
Atlantic southward into the South Atlantic. The plunge of
this great volume of cold, salty water propels the Great
Ocean Conveyor. And on the back end, it creates a void that
actively pulls the Gulf Stream northward to replace the
waters that are sinking.
Its a pretty neat system. We have been operating under
the climate conditions created by this beneficent oceanic
heating, ventilation, and cooling system for centuries.
But what if this system werent operating today? What if
cold North Atlantic waters didnt sink and warm equatorial
waters werent drawn in to replace the sinking waters?
Then the North Atlantic region would be a much different,
much colder place.
The vast majority of us go about our business thinking that
Earths climate system has always been this way, and
always will be. But that is just not the case. In fact, we
know that the Ocean Conveyor has indeed shut downstopped
operatingin the past.
We know this by examining truly interesting and valuable
natural archives that record past changes in Earths
climate. For example, we can see layers within ice sheets in
Greenland or in high elevations in the Andes Mountains in
South America. This layering is caused by variations in the
amount of snow that fell on the top of the ice sheet in the
light and dark bands within the ice sheet show an annual
cycle of dust in the atmosphere, so each couplet reflects
the amount of snow that fell in one year. By analyzing the
chemistry of the ice itself, scientists can determine the
temperature of the atmosphere when the snow was produced.
Ice sheets reveal annual layers, which scientists
can analyze to reconstruct the history of
precipitation and air temperatures 100,000 years in
the past. (Photo by Lonnie Thompson, Ohio State
University) Bottom: Cores of seafloor sediments
reveal the climate history of the ocean. (Photo by
Ken Buesseler, WHOI)
Thus from a single ice sheet record, it is possible to
reconstruct the past history of precipitation and air
temperature in a region. For very old ice sheets, like the
glaciers on Greenland, it is possible to reconstruct this
climate history 100,000 years into the past.
The ocean has similar archives of past climate. By taking
cores from the ocean floor, it is possible to reconstruct
the history of ocean climate back many thousands of years.
We have used our ships to collect samples of sediments from
the seafloor. Preserved in the sediments are the fossil
remains of microscopic organisms that settle to the seafloor.
They accumulate over time in layerssimilar to the ice
corethat delineate many important aspects of past climate.
For instance, certain organisms are found only in colder,
polar waters and never live in warmer waters. They can
reveal where and when cold surface waters existedand
didnt existin the past.
From records like these, we know that about 12,800 years ago,
North Atlantic waters cooled dramaticallyand so did the
North Atlantic region. This large cooling in Earths
climate occurred in about a decade. And the cold spell
lasted for about 1,300 years.
This period is called the Younger Dryas, and it is just one
of several periods when Earths climate changed very
rapidly from warm to cold conditions, and then back to warm
again. So these long-term cold snaps are not unusual.
These shifts almost certainly involved changes in the
oceans circulation. There were shutdowns and restartings
of the Ocean Conveyor. These warm-to-cold transitions happen
in about 3 to 10 years. The cold periods lasted for 500
to 1,000 years. Such oscillations in temperature and
ocean circulation have occurred on a regular basis.
About 1,000 years ago, during a period of unusually warm
temperatures in the North Atlantic, the Norse established
settlements and vineyards in Greenland that would not be
possible today. Those settlements were abandoned about 500
years ago, when we believe the most recent shutdown of the
North Atlantic Ocean circulation system occurred.
During that era, called the Little Ice Age, northern Europe
was much colder than it is today. Glaciers spread outward
and downward in the Alps. Winters, on average, were more
severe. Farming was affected. Famine was frequent.
In the 1730s and 1740s, abrupt European cooling caused
famine across western Europe, especially in Ireland and
France, where farmers depended on wheat and potatoes. In
Ireland, this is known as the forgotten famine. As
many people died during the forgotten famine as died during
the famed potato famine of the 1840s.
The 16th-century Flemish artist Bruegel couldnt have
painted his famous frozen landscapes today, because now
canals in the Netherlands rarely freeze, as they regularly
did back then. And likewise, the winter in Valley Forge
might not have been so cold, and Washingtons crossing of
the ice-bound Delaware River wouldnt have been so
dramatic, if he had done it a century laterbecause our
climate conditions have shifted since then, and today, the
Delaware River rarely freezes.
If you read David McCulloughs biography of John Adams,
you will remember that the British were about to set Boston
on fire when George Washington was able to bring the cannons
of Fort Ticonderoga down from upstate New York in record
time. He was able to do it because the ground was frozen
solid and they could slide the cannons to the Dorchester
hills of Boston in time to persuade the British to retreat
from Boston and to change the course of history.
