Greenland's melting icebergs may not be a threat to ocean currents

You might not have heard of the Atlantic Meridional Overturning Circulation (AMOC), but this vast system of ocean currents is a crucial part of Earth’s climate system.

It acts like a giant conveyor belt, transporting warm surface water from the tropics northward and cold deep water southward. This process helps regulate temperatures, weather patterns, and even marine ecosystems worldwide.

The AMOC is like a house of cards — disrupt one part, and the whole structure could crumble. Scientists worry that climate change could weaken or even shut down the AMOC, leading to catastrophic consequences like drastic temperature shifts, disrupted rainfall patterns, and dramatic changes in marine life.

Collapse of ocean currents

The collapse of major ocean currents, such as the AMOC, poses significant dangers due to their crucial role in regulating Earth’s climate system:

Disruption of heat distribution

Ocean currents act as giant conveyor belts, transporting warm water from the tropics towards the poles and cold water back towards the equator. 

This heat transfer moderates temperatures, making regions like Western Europe warmer than they would be otherwise. 

A collapse would disrupt this process, leading to drastic temperature changes in various parts of the world.

Extreme weather events

Ocean currents influence weather patterns globally. Their disruption could trigger a cascade of extreme weather events, including more intense storms, prolonged droughts, and severe heatwaves. This could devastate agriculture, infrastructure, and ecosystems.

Rising sea levels

The AMOC plays a role in regulating sea levels in the North Atlantic. Its collapse could lead to significant sea-level rise along the eastern coast of North America and in parts of Europe, displacing millions of people and threatening coastal cities.

Ocean currents impact on marine ecosystems

Ocean currents carry nutrients that support marine life. Their collapse would disrupt food chains, leading to a decline in fish populations and other marine species, with cascading effects on the entire ecosystem and industries that depend on it, like fishing.

Socioeconomic consequences

The combined effects of climate change, extreme weather, food shortages, and displacement of populations could lead to widespread social unrest, economic instability, and conflicts over resources.

Heinrich events and ocean currents

The Heinrich Events are periods of rapid iceberg discharge from the massive Laurentide Ice Sheet during the last glacial maximum.

These events dumped huge quantities of cold freshwater into the North Atlantic, weakening the AMOC and throwing the global climate into disarray.

Yuxin Zhou, a postdoctoral researcher at UC Santa Barbara, wondered if the icebergs calving from Greenland’s melting ice sheet could trigger a similar collapse of the AMOC.

To find out, he dove into the past, studying sediments deposited during those ancient Heinrich Events.

Thorium-230

Zhou used a clever trick to estimate the amount of ice released during each Heinrich Event. He looked at the concentration of thorium-230, a radioactive element that naturally occurs in seawater, in the sediment layers.  

Unlike uranium, thorium doesn’t dissolve well in water, so it precipitates out on particles in the water column.

Because thorium-230 is produced at a steady rate, more sediment flux dilutes its concentration. In simpler terms, less thorium-230 means more icebergs were melting and dumping sediment into the ocean.

By comparing the rate of iceberg melting during Heinrich Events to current trends and future projections for Greenland, Zhou made a surprising discovery.

Icebergs may not trigger ocean current collapse

Zhou found that the predicted ice outflow from Greenland is comparable to a mid-range Heinrich Event. That might sound alarming, but there’s a crucial difference.

“During Heinrich Events, the AMOC was already moderately weakened before all the icebergs came in,” Zhou explains. “In contrast, the circulation is very vigorous right now.”

In other words, the AMOC today is starting from a much stronger position than it was during those ancient events, making it less likely to be completely derailed by Greenland’s icebergs. 

Additionally, Heinrich Events lasted for decades or even centuries, while the period of significant human-caused climate change has been much shorter. 

“It is possible that we simply haven’t screwed up badly enough for long enough for it to really mess up the AMOC,” Zhou notes.

Tug-o-war

There’s another factor at play: the type of freshwater released from Greenland. While icebergs have a more significant cooling effect on the ocean, their production is expected to decrease as the ice sheet melts and retreats inland. Instead, more freshwater will be released as runoff from the land.

This sets up an intriguing scenario. “This presents a tug-o-war between these two factors: the more disruptive but decreasing ice discharge and the less effective but accelerating runoff,” Zhou says. “It’s going to be a competition, and the interplay between the two will determine the future of the AMOC.”

Glimmer of hope for ocean currents

While climate change remains a serious threat, Zhou’s research offers a glimmer of hope. “This is a piece of good climate news that hopefully will dissuade people from climate doomism, and give people hope, because we do need hope to fight the climate crisis.”

In summary, this study is a reminder that the Earth’s climate system is complex and full of surprises. While the future of the AMOC is still uncertain, this study suggests that the situation might not be as dire as we feared. It’s a call for continued research and action to protect this vital ocean current and mitigate the impacts of climate change.

The study is published in the esteemed journal Science.

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