The countervailing effects don't cancel each other out. They're both signs of the changes we are bringing to the planet.
In the seas to our north, sea ice is melting at a perilous pace. This year crushed earlier records for the lowest sea-ice extent ever recorded in the Arctic. Some scientists predict that in four years, Arctic sea ice may collapse entirely during the summer months.
It turns out that the northern pole is not the only one setting records with unusual ice patterns. But to the south, the story is different: record-setting sea-ice extent. That is to say: While the ice to the north is melting away, the ice to the south is growing, extended over 19.44 million square kilometers this year, breaking the previous record (set in 2006) of 19.39 million square kilometers..
Now, lest you get all, "Phew. So glad this who global warming thing is being balanced out," let me tell you plainly: That is not how it works. These are not counteracting phenomena that somehow cancel each other out. Both of these sea-ice trends are the result of changes in our planet's climate, but the effects are different because our hemispheres work in different ways.
To begin to understand this, look north first. The ice there is melting for a pretty straightforward reason: It is getting warmer. As the sea ice melts, the dark oceans absorb more of the suns rays than the bright ice (which reflected them), and the process accelerates.
The south has a more complicated story, with several converging reasons for the ice accretion. The first reason is our depleted ozone layer, which has brought cooler winds and stormier weather. Scientists think that the hole in the ozone has brought temperatures in the stratosphere over Antarctica (about 6 to 37 miles overhead) down by 2 to 6 degrees Celsius. The increased winds push the sea ice around, opening up gaps of water called polynyas where more ice develops rapidly.
Another possible contributing force is an increasingly temperature-stratified ocean. As NASA explained in 2009:
Changes in ocean circulation may also play a role. Jinlun Zhang, an oceanographer at the University of Washington, has pieced together a complex computer model that helps explain why Antarctic sea ice is expanding even with signs that ocean and air temperatures are on the rise. The key is that warming temperatures can lead to more stratified ocean layers.
In the Southern Ocean, there's a layer of cold water near the surface and a layer of warmer water below. Normally, convection causes the two layers to mix and exchange water, a process that brings heat from the lower layers to the surface layer and ultimately helps keep sea ice expansion in check. This transfer of heat is the primary reason that first-year ice in the Antarctic is much thinner than in the Arctic.
But if global air temperatures warm, the model indicates that the amount of rain and snowfall could increase, and surface waters could freshen. Since fresh water is less dense and less apt to mix with the heavier, saltier, and warmer water below, the layer at the ocean's surface could become more stratified and mix less. This, in turn, would reduce the amount of heat flowing upward, allowing surface ice to expand.
One last thing to note: Besides that the polar effects are moving in opposite directions (polar opposites, get it?), the magnitude of the changes are not even close. The minimum extent of Arctic sea ice is declining by an average of some 91,600 square kilometers per year. Antarctica, in contrast, is gaining ice by about 17,000 square kilometers a year. But even if the pace *were* the same, where would that leave us? No amount of ice at the South Pole is going to get us out of the jam we're in.