The ice sheets of Greenland are melting, and it could be worse than we thought.

A new study suggests the ice sheets could be much more sensitive to human-driven climate change than previously estimated. Understanding the geological past of Greenland is vital for predicting sea-level rise, as a result of global warming. That’s because its ice holds enough water to cause 23 feet (7 meters) of sea-level rise, which is a risk to every coastal region on Earth, the authors state.

Greenland’s ice sheet is about 2 miles (3.2 km) thick and spans an area three times the size of Texas. And it’s already shrunk in the ancient past. Between 374,000 years and 424,000 years ago, moderate warming caused the ice sheet to melt, leading to a dramatic sea-level rise of around five feet, the study shows.

The new study contradicts previous estimates that the Greenland ice sheet has persisted for most of the last 2.5 million years. The melting of some of Greenland’s ice around 416,000 years ago left behind an ice-free tundra landscape that may have been covered by trees and roaming woolly mammoths. 

The melting also took place despite carbon dioxide levels in Earth’s atmosphere being lower at the time than today: just 280 parts per million (ppm), as opposed to 420 ppm (and rising) in modern times. (Carbon, a greenhouse gas that traps heat easily in Earth’s atmosphere, is one of the drivers of global warming.)

Read morer at Space.com