Rain on Ice

Last week it rained at the summit of the Greenland Ice Sheet. That’s right, it rained, at 3000 m above sea level in the middle of a huge mass of snow and ice. Greenland itself has been breaking summer temperature records this year, with temperatures in the 20⁰C+ range, which is one of the reasons it was warm enough to rain at the ice sheet summit. According to researchers, temperatures have only been above zero at the Greenland summit nine times in the past 2000 years.

It’s another indicator of climate change, as if we haven’t had enough with the wildfires in Canada, Siberia, Greece, and Turkey; flooding in Germany and Belgium and more recently in Tennessee; and severe drought on the Canadian prairies, in the American West, and in Madagascar. And these are just the highlights to give you a sense of the diversity of climate change impacts.

Drought in Canada as of July 31, 2021. Darker colours signify worse drought (from https://agriculture.canada.ca/en/agriculture-and-environment/drought-watch-and-agroclimate/canadian-drought-monitor).

The rain in Greenland dropped 7 billion tonnes of water on the ice sheet, increasing daily surface ice melt by seven times higher than average. Rain saturates the snowpack, thereby reducing the reflectivity of the ice sheet and allowing for more melt. Reflectivity is critical for glacier melt – when I was working in the Arctic in 2000, for example, we had a warm, windy event that stripped the glacier surface down to the previous summer’s ice surface, which was dirty from dust deposition and melted faster because of that reduced reflectivity. Rain can also freeze as it hits the cold snow/ice surface, leaving a layer of ice that will show up in future ice cores as a marker for such a bizarre summer precipitation event.

Researchers are concerned about Greenland melt not just due to sea level rise, but due to the impacts of fresh glacier water on the Atlantic Meridional Overturning Circulation (AMOC). This is an ocean current in the North Atlantic that brings warm water up from the equator and then sinks as it cools and becomes more saline, pulling the warm water up behind it like a conveyor belt. Recent studies have suggested that this current is weakening, and that it might be in part due to increasing fresh water from Greenland ice melt keeping the current from densifying and sinking, and thus preventing it from pulling warm water north. With a weaker AMOC, Europe could get colder and sea levels could rise along the US east coast, while seasonal monsoons could be disrupted.

Currents like the North Atlantic Current bring warm water north. (by R. Curry, CC-BY-3.0)

We are living in strange times indeed when there’s rain on the summit of a cold ice cap.

Note the featured image is the extent of Greenland melt during a high melt event on July 28 2021.

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