Erica Grow is the weekend meteorologist at WNBC in New York City.
Last month, the National Snow and Ice Data Center announced that sea ice in both the Arctic and Antarctic circles had reached a new low.
In the Antarctic, where the summer season just wrapped up, rapid ice melt led to the lowest sea ice minimum ever recorded for the area. At the same time, on the other side of the world, the Arctic Ocean was experiencing Northern Hemisphere winter, a time in which sea ice reforms and thickens.
However, this past winter, the ice extent topped out with the smallest square mileage ever recorded. It was a very mild winter in the Arctic, about 4.5°F above average over the Arctic Ocean during the course of the winter.
That may not sound like a huge temperature surge, but in the icy Arctic, it's enough of a change to keep temperatures above freezing at a time when they usually drop below 32°F, thereby preventing the winter re-freezing cycle from meeting its normal potential.
Sea ice has been measured precisely via satellite imagery since 1979. These data not only allow us to see the full extent of the ice, but the thickness of the ice, as well. Satellite imagery can differentiate between new ice, which forms every cold season and disappears in the warmer months, from what is known as "multi-year ice."
This more mature polar ice is getting thinner, and in some cases, disappearing altogether.
"Multi-year ice is much thicker and stable, so that its loss represents a more severe and persistent change in the atmosphere and ocean," says Ian Howat, a glaciologist and professor at The Ohio State University.
When sea ice gets thinner, it also gets darker. This allows more sunlight to get absorbed, exacerbating the warming process.
Scientists point to the trend of thinning and shrinking sea ice as one of many signs that the Earth is warming. However, this is not the driving force behind one of the most prominent signs of climate change — sea level rise.
That dubious distinction belongs to glaciers, massive sheets of ice that sit on top of land and oftentimes spill out into an ocean. Glaciers form over the course of many thousands of years, and there's evidence that some ice sheets are millions of years old. Sea ice dwarfs in comparison to glaciers.
The West Antarctic ice sheet, for example, is more than a mile thick in some spots — taller than many mountain ranges. This means that a small percentage loss of glacial ice can have a much more profound impact on sea level than this year’s record sea ice minimum.
"It would take a very long time — thousands of years — for (ice sheets) to shrink appreciably. BUT, in terms of sea level, the ice sheets are so large that it doesn't take much of a change to cause a substantial, catastrophic rise in sea level," Howat says.
In fact, a report released by the New York Department of Environmental Conservation in February projects sea level rise as high as 6 feet is possible by the year 2100. This map from Climate Central shows all the coastal regions that will be submerged by 6 feet of sea level rise — including Wall Street and both JFK and LaGuardia airports in Queens.
Even if the worst-case scenario of sea level rise doesn’t manifest, the trend of diminishing sea ice is concerning to climate scientists.
"While the Arctic maximum is not as important as the seasonal minimum, the long-term decline is a clear indicator of climate change," said Walt Meier, a scientist at the NASA Goddard Space Flight Center Cryospheric Sciences.
So, the bigger issue is not that Arctic sea ice was exceptionally low this year; it's that the sea ice has been getting progressively lower and lower, and scientists say that there's no reason to believe this trend will reverse any time soon.