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Ocean Current Switching: Can changes in ocean circulation cause rapid climate change? Richard Karsten Department of Mathematics and Statistics [email protected] IB Camp, Wolfville August 20, 2008 Climate change, Hollywood style Premise: Climate change might occur rapidly, causing untold destruction This year, sweater won'tdidn't do. check the weather. Whoever said "Tomorrow Nature is aanother has spoken. day"... The villain: the Gulf Stream Wikipedia Synopsis: The movie is based on the idea that the Gulf Stream (or North Atlantic drift), an ocean current which circulates warm water from the tropics to the Northern Hemisphere, is disrupted by the melting of the polar ice caps. This leads to catastrophic changes in the Earth's climate, as the temperature of the Earth's atmosphere stabilizes into a new pattern. A little more science on the big screen … Melt water from Greenland, because of its lower salinity, could then halt the currents that keep northern Europe warm and quickly trigger dramatic local cooling there. (source: Wikipedia) Is any of this realistic? • Can the ocean affect climate? • What is the Gulf Stream and how does it affect climate? • Can melting icecaps move ocean currents? • Is the effect of the Gulf Stream changing due to global warming? Is any of this realistic? • Can the ocean affect climate? • What is the Gulf Stream and how does it affect climate? • Can melting icecaps move ocean currents? • Is the effect of the Gulf Stream changing due to global warming? Ocean and weather: Nova Scotia Australia 19 o 15 www.nsac.ns.ca o Ocean and climate: El Nino Australia El Niño La Niña earthobservatory.nasa.gov The villain: the Gulf Stream British Geomorphological Research Group www.bgrg.org Hopedale, 55º27’ N Stornoway, 58º54’ N The ocean stores a lot of heat. A column of ocean water only 3 m thick contains as much heat capacity as the full atmosphere above (Gill, 1982). Depth 5000 m Halifax Bordeaux The ocean stores the heat created by global warming. S. Levitus, J. Antonov, and T. Boyer, GRL 2005 Is any of this realistic? • Can the ocean affect climate? • What is the Gulf Stream and how does it affect climate? • Can melting icecaps move ocean currents? • Is the effect of the Gulf Stream changing due to global warming? Ocean currents The strongest currents in the ocean are a result of the wind blowing over the surface of the ocean. Average surface winds A rotating earth changes dynamics: Coriolis force To observers in the Northern Hemisphere, rotation of the earth deflects motion to the right. o Wind forcing: Flow is 90 to the right! 50 m Wind-driven circulation H Gyre flow H Thermohaline circulation • Changes in the density of sea water can also drive ocean circulation. • Vertical Motion: Light water rises to the top and heavy water sinks. • Density changes are due to changes in temperature (thermo) and salinity (haline). Sea water density Warm water low density (light) Cold Water high density (heavy) Fresh Water low density Salty Water high density Water properties Temperature Salinity Water properties: Salinity Temperature meridional North South depth depth South Salinity meridional North Convection and overturning Cooling at the surface can create heavy water at the surface, which then “sinks.” The net effect is an overturning circulation. Heat loss Gulf Stream transports heat north Warm water pulled north by cooling and sinking. Gulf Stream Warm water “pushed” north by wind. (after W. Broecker, modified by E. Maier-Reimer). Gulf Stream and climate www.theworldwidegourmet.com Australia www.travelblog.org/Bloggers/Dan--NZ/ Thermohaline circulation (Meridional overturning circulation) Is any of this realistic? • Can the ocean affect climate? • What is the Gulf Stream and how does it affect climate? • Can melting icecaps move ocean currents? • Is the effect of the Gulf Stream changing due to global warming? Melting Ice • when ice melts in turns into COLD, FRESH water • If this water enters the north Atlantic it can prevent the sinking of the salty surface waters Melting Ice Water properties: Salinity Global warming is melting polar ice caps, which should make the northern waters fresher. South depth meridional zonal meridional North Fresh water shuts off convection Fresh water from glacial melt makes surface water fresher. If the water is fresh enough, it doesn’t become heavy so it doesn’t sink. Heat loss X Thermohaline circulation (Meridional overturning circulation) x Is any of this realistic? • Can the ocean affect climate? • What is the Gulf Stream and how does it affect climate? • Can melting icecaps move ocean currents? • Is the effect of the Gulf Stream changing due to global warming? Is the Gulf Stream changing? Weaker Gulf Stream threatens Britain's climate By Mark Henderson, Science Correspondent London Times, November 30, 2005 An ominous current event Matthew Hart Globe and Mail, January 14, 2006 Evidence of freshening http://www.whoi.edu/mr/pr.do?id=5098 Evidence of freshening (B. Dickson, et. al., in Nature, April 2002) Northern waters are fresher • North Atlantic water has become fresher, possibly reducing the thermohaline circulation. • If the warm water is no longer pulled into the North Atlantic, it should return south at shallower depths. X (after W. Broecker, modified by E. Maier-Reimer). Are there changes in the circulation? Deep return flow has reduced by about 30%. Shallow return flow has increased by almost 50%. Measurement line Surface warm flow has decreased slightly Reaction: Climate change: A sea change by Quirin Schiermeier in Nature 2006 • The results are a surprise to scientists in the field. • Modelling suggests that increase of fresh water flows large enough to shut down the thermohaline circulation would be an order of magnitude greater than currently estimated to be occurring. • The Bryden results could be caused by natural variation, or "noise", that is, coincidence. • If the results are correct, perhaps thermohaline circulation reductions will not have the drastic effects that have been predicted on European cooling. More … Ocean circulation noisy, not stalling by Quirin Schiermeier in Nature 2007 • “…the seemingly dramatic reduction discovered two years ago in the strength of the Atlantic meridional overturning circulation (MOC) is easily within the range of huge seasonal variability.” Ocean Current Switching: Can changes in ocean circulation cause rapid climate change?