Arctic sea ice reaches its fall minimum

- Now that we are well into October, the Northern Hemisphere is heading into its colder season thanks to the top part of the Earth tilting away from the sun.  This gives us less sunlight and therefore cooler temperatures because the only thing that heats the surface of the Earth is the sun.  (I realize that there is a certain amount of heat that comes from the inside of the Earth, but it’s pretty negligible for daily life). 

Because of our cooling, the Arctic Ocean has reached its warmest temperature in September and is now gaining ice once again.  But if the warmest temperatures here in Minnesota, and through much of the continent are in July, why does the sea ice minimum occur in September?

It all comes down to the specific heat of air versus water.   Specific heat is the amount of energy it takes to heat an object by 1 degree Celsius.  It takes far more energy to heat water than it does air.  Therefore, the air acts more quickly on changes in sunlight than the oceans do.  When we start losing daylight in the latter part of June, it’s not enough to offset temperatures.  But by the end of July, the Northern Hemisphere as a whole is losing enough sunlight day to day that the average temperatures begin to cool.  But that point isn’t reached for many of the oceans until the middle or even end of September.

This year, it was right in the middle with the minimum sea ice extent coming around the 15th of September.  While it was another sub average year for minimum sea ice, at roughly 5 million square miles, it was not a record.  But it goes with the overall theme over the last decade or so.  This year was the 4th lowest sea ice extent on record, with all 4 of those years coming since 2012, but is trending upward.  We will see if that trend continues in the coming years.

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