Melting Antarctic Ice: A cause for concern despite Natural climate cycle

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Our new research in the Antarctic suggests that the vital layer of cold water on the sea bed, which circulates the globe and influences the ocean’s ability to continue absorbing much of the rise in atmospheric heat and greenhouse gas emissions, is heating up and shrinking. But it appears, as far as the past 30 years are concerned, a natural cycle may have been partly responsible for the changes observed.
Most of this exchange of gases and heat between the atmosphere and the ocean happens in the Southern Ocean around Antarctica through the complex vertical movement of water. One of the biggest drivers of this vertical movement is the production of what oceanographers call Antarctic bottom water.
Antarctic bottom water formation relies on polynyas, where sea ice production creates dense water that drives the global ocean conveyor. Recent research using ship and satellite observations shows a significant decrease of over 20% in bottom water volume in the Weddell Sea over the past three decades. This has led to the deep Weddell Sea warming at a rate four times faster than the global average. Weakening offshore winds in the region have caused shrinking polynyas, reducing the production of cold, dense, and salty water that feeds Antarctic bottom water. This could slow down the deep overturning circulation, impacting the global climate system profoundly.
Previous studies have linked the slowing global ocean conveyor with less cold, dense water forming in the Southern Ocean due to increasing meltwater from ice shelves. While man-made climate change is significant, our new research suggests that natural variability in wind and sea ice are also important.

What’s up with the wind in the Weddell Sea?
Weaker winds blowing offshore in the southern Weddell Sea over the past 30 years have limited the size of the coastal polynya, which in turn has produced less sea ice.We found that this change in the wind seems to be linked to surface temperature changes over the tropical Pacific during the same period, part of a natural cycle similar to El Niño, known as the Interdecadal Pacific Oscillation.Oscillating sea surface temperatures in the tropical Pacific are strong enough to affect the local air pressure and even influence the wind on both sides of the Antarctic peninsula. This means that the trend in Weddell Sea winds and consequent Antarctic bottom water formation over the past 30 years may also be part of a longer natural cycle.

If it is natural, should we stop worrying?
Ship-based observations have revealed that the bottom water layer surrounding Antarctica has been warming and thinning, linked to increasing freshwater from melting ice shelves. While the Weddell Sea shows similar changes despite slower ice shelf melting, indicating natural variability, it aligns with an Antarctic-wide trend consistent with ice sheet melting predictions. Models, though incomplete, aid in understanding future climate. Mounting evidence suggests the vulnerable Antarctic ice sheet’s melting will disrupt global ocean circulation, climate, and accelerate sea-level rise. As scientists, we emphasize the need for ongoing research, refined projections, and immediate action to reduce greenhouse gas emissions and curb global warming.