We have all been told that global warming is because of excessive build-up of greenhouse gases in the atmosphere. Here is a remarkable story from the BBC about how the disintegration of one of the largest icebergs ever seen might have actually reduced greenhouse gases. Michael Marshall writes: “In late 2016, scientists spotted a rapidly growing fissure extending across the enormous Larsen C Ice Shelf that spills into the Weddell Sea from the West Antarctica Peninsula. Within a few months, the rift led to one of the biggest icebergs ever observed breaking off into the ocean.
The gargantuan slab of ice was more than twice the size of Luxembourg, covering an area of more than 2,200 sq miles (5,700sq km) and was around 770ft (235m) thick. For a year, this monster barely moved, trapped in the seasonal embrace of the Antarctic sea ice. But then it began to accelerate north, carried by ocean currents and winds.
Iceberg A-68, as it was designated, had embarked on what would be an epic 3.5-year odyssey that took it from the Antarctic sea ice to a remote island in the Southern Ocean…
the iceberg gradually broke apart and melted before the worst could happen. Over the years, it fractured into smaller pieces and released billions of tonnes of chilled, freshwater into the ocean before finally meeting its end in a slushy whimper during April 2021.”
Now, the reason why these epic meltdowns of country-sized icebergs over the course of five years is interesting because of the impact it has on the environment. Michael Marshall’s article tells us that A-68’s meltdown “…transformed the marine habitat around it, creating unique conditions that support an entire ecosystem of life. Scientists following A-68’s birth and demise were able to track just what such giant icebergs do to the surrounding ocean. For its short, transient life as an iceberg, A-68 became a frozen lifeboat for a wide range of species.”
Mr Marshall’s article is worth reading in full not least for the spectacular photographs they contain. In case you are too busy to read it, here is the crux of what happened as A-68 broke up over 5 years: “Over its three-and-a-half-year journey since it broke away from the Antarctic ice shelf, A-68 lost 802 billion tonnes of ice as it thinned from an average thickness of 770ft (235m) to 551ft (168m). Over a three-month period at the end of 2020 and start of 2021, it dumped an estimated 152 billion tonnes of fresh water into the ocean – that is equivalent to almost 61 million Olympic-sized swimming pools. At the peak of its breakup, around 1.5 billion tonnes of fresh water was gushing into the ocean every day.”
Mr Marshall goes on to describe the impact of the release of these colossal amounts of cold water. First off, lots of eating material was created for some really big beasts: “In the surface water around the iceberg, Tarling and his colleagues found elevated levels of nutrients, including nitrate and phosphate. The concentrations were more typical of what is found in deeper waters. What seems to have happened is that the less dense fresh water melting off the underside of the iceberg, which extended up to 463ft (141m) beneath the surface, picked up these nutrients from the deeper seawater and carried them upwards with it.
These nutrient-rich melt waters were dominated by ice-associated algae: species that prefer to live in or near ice. “They can deal with high differences in salinity,” says Tarling, allowing them to survive moving between fresh iceberg water and salty seawater.
“You have this sort of halo effect,” says Tarling. The ice-associated algae bloomed around A-68a and its iceberg “children”. This attracted tiny animals called zooplankton to feed on them. If the research ship had stayed longer, Tarling says, they would probably have seen larger animals arrive to feed on the zooplankton.
This would likely have included baleen whales, the largest animals on Earth. “Whales definitely would be in there and thriving on that productivity given a few more weeks,” says Tarling. “They are amazing at locating patches of productivity. That’s why they thrive.” He speculates they may even be aware that icebergs tend to leave blooms in their wake. “They’re really clever animals.””
Secondly, the melting of A-68 reduced the amount of greenhouse gases! How exactly this happens is a technically complex. Here is how Mr Marshall explains it: “”What was happening is that huge dumps of water were coming in.” The weight of the fresh water pushed down the layers beneath, so conditions that would normally be found 164ft (50m) down were instead found 328ft (100m) down.
Any particles of food drifting in the water were also driven down. “This deepening of the water masses is creating an effect that we’ve never seen before, taking all this particulate material down with it,” says Tarling.
This may actually have increased the amount of carbon that was buried at the bottom of the Southern Ocean. Normally, organic material drifts down slowly through the water and some of it gets eaten, so only a fraction reaches the seabed and gets trapped there. But the crushing weight of the freshwater from A-68a as it melted may have helped force the carbon-based material down more quickly, to depths where it was less likely to be eaten.
“No one’s ever reported this,” says Tarling. The implication is that massive icebergs like A-68 may help drive carbon into the depths of the sea, slightly lowering greenhouse gas concentrations in the atmosphere.”
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