Not all icebergs are equal – and that matters for the climate, new study finds

Photo Credit: Courtesy of British Antarctic Survey

Scientific Frontline: Extended "At a Glance" Summary: Giant Icebergs and Climate Impact

The Core Concept: Not all giant icebergs, or "megabergs," release fertilizing nutrients into the ocean as they melt; their capacity to stimulate marine life and absorb atmospheric carbon varies drastically depending on their individual life cycles and histories.

Key Distinction/Mechanism: While some icebergs act as "phytoplankton factories" by releasing trapped nutrients and triggering upwelling from deeper waters, others have no measurable biological effect. For example, an iceberg that remains grounded for decades may shed its nutrient-rich outer layers through melting before it drifts into open waters, rendering it unable to fuel ocean blooms compared to a recently calved iceberg.

Major Frameworks/Components:

• Nutrient Release: The biological process by which melting icebergs release essential "fertilizers" into the ocean, providing the foundation for marine food chains.
• Upwelling: A physical mechanism where the meeting of melting ice and deep water draws deeper, nutrient-dense water (rich in nitrogen, phosphorus, and iron) up to the surface.
• Carbon Sequestration: The biological pump where growing phytoplankton absorb atmospheric carbon and subsequently sink to the ocean depths when they die, regulating Earth's climate.
• Biogeochemical Cycling: The overall cycle and movement of carbon and nutrients in the Southern Ocean, heavily influenced by glacial dynamics.

Branch of Science: Biogeochemistry, Oceanography, Glaciology, and Climatology.

Future Application: These insights will be utilized to refine and improve predictive global climate models, specifically focusing on accurately mapping the carbon cycle and nutrient distribution in the Southern Ocean as climate change accelerates iceberg calving events.

Why It Matters: The study challenges the previously hopeful hypothesis that an increase in giant icebergs might offer a "silver lining" by uniformly boosting marine life and carbon absorption. Recognizing that icebergs are not created equal is a crucial puzzle piece for accurately predicting the true ecological and climatic impact of warming polar regions.

RRS Sir David Attenborough alongside A23a in December 2023
Photo Credit: Theresa Gossman, Matthew Gascoyne, Christopher Grey

As the world’s largest and most famous icebergs break up and melt in Antarctica, new research shows what impact they have on the ocean and its ecosystems. 

A new study, led by scientists from British Antarctic Survey (BAS), compared the first water samples from the famous giant icebergs A23a and A76a as they travelled north through Antarctic waters. These ‘megabergs’ have captured imaginations worldwide in recent years due to their staggering sizes – both being more than twice the size of Greater London and together carrying enough ice to supply the UK with freshwater for more than 250 years. 

Important nutrients within the ice of these giants allow ocean life to bloom as the iceberg melts. But the research shows that, at the time of sampling, only A76a provided these beneficial nutrients to the waters it passed through, whilst A23a had no measurable effect. 

It’s an important discovery, because in a warming climate, giant icebergs are expected to become more common. Some scientists had hoped this could have a silver lining – more icebergs feeding more ocean life, which absorbs more carbon from the atmosphere. But these findings complicate that picture. Not all icebergs are equal and understanding why it is an important piece of the puzzle in predicting how the climate could change. 

Edge of A23a in April 2024
Photo Credit: Derren Fox

What the icebergs revealed 

Giant icebergs can act as fertilizer for marine life by releasing important nutrients as they melt. This fuels blooms of tiny plant–like creatures known as phytoplankton, which are the foundation of marine food chains for animals like penguins and whales. But the new study, published today in Nature Communications Earth and Environment, shows the size of these blooms can differ dramatically between icebergs, making their effect on marine life and, in turn, the climate, harder to predict. 

The scientists encountered the giant icebergs within a vast current of drifting ice known as Iceberg Alley, located between the Weddell Sea and the sub–Antarctic island of South Georgia. It was here that, with a stroke of luck, the icebergs crossed the paths of UK research ships – A76a with the RRS Discovery in January 2023, and A23a with RRS Sir David Attenborough in December that same year – allowing scientists on board to take seawater samples around the ice giants. 

