Le petit age glaciaire pour demain !

Le Gulf Stream est dangereusement ralenti...

La glace de mer et la glace continentale fondent en arctique, et l'impact sur le ralentissement du Gulf Stream se confirme. Plusieurs scientifiques émettent maintenant l'hypothèse d'une catastrophe climatique majeure qui pourrait arriver rapidement (10 ans), suite à une coupure brutale du Gulf Stream.

 

Pourrait-on revoir des périodes de glaciation ?

Les variations du climat font intervenir différentes échelles de temps.
On sait qu’il existe des périodes régulières de glaciation (cycle de Milankovitch), avec des cycles d’environ 125 000 ans et ce depuis plus d’un million d’années. Il n’y 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 l’activité de l’homme.

Les cycles glaciaires-interglaciaires

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.

  Prévisions saisonnieres

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 :

  • Juste avant un age glacière la température de l'eau augmente, exactement comme ce qui se passe maintenant.

Cela est du à :

  • Un accroissement des éruptions volcanique et comme 80% des volcans sont sous l'eau

  • États-Unis : les précipitations ont augmentées de 20% depuis 1970

  • Plus d'humidité (El Nino)

  • L'activité solaire a un lien direct avec un age glacière

La situation actuelle du tapis roulant serait au bord de l'instabilité

Une 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 ?

La 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 ?

source : "L'incertitude des climats" par Robert KANDEL

 

Des catastrophes d'une rapidité et d'une violence inouies

Des variations brusques apparaissent, dues à des accumulations de poussières et de minéraux, signes de coup de froids violents. 

Périodes : - 115 000 ans chute de la T° de 14° pendant 70 ans

Périodes : - 131 000 ans chute de la T° de 10° pendant 750 ans

( La calotte polaire descendait jusqu'à Londres...)

La cause de ces changement se trouve-t-elle dans les océans ?

Prochains 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

Température à la surface du globe
(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 millénaire

Température de l'hémisphère nord
(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 terrestre.

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

Réchauffement climatique selon les modèles numériques

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 divers scénarios.

 

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 voire plus).

 

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 moyenne 1961-1990

Temperature a la surface du globe
(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

Tendance 1900-2000
(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

Changement des températures hivernales Changement des températures estivales
(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

Atlantic Ocean Warming
(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

Augmentation du niveau de la mer

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).

 

L'oscillation atlantique multidécennale

 


(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).

 

Modulations de l'oscillation nord-atlantique

 

Température et transport océanique relié à l'oscillation nord-atlantique (NAO)

MacCartney
(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).

"Le réchauffement 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

Références

Crowley, T. J., 2000 : Causes of climate change over the last 1000 years. Science, 289, 270-277.
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.

Liens (langue anglaise)

Climatic Research Unit Information sheets
Basics of weather, climate and climate change

 


Reprise sur le front des glaciers

Planète. N'en 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.

L'intégralité sur cette page à surtout ne pas louper !

Consulter sur l'Internet : http://virtedit.online.fr/article

Le 14 avril 2001.

 

Auteurs : Eduardo Costa <dacosta@univ-brest.fr> & Thierry Huck <thuck@univ-brest.fr>
Contenu relu par Bruno Blanke et Alain Colin de Verdière,
Laboratoire de Physique des Océans (Unité Mixte de Recherche CNRS IFREMER UBO), Brest.

 
POSSIBLE "ICE AGE" IN NEAR FUTURE?
 
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 and/or
 
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 planet’s climate control system.
 
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 world’s economy is based. Read more below!

Triggering Abrupt Climate Change
Robert Gagosian A perspective on potential climate changes presented by Dr. Robert B. Gagosian, President and Director of Woods Hole Oceanographic Institution

Over the past two decades, we have heard about greenhouse gases and the idea that our planet is gradually warming. I’d like to throw a curveball into that thinking—specifically the “gradually warming” part.

This new thinking is little known and scarcely appreciated by policymakers and world and business leaders—and 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 different—almost counterintuitive—scenario.

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 region—where, not incidentally, some 60 percent of the world’s 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. That’s 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 drastically different.

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 grandchildren’s grandchildren will still be confronting them.

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, Earth’s climate could switch gears and jump very rapidly—not gradually— into a completely different mode of operation.

global ocean circulation system
animationA 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)
This is not something new under the Sun. It has happened throughout Earth’s history, and it could happen again.

The key to these climate shifts is that Earth’s climate is created and maintained by a dynamic system of moving, interacting parts. Earth’s 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 Sun.

The other half is transported around our planet by a circulation system that is equally important, but far less understood—the ocean. The ocean isn’t 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 Earth’s 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 damage—but El Niño is relatively short-lived. It lasts only a year or two.

