One of the most obvious effects of global climate change is that it’s causing ice to melt all over the world, especially at the planet’s frozen poles. Ice losses have been observed for years now in glaciers and ice sheets, as well as in Arctic sea ice — the ice that floats on top of the ocean. In a confusing twist, though, new research published Monday is showing that there was a large increase in Arctic sea ice in 2013, rather than a decrease.

It’s just the kind of news often seized upon by climate skeptics as a way to undermine the concept of anthropogenic global warming. However, making sense of these observations requires a deeper understanding of long-term trends in sea ice and the factors that affect it from one year to the next.

Overall, Arctic sea ice has experienced a decreasing trend since the 1970s — however, its extent still fluctuates slightly from one year to the next depending on local climate-related conditions, sometimes increasing a bit from one year to the next, and other times decreasing. Despite these little annual fluctuations, the overall trend observed for decades now still shows that we’re losing ice in the long term.

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These are important things to bear in mind when considering the new research on Arctic sea ice. The study, published Monday in the journal Nature Geoscience, examines observations of sea ice volume between 2010 and 2014, derived from measurements taken by the European Space Agency’s CryoSat-2 satellite. These observations indicate that there was an unexpected increase in Arctic sea ice between 2012 and 2013, when sea ice volume increased by a whopping 41 percent — a result that “was really quite surprising,” says Rachel Tilling, a doctoral student at the University College London and lead author of the study.

The sudden increase meant that, overall, there was 33 percent more ice in 2013 and 25 percent more in 2014 compared to the average amount of sea ice that was present between 2010 and 2012. The researchers attribute the surprising increase to unusually cool conditions in 2013, which caused there to be 5 percent fewer “melting days” — warm days in the summer during which sea ice is actually melting — that year. (There are a certain number of days each summer, when temperatures are highest, that cause sea ice to melt — at least some of the ice then refreezes in the winter when temperatures start to cool back down again.)

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The key thing to remember here is that conditions in 2013 were very unusual compared to the overall trend the Arctic has seen the last few decades. In fact, the authors point out that the cool conditions and fewer melting days observed in 2013 were more typical of conditions observed during the late 1990s. This means what happened in 2013 was an anomaly — an effect that is significantly different than the conditions that are usually observed. When looking at graphs that show changes in Arctic sea ice over the course of the last few decades, there’s still a definite downward trend.

“These people are not making any claim that we’re going to have some big recovery [in sea ice],” says Mark Serreze, director of the National Snow and Ice Data Center (who was not involved with the paper), adding that decreasing trends in the sea ice mean we’re still eventually heading toward a seasonally ice-free Arctic ocean. “But it’s going to occur in fits and starts because the sea ice is highly variable — we’re going to go up some years, down in others.”

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Serreze does have some criticism for the paper. This study shows that “sea ice thickness, like extent, is highly variable,” he says. “Now, we’ve already known that it’s highly variable … So it’s really not saying anything new there.” He also has some concerns about the basic methods the researchers used to compute changes in sea ice volume, arguing that they made some “suspect assumptions” about certain important factors that could affect their calculations, like the thickness of snow cover on the ice.

However, Tilling argues that the study offers new information because most data up until now have only looked at sea ice extent — the area the ice covers in the Arctic — and not sea ice volume, or thickness. She says this is the first study to actually measure sea ice volume, rather than get that information from models. “Sea ice is complex —  it doesn’t just grow in area, it grows in thickness,” she says. “To see what’s really happening, you need to look beneath the surface.” And she adds that she and her colleagues performed mathematical tests to see how much these questions about the snow load could affect their results and concluded that “it wasn’t a significant source of uncertainty.”

While we know ice is going to continue melting as long as temperatures in the Arctic keep heating up, Tilling and her colleagues conclude that the big increase in Arctic sea ice after just one cool summer may mean the ice is more resilient than we previously thought. However, she adds, “It is only five years worth of data, so it’s very difficult to say one way or the other,” and she acknowledges that five years is not enough time to establish a trend. She says the goal of future research in the field should be to “keep taking these measurements and start to develop a longer term dataset so we can start to look at more data on interannual variability and see what is the long term trend.” Once researchers are able to recognize long-term trends in sea ice volume, they can plug more detailed information into the models they use to predict what the Arctic’s future will look like.

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For now, though, that future looks the same as it did before this paper was published — that is, a little on the bleak side. “We do not want to suggest that this is a recovery in the long term,” Tilling cautions. “If you look at the long-term temperature trend in the Arctic, it’s upward, and if you look at the long-term volume trend, it’s downward. If temperatures continue to rise, volume is going to continue to decrease.”

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