Laura Taylor, a biogeochemist from BAS who led the study and was on board both ships, said: 

“Being so close to these icebergs is like standing next to a moving cliff face – walls of ice towering above you, stretching to the horizon in every direction. It makes you feel very small.” 

Samples were analyzed by BAS and research collaborators in labs around the UK. By looking at the chemical makeup and freshwater content of the samples, the scientists compared the impact of the icebergs on the growth of phytoplankton in the water, and the results were just as striking. 

“We knew the icebergs could affect the waters around them differently, but the scale of that difference was a real shock,” explains Laura. “One was causing marine life to thrive; the other was having no detectable effect at all. It changes what we know about how icebergs interact with the ocean.” 

A tale of two bergs 

Despite coming from the same ice shelf, and being sampled only 11 months apart, the icebergs themselves have different tales to tell. A23a’s journey began over 30 years before A76a’s. It broke off, or calved, from the Filchner–Ronne Ice Shelf in 1986, at roughly the size of the entire UK county of Norfolk. But for the next three decades, it grounded to a halt in the muds of the Weddell Sea, before finally breaking free in 2020 and making its way north along Iceberg Alley. The scientists believe this 30–year roadblock forms part of the reason why A23a had such a different effect on marine life than A76a. 

“The story of each iceberg’s journey could explain our contrasting results,” explained Laura. “When A23a got stuck, it lost about a quarter of its total area from melting, and with it, potentially lots of the nutrients from its outer layers. By the time we came across it in Iceberg Alley, it may still have contained some of this ‘fertilizer’, but there wasn’t enough melting into the ocean to cause a phytoplankton bloom.” 

A76a, on the other hand, was a phytoplankton factory. It calved from the Filchner–Ronne Ice Shelf in May 2021, and at the time held the title of world’s largest iceberg as it quickly made its way north along Iceberg Alley towards South Georgia. From their samples, the scientists identified huge phytoplankton blooms in the waters surrounding the iceberg, fuelled by the nutrients escaping from the ice. 

But this alone didn’t explain the impressive levels of phytoplankton growth observed around A76a. The scientists found evidence of another process happening beneath the water’s surface. 

Professor Kate Hendry, an ocean scientist at BAS and collaborator on the study, said: 

“Where giant icebergs meet deeper waters, the melting ice can cause water and nutrients to be drawn up along the iceberg’s edge in a process known as upwelling. In the case of A76a, we think that this process pulled up other important nutrients, such as nitrogen, phosphorus and iron – which are more common in deeper Antarctic waters – to the surface, helping phytoplankton bloom to a level that would not be possible with only the nutrients melting from the iceberg.” 

A76a looms through the mist
Photo Credit: Michael Meredith

Icebergs and climate change 

The study challenges the idea that all icebergs affect ocean life in the same way. As phytoplankton grow, they use up carbon from their environment, which is drawn down into ocean depths as they die and sink. This helps to regulate Earth’s climate by absorbing carbon from the atmosphere. 

Professor Michael Meredith, an iceberg expert at BAS and collaborator on the study, said: 

“These new findings could change the way we model climate in Antarctica. If some icebergs boost the growth of phytoplankton, and others have no effect, the role of each iceberg in the carbon cycle is much harder to predict. That means understanding why icebergs behave in different ways is needed to help us make better predictions about their climate impact.” 

There is also concern among scientists that as the climate warms, calving events – where giant icebergs break off from ice shelves – will become more frequent, resulting in more megabergs like A76a and A23a drifting through Antarctic waters. Currently, it is unclear how this could change the cycling of carbon and nutrients in the Southern Ocean, but scientists believe this new research provides significant insights towards accurately predicting such effects. 

Published in journal: Nature Communications Earth and Environment

Title: Giant iceberg behaviour impacts regional biogeochemical cycling in the Southern Ocean

Authors: Laura R. Taylor, Helena Pryer, Katharine R. Hendry, Rachael N. C. Sanders, Michael P. Meredith, Andrew Meijers, Edward Mawji, E. Malcolm S. Woodward, Carol Arrowsmith, Melanie J. Leng, E. Povl Abrahamsen, Helen M. Williams, and Clara Manno

Source/Credit: British Antarctic Survey

Reference Number: es042026_01

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