Today's article is "Global sea ice coverage from satellite data: Annual cycle and 35-yr trends" by Claire Parkinson, published in Journal of Climate, 2014.
Sea ice is one of the hot topics in climate science right now, with a particular emphasis on Arctic sea ice. In recent years, Arctic sea ice has been decreasing, both in how much area the ice covers, and in how thick the ice is. This has serious implications on everything from climate change to global trade to animal welfare.
However, this is only one half of the issue. On the opposite end of the earth, sea ice in Antarctica is increasing. And, given that the average amount of sea ice at each pole is similar in magnitude, many people think (wrongly) that the two poles cancel each other out. And so, instead of focusing on one pole, the author of this paper looked at both poles combined to calculate the global trend in sea ice and set the record straight. (Spoiler: It's still decreasing overall.)
Now, let's back up for a minute and talk about exactly what I mean by "sea ice". The term itself is rather obvious - ice that is on the ocean. This does not count ice that is attached to ice on land - no glaciers, ice shelves, or ice sheets here. When sea ice is measured, a satellite measures the amount of ice in a particular area, essentially one square in a grid that covers the earth. This measurement is returned as a percentage - so the percentage of water that is covered by ice in each particular box. To get the total amount of ice, we say that each box that is more than 15% ice is ice covered. Then we add up the areas of all the ice covered boxes. 15% may seem like a random threshold, but it is widely used in the scientific community as a standard.
In general, sea ice in the Arctic is very different from the Antarctic. There are two main characteristics we can use to compare sea ice at each pole (indeed, any sort of environmental data). When we measure sea ice, we can separate the data into two parts - a seasonal cycle, and a trend. The seasonal cycle is just what it sounds like. As it gets warm in summer, the ice decreases, and then increases again during winter. Almost everything in the environment has a seasonal cycle. We can calculate the seasonal cycle, and subtract it from the data. What we are left with is the trend, or how the amount of sea ice changes from year to year. In terms of magnitude, the seasonal cycle is typically much larger than the trend, so it is hard to see a trend without removing the seasonal cycle.
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| Sea ice seasonal cycles. The seasonal cycle is the same for every year data was taken. Note that while both poles seems to have a similar amount of sea ice, the extremes in the Antarctic are larger than those in the Arctic (compare the maxes and then the mins). Thus, the global seasonal cycle hits a minimum the same time as the Antarctic. Figure 2 from Parkinson, 2014. |
In the Antarctic, the seasonal cycle is large in magnitude. In the (austral) summer, almost all of the ice melts away, but comes back again the following winter. In terms of trend, different regions of the Antarctic have different trends. If you average them out, the overall trend is slightly positive = increasing amounts of sea ice over time (years).
In the Arctic, the seasonal cycle is smaller in magnitude. It is opposite sign of the Antarctic, because the seasons are switched going from southern to northern hemisphere. Ice tends to grow slightly in winter, but sticks around during summer. A good portion of the ice in the Arctic is multi-year ice - it has been around for several years. The trend here is decidedly negative = decreasing amounts of sea ice over time.
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| Sea ice trend. This is the part that is left over after removing the global seasonal cycle from the data. The data is still noisy - not all points fall exactly on the trend line - but it is easy to see that the global trend is clearly negative. (A statistical analysis shows that it is significant as well.) Taken from Figure 3 of Parkinson, 2014. |
Now, if we add the sea ice information from both poles together, we get an idea of what sea ice is doing globally. Looking first at the seasonal cycle, the global cycle follows the same basic pattern as the Antarctic. This makes sense. The Antarctic cycle has more extremes than the Arctic one, and this dominates when you combine the data. But, when you look at the global trend, it follows the negative pattern of the Arctic trend. Since the Arctic is losing sea ice much faster than the Antarctic is gaining sea ice, it follows that globally sea ice is decreasing.
Thus, with a relatively simple (but necessary) analysis, the author showed that the negative sea ice trend in the Arctic is not canceled out by the positive sea ice trend in the Antarctic. Overall, from a climate perspective, global sea ice is decreasing - a solid indicator of climate change.
You may have noticed the title of the paper specifically says a 35-yr record. That's all the longer we have the satellite data to make these sorts of calculations. It is entirely possible that the 35 year trends found here are not trends at all, but small sections of a much longer cycle. This is why climate scientists use other sources of data and climate models to help confirm findings from the satellite record.
