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Introduction What is the oceans role in climate? The oceans play a vital and pivotal role in the distribution of life sustaining water throughout our planet. 86 % of the evaporation that occurs on earth is over the oceans. The oceans are the planets largest reservoir of water transferring huge amounts of water around the hydrological cycle. In fact the oceans dominate the hydrological cycle, for they contain 97 % of the global water inventory. The hydrological cycle can be disrupted by changes in ocean circulation that play such an important role on evaporation and precipitation.
When the ocean circulation system changes it can change how much heat and rainfall is distributed around the world. Changes on a global scale can ultimately lead to flooding and long term drought in various regions. The big questions are can we monitor the oceans circulation and watch for climate changes? Can we predict what might happen if the ocean circulation changed dramatically? We have experienced major climate changes in the past; can we look for evidence of ocean change during these periods? The Conveyor Belt The global ocean circulation system is called the thermocline circulation.
Often called the conveyor belt courtesy of Wallace Broecker who in an article for Natural History in 1987 had an artist draw a simplified version of the thermocline circulation and called it the conveyor belt. Wallace Broecker is the Newberry Professor of Earth and Environmental Sciences at Columbia University. He has taught at Columbia since 1959, and his research interests include paleoclimatology, ocean chemistry, isotope dating and environmental science. He conducts much of his research in Columbia University's Lamont Doherty Earth Observatory.
Broecker has received many awards for his scientific work, including Arthur L. Day Medal from the Geological Society of America (1984), the Alexander Agassiz Medal from the National Academy of Sciences (1986), the Wollaston Medal from the Geological Society of London (1990), and the National Medal of Science (1996). Broecker's image of the conveyor belt is somewhat simplified. The ocean conveyor is propelled by the sinking of cold, salty (and therefore denser) waters in the North Atlantic Ocean. This band of deep water flowing south down through the depths of the Atlantic spreads into the Indian and Pacific oceans where it wells to the surface and mixes with other warm tropical waters, it also helps to pull warm, salty Gulf stream waters northward. The warm waters return back to the North Atlantic where mixed with colder Greenland waters and driven by bitter winds it becomes gradually colder & saltier once again sinking into the deep.
In reality much of the water that sinks in the Atlantic never leaves and other eddies and gyres across the oceans mean that this really is a simplified image of the conveyor belt but essentially the basic idea is true. The conveyor transports heat into the North Atlantic and salt out of it. Past History The earths past is full of dramatic climate changes. Many glacial advances and retreats have occurred during the last billion years of Earth history. Large, important glaciations occurred during the late Proterozoic (between about 800 and 600 million years ago), during the Pennsylvanian and Permian (between about 350 and 250 million years ago), and the late Neogene to Quaternary (the last 4 million years). Somewhat less extensive glaciations occurred during parts of the Ordovician and Silurian (between about 460 and 430 million years ago).
It has been discovered that one of the reasons for large climate changes are the Milankovitch cycles; cyclical changes in earths orbit which change the amount of sunlight falling on the northern hemisphere. We have also experienced smaller more abrupt climate changes. Research has been carried out to find out what caused them. The clues are preserved in ice sheets and deep-sea sediments. Cores of sea-sediments record temperatures and circulation patterns.
Cores of the two mile thick Greenland ice sheet record Oxygen isotopes, CO 2 levels and temperature for tens of thousands of years. Broecker first became very interested in the effect of the conveyor belt on climate change in 1984 after a lecture by Hans Oeschger University of Bern. Oeschger was studying ice-cores from the Greenland ice sheet. These ice cores are a unique record of what has happened to the weather, for the last hundred thousand years.
Oxygen Isotopes analysed in the ice showed that during extreme cold (ice-ages) water containing the heavy isotope oxygen 18 tended to remain deep in the oceans (analyses of the shells of microscopic foraminifera showed they became enriched in Oxygen 18 isotope during heavy ice periods) and the icebergs became depleted in Oxygen 18. The ice core worked like a thermometer measuring how cold the air was over Greenland when the ice was laid. Air trapped in the ice was analysed and showed that the atmosphere only contained about two-thirds as much CO 2 as today. CO 2 levels and temperature fluctuations worked hand in hand. Broecker realised faced with all this evidence that one explanation for atmosphere & temperature changes could be changes in the thermocline circulation. In a documentary for BBC Horizon called The big Chill Broecker said I am convinced that the ocean is at the core of the whole thing.
This theory helped explain the Younger Dryas Ice age. Driven by the Milankovitch variations the ice began its retreat about 16, 000 years ago. Warm, salty water reached the north giving its heat to Europe along the way before sinking near Greenland and starting up the conveyor. As the conveyor gained power and pulled more warm water north the ice retreated but then for some reason around 12, 500 years ago the temperature abruptly plummeted again by nearly 5 OC and remained that way for 1, 300 years before warming again. During this period deep sea cores show that ice-bergs extended as far south as Portugal. Broecker believes that what happened was the conveyor turned off.