So we have solid evidence that the Great Ocean Conveyor has
slowed down or shut down in the past. And we have seen dire
impacts on our climate. It begs the question: Could
something throw a wrench into the Great Ocean Conveyor in
the near future? And could that trigger abrupt, dramatic
climate changes throughout our planet? The answers to those
questions are, indisputably, Yes and yes.
way to look at Earths climate system is a simple mode of
balances (see diagram, left). Like most dynamic systems,
Earths climate seeks a stable mode. And it will tend to
stay in that mode if nothing causes it to change. Thats
the top tier of the diagram. The ball remains ensconced in
most dynamic systems, Earths climate seeks a
stable mode (top tier). If the system is pushed, it
will stagger for a while unitl it recovers (middle
tier). If it is pushed past a threshold, it will
shift into another mode of operation (bottom tier).
(Illustration by Jack Cook, WHOI, adapted from Abrupt
Climate Change, Inevitable Suprises National
The middle tier shows what happens if you push on the
system. The ball will rattle and roll around in different
directions for a while until it settles back down in its cup.
We humans would definitely notice that rattling until the
climate system returned to equilibrium. The balance, forced
to move, staggers but recovers.
Then there is a third situation in which a strong enough
push at the right time could shove the system past a
threshold and into a completely different mode of operation.
In terms of our climate system, that means that a small or
temporary forcing could produce a sudden, large, and
long-lasting change. That begs the next question: What could
do that to our climate system today?
One answer to that is fresh water. If you simply add too
much fresh water to the North Atlantic, the waters there
will become less salty and less dense. They will stop
sinking. Then the Gulf Stream slows down or is deflected
southward. Winters in the North Atlantic region get
Now heres the predicament. In the past year,
oceanographers monitoring and analyzing water conditions in
the North Atlantic, have concluded that the North Atlantic
has been freshening dramaticallyespecially in the past
decade. New data
from Ruth Curry at Woods Hole Oceanographic Institution
and her colleague Robert Dickson at the British Centre for
Environment, Fisheries, and Aquaculture Sciencechronicles
salinity changes in the western North Atlantic since 1960.
The Great Ocean Conveyor transports fresh surface water down
into the depths. The depths can absorb a lot of fresh water
like a sponge. But since 1970, the equivalent of an extra 20
feet of fresh water across the surface of the northern North
Atlantic has been transported down into the ocean depths,
most of that since 1990.
A sponge that is three-quarters saturated can still absorb
more water. But the moment that sponge is fully saturated,
it can absorb no more water.
some point, the North Atlantic will no longer absorb any
more fresh water. It will begin to pile up on the surface.
When that happens, the Great Ocean Conveyor will be clogged.
It will back up and cease functioning.
too much fresh water enters the North Atlantic, its
waters could stop sinking. The Great Conveyor would
cease. Heat-bearing Gulf Stream waters (red arrows
would no longer flow into the North Atlantic, and
winters would become more severe. (Animation by Jack
The very recent freshening signal in the North Atlantic is
arguably the biggest and most dramatic change in ocean
property that has ever been measured in the global ocean.
Already, surface waters in the Greenland Sea are sinking at
a rate 20 percent slower than in the 1970s.
At what percent will the Ocean Conveyor stop? 25 percent? 40
percent? 60 percent? This is not like a dimmer switch, but
more like a light switch. It probably goes from on to
We cant yet determine the precise source or sources of
this additional fresh water. Global warming may be melting
glaciers, or Arctic sea ice. In recent decades, the volume
of Arctic sea ice has decreased by 40 percent. And if North
Atlantic sinking slows down, less salty Gulf Stream waters
flow northwardwhich exacerbates the situation.
In February 2002, at a worldwide meeting of oceanographers,
new data on North Atlantic freshening prompted many
scientists to say that salinity levels in the North Atlantic
are approaching a density very close to the critical point
at which the waters will stop sinking.
One of my colleagues at Woods Hole, Terry Joyce, put it this
way: Im in the dark as to how close to an edge or
transition to a new ocean and climate regime we might be,
he said. But I know which way we are walking. We are
walking toward the cliff.
To that sentiment, I would add this: We are walking toward
the edge of a cliffblindfolded. Our ability to understand
the potential for future abrupt changes in climate is
limited by our lack of understanding of the processes that
New data shows that North Atlantic waters at depths
between 1,000 and 4,000 meters are becoming
dramatically less salty, especially in the last
decade. Red indicates saltier-than-normal waters.
