Global Currents, or "Thank you planet from 1000 years ago for the nice weather!"

Most people know what an ocean current is.  We’ve stood in the Atlantic or Pacific, started swimming, and ended up a quarter mile down the beach.  These currents are called surface currents, and they are largely wind and tide driven.  On the global scale, there are also surface currents which travel the same way that you are used to storms traveling (i.e. on the Pacific Coast: North to South, and on the Atlantic Coast: South to North).  These currents move warm water all over the world. They transfer huge swaths of energy which causes our weather patterns which then form the climates we are used to seeing. 

After water from the equator moves up the east coast of the United States to the North Pole, where does it go?  It can’t simply wrap around the earth like you are in a flying machine in Final Fantasy!  Obviously the currents don’t run out of water, heat is still being distributed around the globe so it has to be going somewhere to get recycled.

Recently (for earth science) scientists have discovered very deep currents along the ocean floor.  They are incredibly cold, and move incredibly slow.  Water may be in one of these currents for 1000+ years and has a max speed of about 0.2 MPH.   The (relatively) warm water at the surface of North and South Pole moves to the ocean floor by Density.

The classic elementary school experiment of combining oil and water is the best example of density.  When the oil and water are combined, the oil rises to the top because it is less dense than the water.  Now imagine you had a cup full of oil and dropped water one drop at a time into it- the water droplets would sink straight to the bottom of the glass.  The ocean around the poles is like this.  

Colder water is denser than warmer water.  Salty water is denser than fresh water.

 Saltier water runs north in the Atlantic Ocean.  That salty water collides with the fresh water melting from the polar ice cap.  This mixed water is saltier (denser) than the fresh water surrounding the ice cap, and colder (denser) than the warm water from the surface current, so like our water droplets it only has one place to go—down.  It is then pushed (very slowly) along the ocean floor by this density gradient until eventually it moves into the Indian ocean or the Northern Pacific- These oceans are less salty than the Atlantic and as the slow deep currents reaches them and distributes its salt (becomes less salty) it also becomes less dense and starts to rise!

As the earth gets hotter and hotter from global warming (from geological causes or man made) a measurable effect is the global ice caps melting.  We hear about it in the news all the time as they show pictures and figures about disappearing ice caps, however they never report why we should care.  The simple answer is… salt! Or lack thereof.  The ice caps are huge repositories of fresh water, and as they melt they dump more and more fresh water into the ocean around them.  This causes the mixed water where the warm salty surface current up the east coast and cold polar water meet to be less salty or less dense than it previously was.  The effect is a “slowing down” of surface water moving to the ocean floor.  The water in both deep and surface ocean currents will now be less dense which will cause a “slowing down” of the deep currents.  This slowing down is not only in terms of speed the water is moving, but the amount of water that gets moved as well.  There is less deep water (yielding less energy) being moved up into the Indian  Ocean and Northern Pacific which results in a slower, less energy dense surface current.  Again these surface currents are the primary way the earth transfers heat and energy from the equator to the poles, as it gets less and less energetic weather patterns and climates are forced to change.

A Note on the Geological Time Scale

Humans live for about 65-85 years we will say.  That means in 10-12 human life times from now the water currently sinking off the coast of Greenland may be surfacing in the Northern Pacific ocean.  10-12 lifetimes.  We have trouble comprehending global warming and climate change as the earth largely operates on a time scale we can’t fathom.  Our lives are but millisecond in the scope of the earth’s age.  This does mean that studying the exact effects of something like the slowing down of global currents can be extremely hard and largely extrapalatory.  These currents are already moving so slow and are already incredibly difficult to measure, we can’t just trend their slowing.  The surface currents we are seeing are the result of the state of the earth from hundreds of years ago.  Scientist do have incredibly clever ways of inferring what the state of the atmosphere was hundreds and thousands of years ago (look it up it’s dope) and can relate that geological existence with the one we are currently living in.  Just because you cannot physically see the changes on a daily basis remember the earth has seen 1642+ Billion Days.  These will be slight changes over a long time that will change our climates in very real ways.

Image Credit
http://act.rsmas.miami.edu/science/global-circulation/
http://cleanet.org/clean/community/activities/oceans.html
http://mail.tku.edu.tw/086138/EnvFutures/WebPages/Global%20warming/Thermohaline%20Circulation.htm