Evidence for this is found in sediment in the Gulf of Mexico (depleted in Oxygen 18) and sediment on the slopes of the Bermuda Rise. During the Younger Dryas the mud-dwelling foraminifera showed the chemical imprint of the Antarctic bottom water not the Atlantic Deep water that normally flows onto the Bermuda Rise. This is evidence for showing that at that time the conveyor belt was weak if flowing at all. Further work has been carried out on sediment cores to determine oceanic influences by various people. In 1995 Delia Open and Scott Lehman of Woods Hole analysed a core from the east flank of the Mid Atlantic Ridge. They analysed carbon isotopes in foram's.
Their results showed major shifts in the operation of the conveyor belt. Gerard Bond and his wife Rusty Lotti analysed a core containing iceberg-deposited sediment from latitude 50 degrees north (latitude of the south coast of England) called DSDP 609. The results from this core showed deposits of light and dark sediment some of which contained limestone. This limestone it turned out came from the Hudson Strait. John Andrews University of Colorado and Hartmut Heinrich had both identified the same layers in core samples from the Labrador Sea and south of DSDP 609. Bond determined that at various points a huge ice sheet had dominated the Hudson strait eventually creating an armada of icebergs that floated and melted in the Labrador Sea before moving into the North Atlantic.
Bond called these Heinrich events. Once the freshwater from the ice-bergs hit the North Atlantic the conveyor starts to weaken. What could cause the conveyor to turn off? At the start of the Younger Dryas a huge lake of glacial meltwater called Lake Agassiz started to flow southwards. At first it flowed into the Gulf of Mexico from the mississippi (oxygen 18 depleted sediment has been discovered there) then through the great lakes to the St Lawrence. All this freshwater flowing into the North Atlantic diluted the saltwater.
The conveyor belt is essentially driven by the dense salty cold water in the North Atlantic sinking. When the waters from Lake Agassiz flowed into the North Atlantic it meant that the water was not salty enough to sink. It is likely that only a 1 % change in salinity is enough to tip the balance. When the conveyor belt weakens or stops it has coincided with disruptions in the convection system in the North Sea. Broecker certainly believes that fresh water is the cause of conveyor shutdown as it stops the convection system.
If the conveyor belt weakens so does the heat it transports back to the North Atlantic, cooling the Northern Hemisphere even more. This is what scientists believe caused the Younger Dryas Ice-age and probably the cause of smaller ice-ages since. Whether the freshwater comes from icebergs moving down like the Heinrich events or the melting of ice sheets the result is the same. As the temperature cools in the Northern Hemisphere the global hydrological cycle is effected. The air becomes drier and eventually less evaporation and precipitation slows the icy freshwater deluge into the North Atlantic.
Cool air holds less moisture. A global cooling will effect the ocean temperatures and currents in the tropical pacific which dramatically affects worldwide weather. This is the system that today produces El Nino and La Nina (cyclical shifts in rainfall, winds etc). Less rainfall means the armada retreats as do the growing ice-sheets. The Atlantic waters become colder and saltier again starting back up the conveyor belt. Conclusion.
The results from the ice and the sea floor sediment cores showed one thing. The conveyor belt does appear to have decreased in power and turned off entirely at times in the past that coincided with large climate changes. When the conveyor belt switches state it changes the amount of heat transported to the North Atlantic and the result is global climate change. Global warming and climate change are a very real concern. Unfortunately our current knowledge of the phenomenon is not advanced enough. We do know that changes in temperature, rainfall and ocean currents which we previously believed took thousands of years actually happened in decades.
Computer models like the sophisticated system developed by Syukuro Manage at the Geophysical Fluid Dynamic Laboratory in Princeton have helped with the analysis of data available and have produced reconstructions of previous climate changes. The models have shown a clear established link between a conveyor cut off and precipitation globally. There are a lot of uncertainties in climate research however and even computer models struggle with possible scenarios for past as well as future. Computer models are limited partly due to the limits of current technology and the fact that there are so many factors to analyse. Some scientists believe we require a more detailed study of the sub-grid-scale processes in the ocean and others say we need to establish the oceanic equivalent of our MET office.
It is fair to say that considerably more research and investment is required if we are to learn more about the fine balance of the atmosphere and the oceans. Global climate changes have had a devastating effect on societies in the past. Rapid climate change in Britain could grind the infrastructure to a halt and effect the economy drastically. Ecosystems, economies and societies adapt better to gradual change and our ability to predict what the consequences are of global warming or changes in the oceanic currents is very important.
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