Global Warming May Alter Atlantic Currents, Study Says John Roach for National Geographic News Article accessed online 10 January 2010

Suffice it to say that the conveyor belt continues to work today. But freshwater runoff into the North Atlantic has increased in recent decades, and runoff is expected to increase further as global temperatures climb higher, Curry said. Curry and research colleague Cecilie Mauritzen of the Norwegian Meteorological Institute in Oslo estimate that it will take about a century, at present rates, for the circulation pattern to be seriously affected by the increase in freshwater runoff. The scientists conclude that it would take about two centuries for freshwater runoff to halt the North Atlantic conveyor belt entirely. Curry and Mauritzen published their findings in the June 17 issue of the research journal Science. Stefan Rahmstorf, a professor of ocean physics at Potsdam University in Germany, said the researchers' calculations appear accurate. But he emphasized that their findings do not provide a forecast; they only give an impression of the amount of freshwater required to alter or halt the North Atlantic ocean current. "It is of course very unlikely that the freshening [freshwater inflow] will simply continue at the same rate it has for the past few decades," he wrote in an e-mail to National Geographic News. Rahmstorf said the freshening could be part of a natural fluctuation in Earth's climate system that will stop and reverse. He added, however, that if the phenomenon is due to global warming, which he said is likely, then the freshening will probably accelerate as glaciers melt and more rain falls at high latitudes in response to rising temperatures.

According to Curry, scientists are uncertain as to the exact course global warming will take and how it will affect the amount of freshwater flowing in the North Atlantic. A particular wild card, she noted, is Greenland.

"As it does melt, it will release freshwater into the Nordic seas"—water bodies found between Iceland, Greenland, and Norway—"and that probably represents the biggest source of freshwater that could have an impact on the conveyor belt," she said. There are a number of mechanisms that could inject large amounts of freshwater into the Nordic seas at the precise region that is critical to the conveyor belt. They include

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  pooling and release of glacial meltwater,
  
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  collapse of an ice shelf followed by a surge in glacier movement, or
  
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  [decreased friction along] a glacier's base through increased melting [basal sliding].
  

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4. Infer from the article what water properties scientists most commonly measure to study ocean circulation. Name at least two properties and explain why they are measured.

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  ________________________ is a measure of the amount of salt dissolved in a liquid. The salt in the oceans comes from ________________________ pounding against the rocks of the lithosphere. Melting glaciers dilute seawater because ________________________ is mixed with the salt water.
  

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  Ocean currents move seawater in a certain ________________________. ________________________ describes the combined effect of all the ocean currents. ________________________ are the main cause of surface currents. Subsurface currents are mostly due to variations in ________________________ between water layers. The ________________________ the water, the deeper it sinks.
  

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  Also, the ________________________ the water, the denser it is, and the saltier the water, the ________________________ it is as well. Temperature and ________________________ therefore affect water density.
  

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  Surface and subsurface currents are closely connected. They form a huge loop like a conveyor belt, and their movement is referred to as “________________________ circulation.” This loop is instrumental in the transfer of ________________________ on Earth. It lessens temperature ________________________ between the equator and the poles.
  

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  The water that is found in solid form on Earth is called the “________________________.” It consists of pack ice, ________________________, large expanses of snow, frozen lakes and rivers and the ice in permafrost.