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Why don't the oceans freeze?
Category: Earth Science
Published: April 29, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
If the temperature is cold enough, ocean water does freeze. The polar ice cap at earth's North Pole is a giant slab of frozen ocean water. At earth's South Pole, the land mass constituting Antarctica complicates the situation, so most of the ice there is compacted snow. Over cold regions such as Antarctica, Greenland, and Canada, the fresh water in the air freezes to snow and falls onto the land without a melting season to get rid of it. Over time, this snow builds up and compacts into an ice mass known as a glacier. Gravity slowly pulls the glacier downhill until it reaches out onto the ocean, forming an ice shelf. The ocean-bound edge of the ice shelf slowly crumbles into icebergs which float off on their own path. For this reason, glaciers, ice shelves, and icebergs are all thick sheets of frozen fresh water and not frozen ocean water. In contrast, when ocean water freezes, it forms a thin flat layer known as sea ice or pack ice. Sea ice has long been the enemy of ships seeking an open route through cold waters, but modern ice breaker ships have no problem breaking a path through the fields of frozen ocean.
Despite the fact that the oceans do freeze when the temperature is cold enough, ocean water does indeed stay liquid under much colder weather than one would first expect. For instance, go to the beach on a winter day and you may be surprised to find that the ocean is still liquid despite the snow and ice on the ground being frozen. There are four main factors that keep the ocean in a liquid state much more than may be expected, as described in the textbook Essentials of Oceanography by Tom Garrison.
1. Salt
The high concentration of salt in ocean water lowers its freezing point from 32° F (0° C) to 28° F (-2° C). As a result, the ambient temperature must reach a lower point in order to freeze the ocean than to freeze freshwater lakes. This freezing-point depression effect is the same reason we throw salt on icy sidewalks in the winter. The salt lowers the freezing point of the ice below the ambient temperature and it melts. Note that if the ambient temperature is lower than 28° F (-2° C), the ocean water would be ice if this were the only effect involved. Such is not the case, so there must be other effects involved.
2. Ocean currents
The gravitational pull of the moon, earth's spinning motion, and thermal convection combine to create large-scale flows of ocean water known as ocean currents. This constant motion of the ocean water helps keep the water molecules from freezing into the somewhat stationary state of ice crystals. More significantly, the ocean currents continuously pump warm water from the equatorial regions to the colder ocean regions.
3. High volume
The larger the volume of water, the more heat has to be removed in order to freeze it. A teaspoon of water placed in the freezer will become completely solid long before a gallon jug of water. More accurately, it is the surface-area to volume ratio for a given external temperature that determines the rate of heat loss and therefore the speed of freezing. Because the heat must be lost through its surface, a small shallow puddle with a large surface will freeze quicker than a deep lake. The immense volume and depth of the oceans keeps them from freezing too quickly, thereby allowing the heating mechanisms to have a larger effect.
4. Earth's internal heating
As miners are well aware, the earth gets hotter and not colder as you dig straight down, despite the fact that you are getting farther away from the warm sunlight. The reason for this is that the earth has its own internal heat source which is driven primarily by the nuclear decay of elements inside earth's mantle. The earth's internal heat is most evident when lava flows and hot springs poke through the surface. Because earth's insulating crust is much thinner under the oceans than under the continents, most of the earth's internal heat escapes into the oceans. Although the temperature of the air at an ocean's surface may be freezing, the temperature of the water deep in the ocean is significantly warmer due to internal heating.
This combination of salt, ocean currents, high volume, and internal heating keeps most of the ocean in liquid form even during cold winters.