Greenland's massive ice sheet may be in more serious peril from climate change than scientists previously thought, a new study has found.

Studies agree that rising global temperatures are causing the ice sheet that covers most of the world's largest island to shrink. If the entire ice sheet were to melt, global sea levels could increase by as much as 23 feet.

But in a study published Monday in the journal Nature Climate Change, researchers found that global warming is also undermining the ability of Greenland's "firn" to limit the effects of climate change.

The firn is the sponge-like snow pack atop the ice sheet, which traps and stores melting water that would otherwise run off into the oceans. It thereby helps to maintain the ice sheet in the face of the usual summer's warmer temperatures.

Past studies had concluded that the firn's storage capability was largely undiminished. But Greenland endured exceptionally warm summers in 2010 and 2012 -- in the latter year, it experienced "the largest observed melt extent" on record.

Now the latest study has found that the firn has become denser and less porous, making it far less absorbent.

What happened? The researchers found that the greater amounts of meltwater from those warmer summers filled up the firn's pores and hardened into an impenetrable layer of ice. Consequently, meltwater in the following years couldn't be absorbed by the firn and "instead drained along the ice sheet surface toward the ocean."

As study author and York University researcher William Colgan explained in a press release, that finding "overturned the idea that firn can behave as a nearly bottomless sponge to absorb meltwater. Instead, we found that the meltwater storage capacity of the firn could be capped off relatively quickly."

The study is a reminder that we don't know all the ways that climate change is affecting our world.

"Basically our research shows that the firn reacts fast to a changing climate," said Horst Machguth, the lead researcher from the Geological Survey of Denmark and Greenland.

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