Blue indicates fresher waters. Oceanographers say we
may be approaching a threshold that would shut down
the Great Ocean Conveyor and cause abrupt climate
changes. (Data from Ruth Curry, WHOI, Bob Dickson,
Centre for Environment, Fisheries, and Aquaculture
Science and Igor Yashayaev, Bedford Institute of
In the past decades, we have made great strides in
understanding Earths atmospheric circulation system
because we established a global network of thousands of
meteorological stations to monitor changing atmospheric
conditions. No observational network exists to continuously
monitor the oceans. If we just had a few more strategically
placed modern instruments in the oceans for an extended
time, we could understand so much more about how the oceans
can cause abrupt climate changes. At present, there is no
national plan for improving our understanding of the issue,
and according to a 2002 National Research Council report, no
policymaking body is addressing the many concerns raised by
the potential for abrupt climate change.
So heres the situation: We have unequivocal evidence of
repeated, large, widespread, abrupt climate changes on Earth.
It is reasonable to assume that greenhouse warming can
exacerbate the possibility of precipitating large, abrupt,
and regional or global climatic changes. We even have strong
evidence that we may be approaching a dangerous thresholdthat
we are squeezing a trigger in the North Atlantic.
We could downplay the relevance of past abrupt events and
deny the likelihood of future abrupt climate changes. But
that could prove costly. With growing globalization, the
adverse impacts of climate changes are likely to spill
across national boundariesthrough migration, economic
shocks, and political aftershocks.
Over human history, one of the major ways that humans have
adapted to changing environmental and economic fortunes has
been to migrate from unproductive or impacted regions to
more productive and hospitable regions. But today, the
worlds population has grown too large. There is less
usable, unpopulated territory to absorb migrants. National
borders are less open, so it is difficult for people to move
to other countries when droughts, floods, famines, and wars
occur. These boundary effects could be particularly severe
for small and poor countries, whose populations are often
unwelcome in richer countries. In the 1840s, more than 1
million Irish people emigrated because of the potato blight.
Can you imagine an equivalent migration of many millions of
people today? Keep in mind that there were only about 1
billion people on Earth then. There are 6 billion now.
As a society, I believe we must face the potential for
abrupt climate change. Perhaps we can mitigate the changes.
If not, at least we can still take steps to adapt to them.
The best way to improve the effectiveness of our response is
to have more knowledge of what can happenand how and when.
Research into the causes, patterns, likelihood, and effects
of abrupt climate change can help reduce our vulnerabilities
and increase our ability to adapt.
If climate changes come abruptly, we will have less time to
adjust. In other words, the more knowledge we havethe
more reliably we can predict changesthe better our
Maybe over the edge of the cliff, theres just a
three-inch drop-off. Or maybe theres a big, fluffy bed
full of pillows. My worry is that we are indeed approaching
this cliff blindfolded.
Are you comfortable and secure with this scenario?
Heat Before the Cold
By Terrence Joyce
Published in The New York Times
April 18, 2002
This weeks unexpected heat wave across much of the
Northeast and Midwest, couple with recent reports about the
surprisingly fast collapse of an Antarctic ice shelf the
size of Rhode Island, has heightened fears of long-term rise
in temperatures brought about by global warming. But this
fear may be misguided. In fact, paradoxically, global
warming could actually bring colder temperatures to some
highly populated areas like Eastern North America and
Heres what might happen: In the North Atlantic, a 10-foot
layer of fresh water - some of which may be coming from
melting ice in the Arctic - has been accumulating and
lowering the salinity of the ocean to depths of more than a
mile for the past 30 years. Fresh water in the ocean may not
sound cataclysmic, but it can upset the ocean currents that
are the key to our planets climate control system.
In February, oceanographers presented new evidence that this
northern freshwater buildup could alter currents in a way
that would cause an abrupt drop in average winter
temperatures of about 5 degrees Fahrenheit over much of the
United States and 10 degrees in the Northeast. That may not
sound like much, but recall the coldest winters in the
Northeast, like those of 1936 and 1978, and then imagine
recurring winters that are even colder, and youll have an
idea of what this would be like. This change could happen
within a decade and persist for hundreds of years.
