The Antarctic Peninsula is setting heat records. They won’t stand long.

Five hundred miles south of South America’s Cape Horn, a narrow strip of land and a smattering of islands form the tip of the Antarctic Peninsula, a forbidding realm of ice and permafrost best known as a popular penguin breeding ground. Recently, however, the southern continent’s northernmost reaches, currently basking in summer, have been in the news for something far more exotic than armies of large, flightless birds: 70-degree weather.

The Antarctic Peninsula is recovering from a heat wave that has redefined t-shirt weather on the world’s largest frozen landmass and raised alarm bells about its future. Weather stations near the northern tip of the peninsula and on neighboring islands recently logged temperatures in the mid-60s and even one near 70-degree reading. If confirmed, they will stand as new high temperature records for the continent.

A mix of meteorological factors, from warm mountain winds to larger patterns in the ocean and atmosphere, are responsible for the balmy blast. But the bizarre weather is also consistent with a long-term trend. While summertime temperatures across the Antarctic Peninsula normally hover around freezing or rise just a few degrees above, the region has experienced dramatic warming in recent decades, making it easier for heat spells to veer into record-breaking territory. And with Earth’s climate continuing to warm as atmospheric carbon levels soar, any newly minted records probably won’t last long.

“I think it’s not at all surprising,” says Peter Neff, an Antarctic glaciologist at the University of Washington. “It’s part of the trend, and we’re going to see more of those warm events than cold events” in the future.

Attack of the warm air

The roots of the recent Antarctic heat wave can be traced hundreds of miles north.

Toward the beginning of February, a ridge of high pressure air migrated over South America’s southern tip, enveloping the region in warm weather. According to Xavier Fettweis, a polar climatologist at the University of Liège in Belgium, this happens several times a summer. Normally, the effects aren’t aren’t felt on the Antarctic Peninsula, which is protected from warm air incursions by the Southern Hemisphere westerlies, a belt of strong winds encircling the continent.

But in recent months, those westerlies have been in a weakened state around the South Pole, part of a recurring pattern known as the Antarctic Oscillation. This has helped warm air spill south in an “exceptional” manner, Fettweis says.

On top of that, ocean temperatures surrounding the northern tip of the Antarctic Peninsula were 3-5 degrees Fahrenheit warmer than usual toward the beginning of the month. According to Harry Hendon, a research scientist with Australia’s Bureau of Meteorology, this oceanic warmth might be the lingering effect of a rare upper-atmospheric warming event that occurred in the spring, which also shifted the westerlies north while it was ongoing.

All of this warmth in the oceans and atmosphere helped set the stage for a record-breaking heat wave. Then, at the end of the first week in February, a quirk of Antarctic Peninsula topography pushed things over the edge.

The western spine of the peninsula is covered in a mountain range: the Antarctic Peninsula Cordillera, which might have once linked up with South America’s Andes. As air flows over the mountains it starts to sink, compressing and heating up as it speeds downslope and creating so-called “foehn winds.” These hot air blasts can cause temperatures to temporarily rise by as much as 55 F (30 C), said Ella Gilbert, a PhD researcher in meteorology with the British Antarctic Survey.

“It’s not unprecedented to get these foehn events,” says Ruth Mottram, a climate scientist with the Danish Meteorological Institute. “Of course, when you have a warmer atmosphere or ocean nearby, it’s easier to break the records when you have them.”

That’s exactly what seems to have happened earlier this month, when the peninsula was enveloped in a foehn wind event that Gilbert described as “quite extreme.” Antarctic research stations took note: On February 6, Argentina’s Esperanza Base clocked a temperature of 65 F (18.3 C), beating the station’s previous high record of 63.5 F (17.5 C) in March 2015, which also stands as the all-time high temperature record for the entire continent.

Then, on February 9, a Brazilian-run research station on nearby Seymour Island logged an even more extreme temperature: 69.4 F (20.75 C), according to several media reports. If verified, it will be the first 20 C-plus temperature measurement not just for Antarctica, but for the entire region south of 60 degrees latitude.

Verifying a record

That’s a big “if,” however. Randall Cerveny, who heads up weather and climate extremes tracking at the World Meteorological Organization, is currently pulling together a committee to investigate both high temperature readings and determine if they meet the WMO’s exacting standards for official records. There’s no guarantee they will.

