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Historic Greenland ice sheet rainfall unraveled

For the first time ever recorded, in the late summer of 2021, rain fell on the high central region of the Greenland ice sheet. This extraordinary event was followed by the surface snow and ice melting rapidly. Researchers now understand exactly what went on in those fateful summer days and what we can learn from it. 

 

The never-before-seen rainfall, on 14 August 2021, made headlines around the world. The upper-most parts of Greenland’s enormous ice cap used to be too cold for anything other than snow to fall, but not anymore.

What caused this extreme event and how did it affect the ice? Researchers from the Department of Glaciology and Climate at the Geological Survey of Denmark and Greenland (GEUS) in collaboration with colleagues from France and Switzerland have since scrutinised these questions and come up with the answers.

It didn’t only rain at Summit Camp – rain was measured by new automatic weather stations placed across the ice sheet by GEUS’ ice-sheet monitoring projects Programme for Monitoring the Greenland Ice Sheet (PROMICE) and Greenland Climate Network (GC-Net).

Studying detailed data from these stations alongside measurements of surface reflectivity, or albedo, from the Copernicus Sentinel-3 satellite mission and information on atmospheric circulation patterns, the researchers discovered that the rain had been preceded by a heatwave at a time of year when seasonal melting is usually slowing down.

From the onset of the August heatwave in 2021, the surface of the ice sheet (here seen east of Kangerlussuaq) became notably darker due to melting snow, lowering the albedo and making it melt way more than expected for that time of year. (GIF: Edited by GEUS, images from Copernicus Sentinel-2, ESA)

It wasn’t the rain

“It turns out that the rain itself wasn’t the most important factor”, says Prof. Jason Box from GEUS and lead author of the paper reporting their results, which were just published in Geophysical Research Letters.

“There is an irony. It’s not really the rain that did the damage to the snow and ice, it’s the darkening effect of the meltwater and how the heat from the event erased snow that had overlaid darker ice across the lower third of the ice sheet.

“Unusually warm atmospheric rivers swept along Greenland in the late summer months, bringing potent melt conditions when the melt season was drawing to a close.”

In fact, this sudden increase of surface ice melt on Greenland could have happened without any rain ever touching the ground.

The main culprit was the heat itself, melting and completely removing the surface snow, thereby changing the surface albedo, Greek for ‘whiteness’, so that Greenland snow and ice absorbed more of the Sun’s rays.

The researchers found that, between 19 and 20 August 2021, this melt caused the altitude of the ice sheet’s snowline to retreat by a whopping 788 metres, exposing a wide area of dark bare ice (see below).

One of the areas of the ice sheet hit by the heat wave, causing the snowline to retreat immensely in a short period of time. This is along the PROMICE/GC-Net Kangerlussuaq transect of automatic weather stations (white dots) from where the researchers documented the very warm, wet ice and snow conditions during their field work. Usually the snow at KAN_U station is not this difficult to walk in during august. (Illustrations: GEUS, edited from Box et al. 2022 by C. Thuesen, photos Dirk van As).

 

Under normal circumstances, snow would cover and insulate this ice, but the snow melted suddenly and exposed the ice to heat, causing even more melting.

Since 2017, Prof. Box and colleagues have been consistently monitoring these melt dynamics with data from the Copernicus Sentinel-3 mission as part of their research supported by ESA’s Earth Observation Science for Society programme.

They found that more bare ice had been exposed on the one day of 19–20 August than any other day since their research began (see below).

Area of bare ice measured from 2017 to 2021, where the heatwave in 2021 greatly increases bare ice extent very late in the melt season. (Illustration: Edited from Box et al. 2022 by C. Thuesen, GEUS)

So, what does this mean?

The authors conclude that the heatwave causing the rain event serves as an excellent example of ‘melt-albedo feedback that amplifies the melt impact of the initial melt perturbation’.

Besides the obvious concern of making the ice sheet melt faster while this was happening, studying the event using detailed measurements from the network of automatic weather stations on the ice set up by GEUS coupled with measurements from the Copernicus Sentinel-3 satellites helps understand the exact processes and dynamics of how ice melts.

Even though the rainfall was a shock and a milestone in climate history, researchers knew it was bound to happen sooner or later, given the rising temperatures of the Arctic.

Therefore, Prof. Box and the co-authors encourage research to look further into the workings behind atmospheric rivers and not just rainfall.

They conclude that understanding the frequency of heatwaves, appears to be a more significant research target than the liquid precipitation that heatwaves may or may not produce.