Under normal circumstances, the famous warm waters of the
Gulf Stream, carrying heat absorbed in the tropics, move up
the East Coast of the United States and southeastern Canada
and then angle toward Europe, warming the overlying
atmosphere and surrounding land as they go. As the Gulf
Stream system carries warm, salty water north, the
atmosphere cools it, making it dense enough to sink to great
depths. The plunge of that great volume of water helps
propel a global system of currents sometimes called the
great ocean conveyor. But add too much fresh water, and
North Atlantic waters become less salty and less dense. They
stop sinking. The Gulf Stream slows or is redirected
southward. Winters in the North Atlantic region get
Changes in the conveyor were responsible for some of the
most noticeable climate changes in scientific history. About
12,000 years ago, as the earth emerged from the most recent
Ice Age and the North Atlantic region warmed, an influx of
fresh water - perhaps from melting ice sheets - shut down
the great conveyor and plunged much of the Northern
Hemisphere back into ice-age conditions that lasted 1,000
years. About 500 years ago a reduction of the ocean
conveyors may have turned the climate in northern Europe and
the northeastern United States much colder, during what
became known as the Little Ice Age, which lasted for about
300 years. In America, the Little Ice Age coincided with the
notorious winter at Valley Forge.
There is not enough evidence for scientists to know for sure
whether the influx of fresh water in the North Atlantic has
come from an already altered ocean circulation, changes in
rainfall patterns or rivers, or glacial and Arctic Ocean ice.
We dont know the exact threshold at which sinking, and
the great ocean conveyor, could stop. A global
ocean-observing system would greatly enhance our ability to
monitor changes that can spawn major, long-lasting climate
shifts like these and lead to reliable predictions of what
may follow. But the evidence we do have suggests that global
warming could actually lead to a big chill.
Terrence Joyce is a senior scientist and chairman of the
department of physical oceanography at Woods Hole
Vol. 23 No. 9 (September 2002)
Worried about global warming? Talk to
a few scientists at Woods Hole. Oceanographers there are
seeing big trouble with the Gulf Stream, which warms both
North America and Europe
By Brad Lemley
analysis of fossilized foraminifera, shell-building
one-celled creatures, helps climate researchers
determine oceanic temperatures during a mini- ice age
hundreds of years ago. G. sacculifera (top left)
and G. ruber (bottom right) are planktonic
organisms that spend their lives floating near the
surface but fall like sand grains to the bottom of the
ocean when they die. U. peregrina (top right)
and C. wuellerstorfi (bottom left) are
benthonic organisms that live and die on or in
sediments on the seafloor.
Photographs courtesy of Woods Hole Oceanographic
William Curry is a serious, sober climate scientist, not an
art critic. But he has spent a lot of time perusing Emanuel
Gottlieb Leutze's famous painting "George Washington
Crossing the Delaware," which depicts a boatload of
colonial American soldiers making their way to attack English
and Hessian troops the day after Christmas in 1776. "Most
people think these other guys in the boat are rowing, but they
are actually pushing the ice away," says Curry, tapping
his finger on a reproduction of the painting. Sure enough, the
lead oarsman is bashing the frozen river with his boot.
"I grew up in Philadelphia. The place in this painting is
30 minutes away by car. I can tell you, this kind of thing
just doesn't happen anymore."
But it may again. Soon. And ice-choked
scenes, similar to those immortalized by the 16th-century
Flemish painter Pieter Brueghel the Elder, may also return to
Europe. His works, including the 1565 masterpiece
"Hunters in the Snow," make the now-temperate
European landscapes look more like Lapland.
Such frigid settings were commonplace
during a period dating roughly from 1300 to 1850 because much
of North America and Europe was in the throes of a little ice
age. And now there is mounting evidence that the chill could
return. A growing number of scientistsincluding many here
at Curry's base of operations, the Woods Hole Oceanographic
Institution on Cape Cod in Massachusettsbelieves conditions
are ripe for another prolonged cooldown, or small ice age.
While no one is predicting a brutal ice sheet like the one
that covered the Northern Hemisphere with glaciers about
12,000 years ago, the next cooling trend could drop average
temperatures 5 degrees Fahrenheit over much of the United
States and 10 degrees in the Northeast, northern Europe, and
"It could happen in 10 years,"
says Terrence Joyce, who chairs the Woods Hole Physical
Oceanography Department. "Once it does, it can take
hundreds of years to reverse." And he is alarmed that
Americans have yet to take the threat seriously. In a letter
to The New York Times last April, he wrote, "Recall
the coldest winters in the Northeast, like those of 1936 and
1978, and then imagine recurring winters that are even colder,
and you'll have an idea of what this would be like."
A drop of 5 to 10 degrees entails much more
than simply bumping up the thermostat and carrying on. Both
economically and ecologically, such quick, persistent chilling
could have devastating consequences. A 2002 report titled
"Abrupt Climate Change: Inevitable Surprises,"
produced by the National Academy of Sciences, pegged the cost
from agricultural losses alone at $100 billion to $250 billion
while also predicting that damage to ecologies could be vast
and incalculable. A grim sampler: disappearing forests,
increased housing expenses, dwindling freshwater, lower crop
yields, and accelerated species extinctions.
