“We keep on [seeing] rising temperatures between the 1990s and 2011,” said study lead author Maria Hoerhold, a glaciologist at the Alfred Wegener Institute in Germany. “We have now a clear signature of global warming.”

It takes years to analyze ice core data. Hoerhold has new cores from 2019 but hasn’t finished studying them yet. She expects the temperature rise to continue as Greenland’s ice sheet and glaciers have been melting faster recently.

“This is an important finding and corroborates the suspicion that the ‘missing warming’ in the ice cores is due to the fact that the cores end before the strong warming sets in,” said climate scientist Martin Stendel of the Danish Meteorological Institute, who wasn’t involved in the research.

The ice cores are used to make a chart of proxy temperatures for Greenland running from the year 1000 to 2011. The chart shows temperatures gently sloping cooler for the first 800 years, then wiggling up and down while sloping warmer until a sharp and sudden spike hotter from the 1990s on.

The jump in temperature after 1995 is so much larger than pre-industrial times before the mid-19th century that there is “almost zero” chance that it is anything but human-caused climate change, Hoerhold said. The warming spike also mirrors a sudden rise in the amount of water running off from Greenland’s melting ice, the study found.

What had been happening in Greenland is that natural weather variability and undulations due to an occasional weather system called Greenland blocking in the past had masked human-caused climate change, Hoerhold said.

But about 25 years ago, the warming became too big to be hidden, she said.

Past data also showed Greenland not warming as fast as the rest of the Arctic, which scientists say is now warming four times faster than the global average. But the island appears to be catching up.

Hoerhold’s team drilled five new cores near old cores so as to match established ice core records. They used the difference between two different types of oxygen isotopes found in the ice to calculate temperatures, using an already established formula that is checked against observed data.

“We should be very concerned about North Greenland warming because that region has a dozen sleeping giants in the form of wide tidewater glaciers and an ice stream,” said Danish Meteorological Institute ice scientist Jason Box. And when awakened, they will ramp up the melting of Greenland’s ice, he said.