Snow cover – the extent of the Earth’s surface that is covered with snow — was the third-lowest on record this May for North America, according to researchers at Rutgers University who track snow cover each year based on an analysis of satellite images.

It’s part of a general downtrend that scientists say could have vast climatic consequences.

“Whether we have a record or not, it’s still indicative of a period of time where May and June snow extents are still much lower than they were earlier in the satellite era,” says Rutgers geography professor David Robinson, who tracks snow cover each year, adding to a record that now goes back 49 years (to 1967). “It’s just another year at the low end. You’re not going to break a record every year, but it’s going to be low every year.”

Scientists consider the extent of northern hemisphere snow cover in May to be perhaps the most significant because that’s when the sun starts to really shine down, and snow cover determines how much of it is reflected back to space. The longer snow cover lasts in May, meanwhile, the more its melt can contribute to streams, rivers, and drinking water supplies over the summer, says Robinson.

The actual data, per an e-mail from Robinson:

The May 2015 snow cover extent over North America was the third lowest on record (period of record is 49 years: 1967-2015).  On average, 8,070,000 million square kilometers (3,115,844 sq. miles) of the continent was covered.  This is slightly above the 8,010,000 sq. km. in 1968, with 2010 cover lowest at 7,800,000 sq. km.  The mean extent is 9,296,000 (based on the 1981-2010 period).  For comparison sake, North America covers 27,709,000 sq. km. from Panama northward and including Greenland.

Here’s an image from Rutgers of the departure from normal for May, showing that the snow loss was particularly intense in Alaska and stretched across much of Canada:

Robinson also tracks the snow cover over Eurasia, which wasn’t as low this year but which set a record low in May  2013.

We’ve seen one manifestation of snow cover lost in California, where the mountain snowpack is essentially gone this year — knocking out a previously vital source of water supplies. But the concern about declining snow cover goes much farther.

Because it is white, snow reflects sunlight away from the planet. But darker ground that is no longer covered with snow absorbs more radiation. This leads to more planetary heating. It’s the land-based equivalent of losing sea ice over the Arctic, which heats up the ocean.

“Take the north slope of Alaska,” explains Robinson. “They lost their snow cover about two weeks early this year. For those two weeks, you’ve got the sun shining 24 hours a day, and instead of that sun falling on snow cover, and some of it being reflected right back out to space, and some of it going to melting the snow, well, it’s now falling on darker, snow-free surfaces, and being absorbed, warming the ground, starting to melt permafrost, and also warming the atmosphere.”

Rafe Pomerance, a former deputy assistant secretary of state, is chairman of Arctic 21, a collection of Arctic-focused groups. “One of the elements that we cite in the unraveling of the Arctic is the loss of spring snow extent at the high latitudes,” he says.  “Unlike sea ice, it gets no attention.”