The reason for such huge effects is simple.
A quick climate change wreaks far more disruption than a slow
one. People, animals, plants, and the economies that depend on
them are like rivers, says the report: "For example, high
water in a river will pose few problems until the water runs
over the bank, after which levees can be breached and massive
flooding can occur. Many biological processes undergo shifts
at particular thresholds of temperature and precipitation."
Political changes since the last ice age
could make survival far more difficult for the world's poor.
During previous cooling periods, whole tribes simply picked up
and moved south, but that option doesn't work in the modern,
tense world of closed borders. "To the extent that abrupt
climate change may cause rapid and extensive changes of
fortune for those who live off the land, the inability to
migrate may remove one of the major safety nets for distressed
people," says the report.
Still, climate science is devilishly
complex, and the onslaught of a little ice age is not certain,
at least at this stage of research. Scientists the world over
are weighing the potential for rapid North Atlantic cooling,
but perhaps nowhere in the United States is more energy,
equipment, and brainpower directed at the problem than here at
Woods Hole. The oceanographers on staff subsist largely on
government grants and are beholden to no corporation, making
the facility "uniquely independent," says David
Gallo, director of special projects. Consequently, it should
be as likely as any research facility or university to get at
The task is huge. Down on the docks where
the institution keeps its three research ships, gulls swoop
around a collection of massive metal frameworks; these are
core samplers that, dropped over a ship's side, can extract
long columns of layered sediments from the undersea muck. In a
workshop nearby, technicians tinker with arrays of multiple
independent water samplers, which at four feet long and eight
inches thick look rather like giant scuba tanks. Out on the
water, researchers drop these instruments into the North
Atlantic, hoping to get a sharper picture of the potential for
a little ice age. A sense of urgency propels the efforts.
"We need to make this a national priority," says
Joyce. "It's a tough nut to crack, but with enough data,
I think we can make a more specific and confident prediction
about what comes next." Policymakers armed with a
specific forecast could make adjustments to prepare for the
continue to pile on atmospheric carbon dioxide, we're
going to have more unintended consequences," says
William Curry, a climate scientist. "We need to
seriously consider steps to curb greenhouse gases."
Photograph by Greg Miller
But first things first. Isn't the earth actually warming?
Indeed it is, says Joyce. In his cluttered
office, full of soft light from the foggy Cape Cod morning, he
explains how such warming could actually be the surprising
culprit of the next mini-ice age. The paradox is a result of
the appearance over the past 30 years in the North Atlantic of
huge rivers of freshwaterthe equivalent of a 10-foot-thick
layermixed into the salty sea. No one is certain where the
fresh torrents are coming from, but a prime suspect is melting
Arctic ice, caused by a buildup of carbon dioxide in the
atmosphere that traps solar energy.
The freshwater trend is major news in
ocean-science circles. Bob Dickson, a British oceanographer
who sounded an alarm at a February conference in Honolulu, has
termed the drop in salinity and temperature in the Labrador
Seaa body of water between northeastern Canada and
Greenland that adjoins the Atlantic"arguably the
largest full-depth changes observed in the modern instrumental
The trend could cause a little ice age by
subverting the northern penetration of Gulf Stream waters.
Normally, the Gulf Stream, laden with heat soaked up in the
tropics, meanders up the east coasts of the United States and
Canada. As it flows northward, the stream surrenders heat to
the air. Because the prevailing North Atlantic winds blow
eastward, a lot of the heat wafts to Europe. That's why many
scientists believe winter temperatures on the Continent are as
much as 36 degrees Fahrenheit warmer than those in North
America at the same latitude. Frigid Boston, for example, lies
at almost precisely the same latitude as balmy Rome. And some
scientists say the heat also warms Americans and Canadians.
"It's a real mistake to think of this solely as a
European phenomenon," says Joyce.
Having given up its heat to the air, the
now-cooler water becomes denser and sinks into the North
Atlantic by a mile or more in a process oceanographers call
thermohaline circulation. This massive column of cascading
cold is the main engine powering a deepwater current called
the Great Ocean Conveyor that snakes through all the world's
oceans. But as the North Atlantic fills with freshwater, it
grows less dense, making the waters carried northward by the
Gulf Stream less able to sink. The new mass of relatively
fresh water sits on top of the ocean like a big thermal
blanket, threatening the thermohaline circulation. That in
turn could make the Gulf Stream slow or veer southward. At
some point, the whole system could simply shut down, and do so
quickly. "There is increasing evidence that we are
getting closer to a transition point, from which we can jump
to a new state. Small changes, such as a couple of years of
heavy precipitation or melting ice at high latitudes, could
yield a big response," says Joyce.