The climate changes I’m concerned about last longer and involve the ocean circulation system that spans the entire globe—which we often call the Great Ocean Conveyor.

The Great Ocean Conveyor is the ocean’s 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 Hemisphere.

Great Ocean Conveyor
animationThe 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 Cook, WHOI)
The 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.

When the Gulf Stream’s 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. That’s one reason London, England—which is located at the same latitude as Calgary and Edmonton—has warmer winters than New York—which is hundreds of miles farther south.

When the Gulf Stream’s waters reach the Labrador, Greenland and other northern seas, and lose their heat to the atmosphere, they become colder—and hence denser. The waters are also relatively salty. Salty water is denser than fresher water, so the whole salty mass begins to sink to great depths.

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.

It’s 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 weren’t operating today? What if cold North Atlantic waters didn’t sink and warm equatorial waters weren’t 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 Earth’s 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 down—stopped operating—in the past.

We know this by examining truly interesting and valuable natural archives that record past changes in Earth’s 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 past.

ice sheets
ice sheets
Top: 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)
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.

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 layers—similar to the ice core—that 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 existed—and didn’t exist—in the past.

From records like these, we know that about 12,800 years ago, North Atlantic waters cooled dramatically—and so did the North Atlantic region. This large cooling in Earth’s 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 Earth’s 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 ocean’s 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 couldn’t 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 Washington’s crossing of the ice-bound Delaware River wouldn’t have been so dramatic, if he had done it a century later—because our climate conditions have shifted since then, and today, the Delaware River rarely freezes.

If you read David McCullough’s 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.”

Great Ocean Conveyor
Like most dynamic systems, Earth’s 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 Research Council)
Another way to look at Earth’s climate system is a simple mode of balances (see diagram, left). Like most dynamic systems, Earth’s climate seeks a stable mode. And it will tend to stay in that mode if nothing causes it to change. That’s the top tier of the diagram. The ball remains ensconced in its cup.

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 significantly colder.

Now here’s 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 dramatically—especially 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 Science—chronicles 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.

freshwater leading to shut-down
animationIf 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 Cook, WHOI)
At 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.

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 “off.”

We can’t 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 northward—which 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: “I’m 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 cliff—blindfolded. Our ability to understand the potential for future abrupt changes in climate is limited by our lack of understanding of the processes that control them.

freshwater v. saltwater, 1960-2002
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 Oceanography)


In the past decades, we have made great strides in understanding Earth’s 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 here’s 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 threshold—that 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 boundaries—through 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 world’s 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 happen—and 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 have—the more reliably we can predict changes—the better our chances.

Maybe over the edge of the cliff, there’s just a three-inch drop-off. Or maybe there’s 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? 

SOURCE : http://www.whoi.edu/home/about/whatsnew_abruptclimate.html

The Heat Before the Cold
By Terrence Joyce
Published in The New York Times
April 18, 2002


This week’s 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 Western Europe.

Here’s 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 planet’s 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 you’ll 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 significantly colder.

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 don’t 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 Oceanographic Institution.  

DISCOVER Vol. 23 No. 9 (September 2002)
Table of Contents

The New Ice Age
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

 
Chemical 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 Institute.

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 scientists—including many here at Curry's base of operations, the Woods Hole Oceanographic Institution on Cape Cod in Massachusetts—believes 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 northern Asia.
    "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 truth.
    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 inevitable.

 
"As we 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 freshwater—the equivalent of a 10-foot-thick layer—mixed 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 Sea—a body of water between northeastern Canada and Greenland that adjoins the Atlantic—"arguably the largest full-depth changes observed in the modern instrumental oceanographic record."
    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.

 
Opposite: "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 lead researchers.
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 isotopes—especially the ratio of oxygen 16 to oxygen 18—he 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," says Joyce.
    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."

 


Cold Brutality

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 Largentire. 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.
— B. L.



RELATED WEB SITES:

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/ 0309074347/html/index.html.

 


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Overview of the Physical Oceanography Department


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What’s New

Project Highlight:
North Brazil Current Rings Engulf the Windward Islands
by Dave Fratantoni


Cruise Highlight:
Edge of the Arctic Shelf
July 14 - August 13, 2002 USCGC Polar Star

Physical oceanography 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 carbon dioxide.

The Physical 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:

PO ‘Research Groups’
The following list contains the present groups in our department:

Rigging & 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 (UOP)
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 air-sea fluxes.

Sub-Surface Mooring 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.

CTD/hydrography 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 vessels.

Calibration Laboratory
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.

Float Group
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.

Seasoar Group
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 work).

Ocean Mixing Group
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.

POOL
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.

CICOR
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 CLIVAR.

GFD Laboratory
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

 

 

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