Average temperatures around the world may be slowly ticking upward, but their effects on the environment can be sharp and quick.

Earlier this week, we learned that Arctic sea ice extent is declining at its fastest rate in 1,500 years. On Wednesday, scientists reported that in 2003, the ice sheet on Greenland suddenly began melting at a much faster rate.

Greenland’s ice is hugely important for the planet. The sheet of ice that covers 660,000 square miles is more than a mile thick and has a volume of 684,000 cubic miles.

If it all melts, it would raise global sea levels by 25 feet. On Thursday, scientists published a new map of the land mass beneath Greenland’s ice, showing that the ice is deeper than previously thought, so losing it all would lead to an additional 2.7 inches of sea level rise above past estimates.

Already, about 269 billion tons of ice have melted away each year since 2002.

In a paper published Wednesday in the journal Science Advances, scientists reported that in one catchment in Southwestern Greenland, the melt rate surged 80 percent between 2003 and 2014 compared to the 26-year period beforehand, hinting at changes in the overall melt rate of the ice sheet.

It’s yet another sign that the rising average temperatures of climate change across the planet are not just having a gradual effect on ecosystems. Some of the biggest changes are abrupt, massive, and have already happened.

Scientists now have a clearer picture of Greenland’s melting ice

What’s unique about this study is that researchers were able to use real-world measurements spanning 40 years on the ground, which provide a sharper picture of the Greenland ice sheet’s melt than the satellite snapshots or computer models used in prior assessments.

Author Andreas P. Ahlstrøm, chief research consultant at the Geological Survey of Denmark and Greenland, noted that there are few places in Greenland that collect data on ice sheet runoff, but in the case of the Tasersiaq catchment, the team got lucky.

Greenland is vast and unforgiving for scientific instruments, so there aren’t many accurate measurements over the long periods of time needed to study changes in the climate. But developers conducted just these kinds of measurements in the Tasersiaq catchment, albeit to harness its commercial potential.

Communities in Greenland were hit hard by the oil embargo crisis in the 1970s, since many relied on imported fuel, so they started investigating the possibility of developing hydroelectric electricity from various ice melt runoff routes.

That’s why the research team ended up studying this particular catchment.

“It was not really a deliberate choice,” Ahlstrøm said. “It was more a matter of a dataset being collected because of interest in hydropower.”

Ahlstrøm and his team started looking at these measurements and realized that the discharge rates in the Tasersiaq catchment, which spans more than 2,500 square miles, double the area of Rhode Island, showed a sharp, massive shift.

Climate change left its mark

Around 2003, the ice sheet runoff into the catchment increased by 80 percent compared to the average runoff rate in the decades prior.

That meant something about the ice sheet changed drastically all of a sudden, and researchers set out to figure out what it was.

From previous studies, the scientists knew that the ice sheet melt rate in Greenland is linked to global atmospheric circulation. They started examining changes in the climate around 2003 to see if there was something different happening that year over Greenland compared to prior years.

“We see that it did correspond with a change in the weather pattern,” Ahlstrøm said.

Looking at the dataset, the team could see the fingerprints of volcanic eruptions in other parts of the world, like the 1991 Mount Pinatubo eruption in the Philippines, for example.

But what they also found was that the air moving over Greenland changed course. The team conducted an atmospheric trajectory analysis, using measurements and computer simulations to trace backward where the air over Greenland came from in a given year. Air parcels rolling over the ice sheet used to arrive in the summer from the North, bringing in cold arctic air and keeping the melt rate in check.

In 2003, that changed. The air currents started arriving from the South, bringing in warmer air, and the jet stream became wavier, making the melt rate increase while shifting erratically from year to year.

Thomas Mote, a climatology researcher at the University of Georgia who was not involved in the study, explained that the findings build a robust bridge between changes in the climate and melting ice using the longest dataset available.

“As scientists, we need to ‘connect the dots’ from the atmospheric circulation through the ice sheet mass balance to rising sea level and other impacts on ocean circulation and productivity,” he wrote in an email. “This long-term record of meltwater runoff gives us a better understanding of one of these connections.”

Researchers are keeping an eye on global sea levels

There are still many other connections scientists want to make and gaps they need to fill in about what the melting of Greenland’s ice sheet will mean for the global climate system and environment. Ahlstrøm said he wants to continue tracing these effects backward one step further, from the melting ice to the changes in the climate to the emissions of greenhouse gases driving these trends.

But while researchers investigate how we got here, there may still be bigger changes that lie ahead. In particular, sea level rise is one of the biggest consequences of the melting ice sheet. But, there’s a caveat: Ahlstrøm noted that measuring the rising oceans and connecting them back to Greenland is difficult since oceans rise at different rates around the world. And baked into the higher water levels are the effects of other melting ice sheets, like the much more massive Antarctic ice sheet, as well as the expansion of the water itself as its temperature goes up.

According to the National Snow and Ice Data Center, “If the Greenland Ice Sheet melted, scientists estimate that sea level would rise about 6 meters (20 feet). If the Antarctic Ice Sheet melted, sea level would rise by about 60 meters (200 feet).”

A study earlier this year found that Greenland’s melting ice accounted for 25 percent of global sea level rise in 2014, a share that’s grown from just 5 percent in 1993. This means that Greenland is becoming an increasingly dominant driver of rising water levels around the world, so scientists are trying to get better measurements of the melting ice to figure out what’s in store for the future.