In her sunny office down the hall,
oceanographer Ruth Curry shows just how extensive the changes
have already become. "Look at this," she says,
pointing to maps laid out on her lab table. "Orange and
yellow mean warmer and saltier. Green and blue mean colder and
fresher." The four-map array shows the North Atlantic
each decade since the 1960s. With each subsequent map, green
and blue spread farther; even to the untrained eye, there's
clearly something awry. "It's not just in the Labrador
Sea," she says. "This cold, freshening area is now
invading the deep waters of the entire subtropical Atlantic."
"You have all this freshwater sitting
at high latitudes, and it can literally take hundreds of years
to get rid of it," Joyce says. So while the globe as a
whole gets warmer by tiny fractions of 1 degree Fahrenheit
annually, the North Atlantic region could, in a decade, get up
to 10 degrees colder. What worries researchers at Woods Hole
is that history is on the side of rapid shutdown. They know it
has happened before.
"The physics of El Niño are simple compared to
the physics of this climate change," says
Terrence Joyce, chairman of the Woods Hole Department
of Physical Oceanography, with Ruth Curry, one of the
Photograph by Greg Miller
On the northwest side of Woods Hole's Quissett campus, in a
dim laboratory that smells like low tide, about 24,000
polycarbonate tubes full of greenish-tan mud rest in wire
racks, as carefully cataloged as fine wines. They are core
samples collected from seafloors, many collected during
expeditions by the Knorr, one of Woods Hole's three largest
research ships. Each core tells a story about time and
temperature spanning thousands of years.
But one particular core, kept carefully
refrigerated at 39 degrees Fahrenheit, was pivotal for
reaching the conclusion that little ice ages can start
abruptly. The Canadian ship CSS Hudson collected the
core in 1989 from a seafloor plateau called the Bermuda Rise
in the northern Sargasso Sea, roughly 200 miles northeast of
Bermuda. "It's a peculiar place on the seafloor where mud
accumulates rapidly," says Lloyd Keigwin, a senior
scientist in the Woods Hole Geology and Geophysics Department.
Most of the sediment was washed out of Canadian rivers before
settling, so it bears witness to the vagaries of climate in
the North Atlantic.
Seafloor sediments are peppered with tiny
invertebrates called foraminifera, which Keigwin describes as
"amoebas with shells," that can yield clues about
the temperature of the ocean in which they lived. Clay and
silt from the Nova Scotia region cause the little creatures to
accumulate in neatly distinguishable layers, which means a
wealth of information.
Keigwin subjected the foraminifera in
various layers of this core to mass spectroscopic analysis. By
measuring the proportions of oxygen isotopesespecially the
ratio of oxygen 16 to oxygen 18he was able to peg the
temperature at which the tiny animals in each layer formed
their calcium carbonate shells to an accuracy of less than 1
degree Fahrenheit. He coupled that with carbon dating to
determine each sediment layer's age.
Keigwin had expected to find evidence of
climate swings during the past few thousand years. But in the
CSS Hudson's prize sample, which was drilled with a
more precise corer than oceanographers had used previously, he
uncovered plenty of data about abrupt temperature changes over
the past 1,000 years, including for a little ice age that
averaged about 4 degrees Fahrenheit colder than the present.
"And because the Sargasso Sea is pretty well mixed, the
cooling must have been widespread," Keigwin says. More
ominously, "I found evidence that proves the climate
cycles continue right up until today."
Clearly, the little ice age from 1300 to
1850 wasn't kicked off by humans releasing greenhouse gases
into the atmosphere. But natural climate cycles that melted
Arctic ice could have caused thermohaline circulation to shut
down abruptly. "We are almost certain that this was the
cause of the last little ice age," says Ruth Curry,
"although we'd need a time machine to be sure."
"I was aware that this could be a
bombshell, but I stuck my neck out," says Keigwin, who
first published his findings in 1996. Since then, similar
high-sediment locations have bolstered his early conclusions.
"As it turns out, there are probably at least 10 places
in the North Atlantic that can give you pretty good core
evidence of mini-ice-age cooling," he says.
