The downward trend in Arctic sea ice -- in both extent and volume -- is an unmistakable and oft-cited early sign of the impact of climate change. But of course, while the trend lines are clear, there are variations from year to year. For example, 2013 saw the sixth-lowest Arctic sea ice minimum on record, but because that extent was 1.69 million square kilometers (653,000 square miles) above the record minimum -- which was set just the year previously -- climate-change deniers promptly bounced up and down with glee and claimed that Arctic sea ice was “recovering.”

In contrast, 2012 was not only the lowest extent on record, it was the lowest by a lot, much lower even than the previous-record low of 2007, which had itself shattered existing records. The ultimate cause of this sharp decline, of course, was rising temperatures; but according to new research by NASA scientists, the approximate cause was the influx of warm water from Arctic rivers.

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Writing in the journal Geophysical Research Letters, a team led by Son Nghiem of NASA’s Jet Propulsion Laboratory describe a sudden influx of warm river waters into the sea in 2012 that rapidly warmed the surface layers of the ocean, enhancing the melting of sea ice.

The team used data from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA’s Terra satellite to examine sea ice in the Beaufort Sea, then compared it to reports of river discharge from the Mackenzie River in western Canada.

They found that on June 14, 2012, a stretch of landfast sea ice (sea ice that is stuck to the coastline) formed a barrier that held the river discharge close to its delta. Sometime over the next three weeks, that barrier had disappeared and the average surface temperature of the area of open water in the area increased by 11.7 degrees Fahrenheit (6.5 degrees Celsius).

“When the Mackenzie River’s water is held back behind the sea ice barrier, it accumulates and gets warmer later in the summer,” said Nghiem. “So when it breaks through the barrier, it’s like a strong surge, unleashing warmer waters into the Arctic Ocean that are very effective at melting sea ice. Without this ice barrier, the warm river waters would trickle out little by little, and there would be more time for the heat to dissipate to the atmosphere and to the cooler, deeper ocean.”

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According to Nghiem and team, the enhanced river flow is both a result of and contributor to a feedback system of Arctic warming. Increased temperatures lead to ice melt, which causes the collapse of barriers of landfast ice, increasing the volume of river water that enters the Arctic Ocean.

That river water would increase the temperature of the sea water anyway, but is having a greater effect because the rivers are also warming as a result of climate change. In addition, as river heating contributes to earlier and greater loss of the Arctic’s reflective sea ice cover in summer, the amount of solar heat absorbed into the ocean increases, causing even more sea ice to melt.

“If you have an ice cube and drop a few water droplets on it, you’re not going to see rapid melt,” said co-author Dorothy Hall of NASA’s Goddard Space Flight Center in a NASA press release. “But if you pour a pitcher of warm water on the ice cube, it will appear to get smaller before your eyes. When warm river water surges onto sea ice, the ice melts rapidly.”

Photo: NASA