The study explained by Prof. Jason Box

 


The 2021 august melt event

  • The ‘atmospheric river’ heat wave causing the rain event in mid-August 2021 was the actual cause of the following surface melt, not the rain itself.
  • The heat wave caused extensive melting to happen unusually late in the season.
  • The melt event caused the snow line to elevate by an extreme amount (788 m), exposing a wide area of dark bare ice.
  • The ice sheet had more than four days of wet snow cover in August which is double the normal amount.
  • The Watson River rose abruptly to record high values for that late part of the melt season, demonstrating independently the off-ice impacts of the heatwave in increasing meltwater delivery into the ocean, rising sea level.

Read the paper

The Mid-August 2021 Greenland Ice Sheet Heatwave with Rainfall: Extreme Melting Sustained by Albedo Feedback (DOI:10.1029/2021GL097356) published in Geophysical Research Letters.

Authors and institutions:

  • Jason E. Box, Dirk van As, Robert S. Fausto, Kristian K. Kjeldsen, Armin Dachauer, Andreas P. Ahlstrøm, the Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark
  • Adrien Wehrlé, Institute of Geography, University of Zurich, Zurich, Switzerland
  • Ghislain Picard Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark and UGA, CNRS, Institut des Géosciences de l’Environnement (IGE), UMR 5001, 8 38041 Grenoble, France

Algae blooms: “I never saw the ice as dark as this”

GEUS and PROMICE professor Jason Box recently made the front page in Danish national media with testimony of algal blooms and rain on the Greenland ice sheet. PROMICE field work helps quantify the ice algae blooms and their effect on melting.

 

Autumn 2021, Professor Jason Box and two PROMICE colleagues landed on what could appear to be tarmac during a field trip to a glacier near Narsaq in the Southwest of Greenland. However, the dark surface was in fact glacier ice hit by a heavy bloom of algae adapted to ice surfaces, called ice algae.

Recently the story was picked up by one of Denmark’s major newspapers, Politiken, who ran it as front cover just before New Year’s Eve of 2021. In the article, Jason Box explained how the algae blooms makes the ice darker causing it to absorb more sunlight and thus melt faster.

“I never saw the ice as dark as this. It was a surprise and made me concerned,” Jason Box said to Politiken, adding that where the ice algae grow particularly well, they can speed up melting of the surface ice by as much as 20 percent.

See albedo data for the location here:

PROMICE climate station recordings of surface darkness (a.k.a. albedo) for 2021 compared to the average since 2008. Melt season 2021 albedo was at or near the lowest on record for more than three months (June through mid-September). The cause is thought to be from persistent cloudy and rainy conditions that promoted ice algae growth. In May however, snowfall caused the albedo to increase shortly before decreasing during the summer melt season. (Illustration: PROMICE)

 

PROMICE helps collect algae data

These years the algal growth on the Greenland Ice Sheet is on the rise, which Professor Box and other PROMICE and GC-Net researchers have helped quantify. Since much of the research done by the team require travelling all over the ice sheet every year, they are starting a new PROMICE activity “PROM-BIO” in collaboration with Aarhus University Professor Alexander Anesio.

The work supports ice algae research by collecting biological samples from the ice from places that would otherwise be difficult to reach for other researchers.

Jason Box collecting samples from the alge bloom seen as dark spots on the ice. Photo: GEUS

This helps other researchers understand the development and biology of ice algae with some emphasis on the dark ice relationship with melting which in turn helps PROMICE and GC-Net understand how the darkening ice surface effect the overall mass balance of the ice sheet.

“Ice algae have started to colonize larger parts of Greenland. They’ve become an x-factor in the melting process,” Box said in the Politiken article.

Unprecedented rainfall

Along with the algae blooms, 2021 was also the year of the first ever witnessed rain falling on the highest and coldest areas of the ice sheet. An event also measured by PROMICE and GC-Net automated weather stations.

As Professor Box explains in Politiken, he and the rest of the researchers studying ice and climate dynamics in the Arctic constantly find new factors driving the melt of the ice sheet:

“Algae blooms existed before humanity. One could observe dark ice in Greenland 200 years ago as well, but what we see now is that the heating of the Earth allows for many different factors to enhance each other. We have several climate models predicting a warming, but reality surprises us over and over because the models haven’t taken all of the contributing factors into account.”

Therefore, the work done by PROMICE and GC-Net becomes all the more important, trying to understand all of these factors effecting each other and enhancing the melt.