A more recent event is perhaps better
evidence that a climate can cool quickly because of
thermohaline shutdown. In the late 1960s, a huge blob of
near-surface fresher water appeared off the east coast of
Greenland, probably the result of a big discharge of ice into
the Atlantic in 1967. Known as the Great Salinity Anomaly, it
drifted southward, settling into the North Atlantic in the
early 1970s. There it interfered with the thermohaline
circulation by quickly arresting deepwater formation in the
Labrador Sea. It continued to drift in a counterclockwise
direction around the North Atlantic, re-entering the Norwegian
Sea in the late 1970s and vanishing soon after.
"I believe it shut the system down for
just a few years. The result was very cold winters,
particularly in Europe," says Ruth Curry.
That fresher-water mass, fortunately, was
small enough to disperse in a short period of time. The one
accumulating up there now, however, "is just too big,"
Climate science is extraordinarily complex
because it is dependent upon the gathering and interpretation
of millions of data points. If the National Weather Service
has trouble predicting tomorrow's weather, how can anyone
forecast a change in global climate a few years hence? One
answer is even more data. At the moment, there are about 450
floating sensors bobbing around in the Atlantic monitoring
temperature and salinity changes, and that is not enough, says
Ruth Curry. "The models don't have enough resolution to
capture all the physics yet. Prediction is tough."
Or maybe Woods Hole researchers are
adhering to a flawed model. That's the view of Richard Seager,
a climate scientist at Columbia University's Lamont-Doherty
Earth Observatory. In a paper titled "Is the Gulf Stream
Responsible for Europe's Mild Winters?" to be published
this year in the Quarterly Journal of the Royal
Meteorological Society, he casts doubt on the notion that
warmth transported by the Gulf Stream has a significant impact
on either continent. Europe would be warmer, he says, "even
if the Atlantic were just a big, stagnant ocean" because
the prevailing westerly winds would still blow heat stored in
the Atlantic in the summer to Europe in the winter.
Transported Gulf Stream heat, he says, accounts for less than
10 percent of England's warmth relative to the United States.
In Seager's view, prolonged winter warmth
is more likely than a little ice age. "The thousand-pound
gorilla in eastern North America and Europe is the North
Atlantic Oscillation," he says. This is a complex, poorly
understood variation in the strength of air-pressure cells
over Iceland and the Azores. When pressure over Iceland is
high, the pressure over the Azores tends to be low, and vice
versa. During the winter, a lower-than-usual low over Iceland
and a higher-than-usual high over the Azores forces cold air
to eastern Canada and warm, moist air to northwestern Europe
and the eastern United States.
That's precisely what has happened from the
1960s to the late 1990s, says Seager, which gave rise to
relatively balmy winters in the high-population regions on
both sides of the Atlantic. "If this phase continues, as
some models predict will occur as the result of rising
greenhouse gases, this would make these changes in winter
climate persist for years to come," he says.
Seager's viewpoint is in the minority. In
other models, and climate science is ultimately a battle of
different computer models, the Gulf Stream is a major source
of warmth for the lands that border the North Atlantic. In
Ruth Curry's view, the science as it stands is more than
strong enough to warrant thinking ahead.
"We can't know the point at which
thermohaline shutdown could actually start," she says.
"But we should plan for it."
Should a little ice age
arrive, its impact will be told in human suffering, not
scientific terminology. The Little Ice Age (Basic
Books, 2000), by anthropology professor Brian Fagan of the
University of California at Santa Barbara, is replete with
tales of woe depicting the plight of European peasants during
the 1300 to 1850 chill: famines, hypothermia, bread riots, and
the rise of despotic leaders brutalizing an increasingly
dispirited peasantry. In the late 17th century, writes Fagan,
agriculture had dropped off so dramatically that "Alpine
villagers lived on bread made from ground nutshells mixed with
barley and oat flour." Finland lost perhaps a third of
its population to starvation and disease.
Life was particularly difficult for those who
lived under the constant threat of advancing glaciers in the
French Alps. One, the Des Bois glacier on the slopes of Mont
Blanc, was said to have moved forward "over a musket shot
each day, even in the month of August." When the Des Bois
threatened to dam up the Arve River in 1644, residents of the
town of Chamonix begged the bishop of Geneva to petition God
for help. In early June, the bishop, with 300 villagers
gathered around him, blessed the threatening glacier and
another near the village of Largentire. For a while,
salvation seemed at hand. The glaciers retreated for about 20
years, until 1663. But they had left the land so barren that
new crops would not grow.
For more about the work of
the Woods Hole Physical Oceanography Department, see www.whoi.edu/science/PO/dept.
