Follow That Puppy!
Sea ice grows in the Arctic each winter, but Arctic sea ice extent has been declining.
How is that possible? Think about walking your puppy down a hill.
Imagine that you live atop a hill and you decide to take your new puppy for a walk. At first he is excited and begins to race down the hill. When he feels the tug on the leash because you are trailing behind, he hops back up the hill to greet you and encourage you to join him in his race down the hill. But you are comfortable moving at your current pace and intend to do so for the entire walk to the bottom of the hill. Your puppy however, continues to meander uphill and down. If we were to create a visualization the erratic changes in elevation of your excited little puppy, it might look something like the black line in the figure below:
In reality, this black line represents the average Arctic sea ice extent for every November since 1978 (in millions of square kilometers). Sea ice forms when ocean water freezes (1). Extent is essentially the area of the ocean that is covered with at least 15% sea ice (2). We can see that like your puppy, the ice extent goes up and down over time, but the overall direction is downward (blue line). This is what is known as a trend.
So while the Arctic sea ice extent has varied from one November to the next, over the last several decades it has trended downhill. In fact, Arctic sea ice extent displays this trend for every month of the year (3). Now imagine that halfway through your trip, the hill gets even steeper. You and your dog would likely march downhill even faster. Similarly, there has been an increase in the speed at which sea ice extent is decreasing in recent years, with the greatest rate of decline happening in September (4).
We can also look at the seasonal cycle of Arctic sea ice, or how the Arctic sea ice extent changes throughout the course of a year. A seasonal cycle is comparable to how an adult’s weight might change throughout one year. For example, a six foot tall young adult male of medium build would, on average, weigh 170 pounds. He might weigh a bit more in winter because he spends more time indoors indulging in holiday treats, but in the summer he is more active and thus lighter. But from year to year, his weight remains around 170 pounds. If we look at the sea ice extent for each month of the year and average the values between 1981 and 2010, we would get the black line below which shows the seasonal cycle of Arctic sea ice extent: it grows to a maximum in March (hilltop) and retreats to a minimum in September (valley).
Comparing an individual year to the black line tells us if the sea ice extent is above or below average. The red line represents Arctic sea ice extent last year (2016). Do you see how the red line is located below the black line? This tells us that sea ice extent for 2016 fell nearly 30% below the 1981-2010 average. Remember our adult male whose weight remained around 170 pounds? Imagine if his weight in 2016 fell 30% below that value- he would weigh, on average, 119 pounds! We would conclude that something about this man’s lifestyle is definitely out of balance. Similarly, the rapid decline in Arctic sea ice is telling us that something is out of balance in our climate system.
While winds and weather systems play a role in decreasing sea ice extent, they are not enough to explain the rate at which Arctic sea ice extent has declined in recent decades. The rapid loss of Arctic sea ice is strongly linked to rising temperatures which are being caused by the massive amounts of carbon dioxide (CO2) that we are spewing into our atmosphere (5).
But if these changes aren’t happening in our backyards, why should we care? Even though it seems like we aren’t connected to what is happening in the Arctic, sea ice is an extremely important part of the global climate system. The circulation of the world’s oceans is driven by the production of dense water in the Arctic and North Atlantic Oceans which is formed, in part, when salt is expelled into the ocean as sea ice forms. When sea ice melts (and land ice too), the water becomes fresher and less dense and this disrupts the Global Ocean Conveyor Belt, which is responsible for carrying vast amounts of heat across our planet and plays a vital role in maintaining our current climate (6,7).
Recall our adult male who lost so much weight? He needs a swift overhaul of his current lifestyle to restore him to a healthy state. Finding an even balance between eating and exercising would be key. Well our climate system is out of balance because we are forcing our atmosphere to take on way too much carbon dioxide.