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“When we are looking at an extreme record, we have to get all the info for that sensor, location, [and] station,” Cerveny says. “Was it at the right height? Had it been calibrated? Was the person, if it’s a manual station, taking the readings properly? Was the site proper? All of those are things we have to take a look at.”

Many small things can throw a high temperature reading off in extreme environments like the Antarctic. One of them, surprisingly, is ice. On sunny days, reflective white surfaces scatter light onto sensors, causing them to heat up more. In 2015, a 64.2 F (17.9 C) degree temperature reading on James Ross Island almost took the title of warmest temperature ever recorded in Antarctica before the WMO decided to downgrade the measurement to 62.6 F (17 C) to account for this solar heating effect.

Cerveny is fairly confident that the Esperanza Base reading from February 6 will pass muster. The weather station is part of the WMO’s official monitoring network, and it’s been logging measurements since the late 1950s, giving meteorologists high confidence in its readings. “Unofficially, I think it will be a record,” he says.

He’s far more skeptical of the higher temperature measured a few days later on Seymour Island, which wasn’t taken at a permanent WMO station but as part of a Brazilian permafrost-monitoring project. The WMO will be looking at this reading closely in order to determine how the temperature sensor was affixed — it needs to be several feet off the ground to avoid the confounding influence of surface heating, Cerveny says—how long it has been taking measurements, and how well calibrated it was.

Cerveny warned that it will take time to get all the necessary data from the station, much less suss out whether it’s the stuff of records. “We will be looking at the Brazilian research [station] with quite a bit of scrutiny,” he says.

The bigger picture

While records tend to grab headlines, to scientists studying the Antarctic they’re far less important than the long-term trend. For the Antarctic Peninsula that trend is complex, but the overall direction is clear: Things are heating up.

While Antarctica as a whole has only warmed slightly since the mid-20th century, at Esperanza Base and other long-term research stations on the Antarctic Peninsula, temperatures rose a whopping 5 degrees Fahrenheit between the 1950s and the early 2000s, far outpacing the globally averaged rate of warming.

Then, in the late ‘90s, this rapid warming abruptly stalled, triggering a raft of scientific interest into what had caused it. Researchers eventually concluded that a mix of factors were at play, including local sea ice losses, enhanced westerly winds, and the knock-on effects of the man-made ozone hole over Antarctica. Eventually, those processes were supplanted by circulation changes that caused the peninsula’s fever to back off slightly.

Still, with humans dumping more and more heat-trapping carbon into the atmosphere with each passing year, scientists expect the peninsula will start to heat up again in the not-so-distant future. And if recent trends are any indicator, that’s bad news for its ice.

While West Antarctica’s icy perimeter is being attacked from below by warm ocean water, on the Antarctic Peninsula warmer air temperatures are destabilizing ice from above, causing meltwater to pool up and occasionally drain into floating ice shelves, cracking them apart. As the protective, icy floodgates crumble and retreat, land-bound glaciers are becoming unshackled, flowing into the ocean faster and triggering global sea level rise. From 1992 to 2017, the rate of ice loss from the Antarctic Peninsula nearly quintupled, from 7 to 33 billion tons a year.

Short-term heat waves play a role. If an ice shelf has already been weakened by persistent warmth, a freak hot spell can give it a “final push” over the edge, Mottram says. She noted that this is effectively what happened to the Larsen B ice shelf, which broke up in spectacular fashion in a matter of days in 2002.

One key limitation to understanding the effects of heat waves is the scarcity of long-term temperature records for Antarctica. Most of the continent’s roughly 180 weather stations date back to the mid-20th century at the earliest, but to truly understand the significance of an extreme event, it would be better to go much further back in time.

With ice cores, which contain yearly snapshots of the atmosphere going back centuries to millennia, we can. And one core in particular—drilled on the James Ross Island ice cap in 2008—puts current conditions on the peninsula in dramatic relief. It shows that recent levels of summer melting are unprecedented in the last thousand years. According to a 2013 study, it suggests even a modest future rise in air temperatures could translate to “rapid increases” in summertime melting and ice shelf decay.

What, then, would an uptick in 60-degree days do? Nobody’s sure, but for Antarctica’s northernmost reaches, the question is starting to feel less outlandish.

“I think it’s fair to say we might see more of these extreme warming events in the future,” Mottram says.