For an exploration of the
science behind another little ice age, see Abrupt Climate
Change: Inevitable Surprises from the National Academy Press,
2002, at books.nap.edu/books/
© Copyright 2002 The Walt
Disney Company. Back to Homepage.
of the Physical Oceanography Department
Brazil Current Rings Engulf the Windward Islands
by Dave Fratantoni
of the Arctic Shelf
July 14 - August 13, 2002 USCGC Polar Star
is the exploration and study of the physics and
geography of the ocean currents and water properties.
Some major components of physical oceanography are the
dynamics of ocean currents on scales from centimeters to
global, the variability of these currents on time-scales
from seconds to millennia, ocean wave phenomena, the
distribution of heat and salt and their transport
through the ocean basins, the exchange of momentum, heat
and freshwater between the ocean and the atmosphere, and
interactions between oceans and rivers, estuaries, ice
and marginal seas. Physical oceanography has important
applications in global climate, oceanic mixing, and
coastal studies, as well as being a key element in
interdisciplinary studies of primary production,
hydrothermal vents, and oceanic flux and storage of
Oceanography department began as a separate entity at
WHOI in 1962 with a total scientific staff of 20 and
Fritz Fuglister as our first chairman. Since then, the
department has grown to its highest ever population of
34 scientists at present. The scientific foci include
the general circulations of the oceans, climate
variability, shelf/slope dynamics, mixing, and air-sea
interaction. Our department is an active participant in
the MIT/WHOI Joint Program with staff giving courses and
advising graduate students. Besides a population of
students, we have a number of Post Doctoral Scholars/Investigators
who provide an influx of new ideas and sometimes become
new members of our scientific staff. The department
continues to maintain leadership in blue water
oceanography and has grown to be one of the leaders in
coastal oceanography as well.
While ocean observations remain one of our principal
missions, we have increasingly developed modeling
expertise, both analytical and numerical, to support
seagoing science and to better understand fundamental
ocean processes. The seagoing groups have evolved into a
number of technical and scientific groups having
specialized equipment. Below we present some of our
specialized research working groups:
The following list contains the present groups in our
& Mooring Shop
This is a joint PO/AOPE entity which services the
entire Institution and a large number of outside users,
locally, nationally, and internationally. It is based
largely in the Village, but uses the Clark South high
bay for staging.
Upper Ocean Processes
The part of the old Buoy Group that dealt with surface
moorings has expanded to include instrumentation (eg.
VMCMs, met packages) for use on these moorings. As a
result of the WOCE effort, the group has expanded into
standardized met packages (IMET) for research and
volunteer observing ships. It has also focused on
understanding how the surface mixed layer responds to
Operations Group (SSMOG)
Moored instrumentation that does not penetrate the
surface layer differs from that of the UOP. Deployments
are generally longer term and consist of arrays of more
than one mooring. Some instrumentation is common such as
releases. These first three groups are the core of the
now disbanded PO Buoy Group.
The extended 7 year observational period of WOCE really
brought the water sampling and CTD parts of this group
together. It continues to provide support to PO
scientists and has, in recent years, also provided
support for CTD/hydrography on our Institution research
Our department has the capability to calibrate
temperature and pressure sensors to the highest
standards required for our field. The sensors are used
on CTDs, moorings and increasingly on floats. The float
group has developed a ballasting facility that is part
of our departmental calibration laboratory.
The first active float group in the US was developed at
WHOI using SOFAR technology. Since then, it has evolved
to use RAFOS and ALACE floats and is active in pursuing
new developments such as the glider and ARGO.
The Seasoar is a commercial instrument for making
undulating profiles from an underway vessel. Our group
made significant improvements to the control system (which
have been incorporated by the manufacturer) and
continues to make new developments (such as bottom
avoidance for shallow water, fiber optic data links for
realtime video plankton, and other interdisciplinary
This departmental group uses as its main instrument a
high resolution profiler (HRP) built at WHOI and now
being rebuilt with a new generation of sensors. They
have worked extensively with a purposeful tracer release
technique used in AOPE.
Physical Oceanographic Observing Laboratory, is a new
oversight group providing a single point of contact for
potential users of the various seagoing groups, and
their specialized equipment.
The newly established Cooperative Institute for Climate
and Ocean Research is a joint NOAA/WHOI facility. Two of
its three foci: climate research and coastal ocean/near
shore processes are central themes of our department.
While just formed in 1998, this facility is expected to
play a larger role in the future as we begin GOOS and
Laboratory studies have been a long tradition at WHOI
beginning with the rotating table experiments of Alan
Faller & Bill von Arx. Two labs are presently used
by members of our department and students of the summer
GFD Program. One lab is in Clark and the other in the
Coastal Research Center.
Nelson G. Hogg
Chairman, Physcial Oceanography Department
01 July 2002