Antarctica's Record High: 18.3°C at Esperanza, Explained

Originally published 2021-07-03.

The highest temperature ever officially recorded on the Antarctic continent is 18.3 °C (64.9 °F). It was measured on 6 February 2020 at Argentina's Esperanza research station, near the northern tip of the Antarctic Peninsula. The World Meteorological Organization (WMO), the United Nations agency that keeps the global register of weather extremes, confirmed the record on 1 July 2021. At the same time it rejected a higher reading of 20.75 °C taken a few days later on nearby Seymour Island, because faulty equipment had skewed it.

Here is how the record was set, why one number was accepted and the other thrown out, and what it tells us about a fast-changing corner of the planet.

The two readings of February 2020

Summer 2020 brought unusual warmth to the Antarctic Peninsula, the long arm of land that reaches north towards South America. Two stations reported numbers that made headlines around the world.

• Esperanza, 6 February 2020: 18.3 °C. The station is run by Argentina's national meteorological service. The previous continental record, 17.5 °C, had been set at the same station on 24 March 2015.
• Seymour Island, 9 February 2020: 20.75 °C. This came from an automated station operated by a Brazilian team to monitor permafrost. Some outlets reported it as the first time Antarctica had passed 20 °C.

Only the first now stands in the official record books.

How the WMO checks a record

An extreme temperature is not accepted simply because a thermometer displayed it. The WMO assembles an international committee of specialists, in this case polar and climate scientists from Argentina, Brazil, New Zealand, Spain, the United Kingdom and the United States. They examine the instrument, how it was mounted and shielded from direct sun, its calibration, its maintenance history and the weather conditions at the time.

Esperanza passed that test with no major concerns. The Seymour Island reading did not. The station's original radiation shield, the housing that stops sunlight heating the sensor directly, had been lost and replaced with an improvised one. The committee found that the makeshift shield created a warm bias. The station remained perfectly useful for studying permafrost, but its number could not count as an official weather observation.

The WMO's rapporteur on extremes, Randall Cerveny, called the episode a "teachable moment" about how quickly unverified figures spread. Partly as a result, the organisation began offering rapid initial assessments of claimed records so that the public gets early guidance before a full investigation, which can take more than a year.

Why it got so warm: föhn winds

Both readings came during the same weather pattern. A large high-pressure system sat over the region and drove strong winds across the peninsula's mountains. On the far side, the air sank rapidly.

That produces what meteorologists call a föhn (or foehn) wind. As moist air is forced up a mountain range it cools and drops its moisture as snow or rain. When the now-dry air descends the other side, it is compressed and heats up quickly, arriving as a warm, dry wind. Föhn events are well known in the Alps and the Rockies, and on the Antarctic Peninsula they can push temperatures far above normal for a day or two and contribute to the melting and break-up of ice shelves.

So the record was a short-lived spike driven by local weather, not the normal summer temperature. But such spikes happen against a background that is shifting.

"Continent" versus "region": a common mix-up

Coverage of Antarctic records often blurs two different categories:

• The Antarctic continent means the mainland and its closely attached islands. Its record is the 18.3 °C at Esperanza.
• The wider Antarctic region, as the WMO defines it, includes all land and ice south of 60° S. Its record is 19.8 °C, measured on Signy Island in the South Orkney Islands in January 1982.

At the other extreme, Antarctica holds the record for the coldest air temperature ever reliably measured on Earth: −89.2 °C (−128.6 °F) at Russia's Vostok station on 21 July 1983. Satellites have detected even colder surface "skin" temperatures, around −93 °C, but these are measured differently and are not directly comparable. In the interior, the mean annual temperature is about −43.5 °C, while the warmest coastal areas average around −10 °C.

The bigger picture: a fast-warming peninsula

The Antarctic Peninsula is among the fastest-warming places on Earth, with temperatures up by almost 3 °C over the past half century. Secretary-General Petteri Taalas said the new record was consistent with that trend, and stressed that Antarctica, even more than the Arctic, is thinly covered by continuous weather observations, despite its large influence on ocean circulation and sea level.

That matters because of how much ice is at stake. If all of Antarctica's land ice melted, global sea level would rise by more than 60 metres. Nobody expects that this century, but even small losses from the edges of the ice sheet translate into higher seas for coastal cities worldwide.

Celeste Saulo, head of Argentina's weather service and then the WMO's first vice-president, used the announcement to call for urgent climate action and for stronger observation, forecasting and early-warning systems to deal with increasingly frequent extremes.

Part of a year of extremes

The confirmation arrived in the middle of a brutal northern summer. Between 25 June and 7 July 2021, a heatwave gripped western North America. On 29 June the village of Lytton in British Columbia reached 49.6 °C, the highest temperature ever measured in Canada; the next day a wildfire swept through and destroyed most of the village. Well over a thousand deaths were linked to the heat across western Canada and the US Pacific Northwest. A rapid attribution study concluded that the event would have been highly unlikely without human-caused climate change. Siberia had also seen exceptional heat in 2020.

Key points to remember

• Antarctica's official continental record high is 18.3 °C, at Esperanza on 6 February 2020.
• The 20.75 °C Seymour Island reading was rejected because of a faulty sun shield, not because the warmth was imaginary.
• The record was driven by föhn winds during a high-pressure event, on a peninsula that has warmed by nearly 3 °C in 50 years.
• Careful verification protects the credibility of climate data, and in this case the verified number still tells a worrying story.

Sources: WMO press release, 1 July 2021; Wikipedia: Climate of Antarctica; Wikipedia: Foehn wind; Wikipedia: 2021 Western North America heat wave; ScienceAlert.