Ocean Current Switching: Can changes in ocean circulation cause rapid climate change? Richard Karsten Department of Mathematics and Statistics IB Camp, Wolfville August 20, 2008

Climate change, Hollywood style

Premise: Climate change might occur rapidly, causing untold destruction This year, a sweater won't do. Whoever said "Tomorrow is another day"... didn't check the weather.Nature has spoken.

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.Gulf StreamNorth Atlantic driftocean current tropicsNorthern Hemispherepolar ice Earthclimate

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.salinity (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 15 o 19 o

Ocean and climate: El Nino Australia earthobservatory.nasa.gov El Niño La Niña

The villain: the Gulf Stream British Geomorphological Research Group

Hopedale, 55 º 27’ 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.

Wind forcing: Flow is 90 o 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 waterlow density (light) Cold Waterhigh density (heavy) Fresh Water low density Salty Waterhigh density

Water properties TemperatureSalinity

Water properties: Salinity South North meridional depth South North meridional depth TemperatureSalinity

Convection and overturning Cooling at the surface can create heavy water at the surface, which then “sinks.” Heat loss The net effect is an overturning circulation.

Gulf Stream transports heat north (after W. Broecker, modified by E. Maier-Reimer). Gulf Stream Warm water “pushed” north by wind. Warm water pulled north by cooling and sinking.

Gulf Stream and climate Australia

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?

when ice melts in turns into COLD, FRESH water Melting Ice If this water enters the north Atlantic it can prevent the sinking of the salty surface waters

Melting Ice

Water properties: Salinity zonal meridional South North meridional depth Global warming is melting polar ice caps, which should make the northern waters fresher.

Fresh water shuts off convection Heat loss 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. 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?

An ominous current event Matthew Hart Globe and Mail, January 14, 2006 Weaker Gulf Stream threatens Britain's climate By Mark Henderson, Science Correspondent London Times, November 30, 2005 Is the Gulf Stream changing?

Evidence of freshening

Evidence of freshening (B. Dickson, et. al., in Nature, April 2002)

Northern waters are fresher (after W. Broecker, modified by E. Maier-Reimer). 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

Are there changes in the circulation?

Surface warm flow has decreased slightly Shallow return flow has increased by almost 50%. Deep return flow has reduced by about 30%. Measurement line

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?