Washington: Antarctica is currently gaining enough ice to outweigh the increased losses from the continent's thinning glaciers, a new NASA study has found. The research challenges the conclusions of other studies, including the Intergovernmental Panel on Climate Change's (IPCC) 2013 report, which says that Antarctica is overall losing land ice. According to the new analysis of satellite data, the Antarctic ice sheet showed a net gain of 112 billion tons of ice a year from 1992 to 2001. That net gain slowed to 82 billion tons of ice per year between 2003 and 2008. "We're essentially in agreement with other studies that show an increase in ice discharge in the Antarctic Peninsula and the Thwaites and Pine Island region of West Antarctica," said lead author Jay Zwally, a glaciologist with NASA Goddard Space Flight Centre in US. "Our main disagreement is for East Antarctica and the interior of West Antarctica — there, we see an ice gain that exceeds the losses in the other areas," said Zwally. But it might only take a few decades for Antarctica's growth to reverse, according to Zwally. "If the losses of the Antarctic Peninsula and parts of West Antarctica continue to increase at the same rate they've been increasing for the last two decades, the losses will catch up with the long-term gain in East Antarctica in 20 or 30 years," Zwally said. The study analysed changes in the surface height of the Antarctic ice sheet measured by radar altimeters on two European Space Agency European Remote Sensing (ERS) satellites, spanning from 1992 to 2001, and by the laser altimeter on NASA's Ice, Cloud, and land Elevation Satellite (ICESat) from 2003 to 2008. The extra snowfall that began 10,000 years ago has been slowly accumulating on the ice sheet and compacting into solid ice over millennia, thickening the ice in East Antarctica and the interior of West Antarctica by an average of 1.7 centimetres per year. This small thickening, sustained over thousands of years and spread over the vast expanse of these sectors of Antarctica, corresponds to a very large gain of ice - enough to outweigh the losses from fast-flowing glaciers in other parts of the continent and reduce global sea level rise. "The good news is that Antarctica is not currently contributing to sea level rise, but is taking 0.23 millimetres per year away," Zwally said. "But this is also bad news. If the 0.27 millimetres per year of sea level rise attributed to Antarctica in the IPCC report is not really coming from Antarctica, there must be some other contribution to sea level rise that is not accounted for," Zwally said. The study was published in the Journal of Glaciology. — PTI. Source: Article
As snow falls on Antarctica, layers build up and turn to ice. Over time, this compressed snow has become a continent-sized glacier, or ice sheet. It’s enormous – almost double the size of Australia and far larger than the continental United States.
As the weight of ice builds up, the ice sheet begins to move towards the oceans. When it reaches the sea, the ice floats. These floating extensions are known as ice shelves. The largest is over 800 kilometres wide.
When the ocean water has a temperature close to 0°C, these ice shelves can persist for a long time. But when temperatures rise, even a little, the ice melts from below. Antarctic ice shelves are now losing an alarming 150 billion tons of ice per year, adding more water to the ocean and accelerating global sea level rise by 0.6 mm per year. Ice shelves in West Antarctica are particularly prone to melting from the ocean, as many are close to water masses above 0°C.
While the melting trend is clear and concerning, the amount can vary substantially from year-to-year due to the impact of both natural climate fluctuations and human-made climate change. To figure out what is going on and to prepare for the future, we need to tease apart the different drivers – especially El Niño-Southern Oscillation, the world’s largest year-to-year natural climate driver.
Our new research explores how heat brought by El Niño can warm the ocean around West Antarctica and increase melting of the ice shelves from below.
Antarctic Ice Mass Loss 2002-2023. Credit: NASA Climate Change.
How can El Niño-Southern Oscillation affect Antarctica?
Australians are very familiar with the two phases of this climate driver, El Niño and La Niña, as they tend to bring us hotter, dryer weather and cooler, wetter weather, respectively. But the influence of this cycle is much larger, affecting weather and climate all around the Pacific.
Can it reach through Antarctica’s cold, fast currents of air and water? Yes.
Giant convective thunderstorms in the Pacific’s equatorial regions move east during El Niño and intensify in the West during La Niña. As these storm systems change, they excite ripples in the atmosphere that are able to travel large distances, just as waves can cross oceans. Within two months, these atmospheric waves reach the Antarctic continent, where their energy can affect the coastal atmosphere and ocean circulation. During El Niño, the energy from these waves weakens the easterly winds off West Antarctica (and vice versa for La Niña).
Using satellite data, researchers recently found that West Antarctic ice shelves actually gain height but lose mass during El Niño. That’s because more low-density snow falls at the top of the ice shelves, while at the same time more warm water flows under the ice shelves where it melts compressed high-density ice from underneath.
What we don’t yet know is how this warmer water (above zero) comes up from below. Similarly, we don’t know what happens during La Niña.
Answering these questions with the few observations we have from Antarctica is challenging because this climate driver doesn’t happen in isolation. Storms, tides, large eddy currents and other climate drivers such as the Southern Annual Mode can change the temperatures of the water under ice shelves too, and they can occur at the same time as El Niño.
Finding a needle in the ice stack
So how did we do it? Modelling.
We take a high-resolution global ocean circulation model and added El Niño and La Niña events to the baseline simulation. By doing so, we can examine what these anomalies do to the currents and temperatures around Antarctica.
The energy brought by El Niño’s atmospheric waves to West Antarctica weakens the prevailing easterly winds along the coasts.
Normally, most of the warm water reservoir is located off the continental shelf rather than on the continental shelf. As the winds weaken, more of this warmer water – known as Circumpolar Deep Water – is able to flow onto the continental shelf and near the base of the floating ice shelves.
During El Niño, weaker winds along the coasts push less cold Antarctic surface waters towards the continent, allowing warmer Circumpolar Deep Water to flow to the base of the ice shelves. During La Niña, stronger winds drive a wedge of cold water up towards the continent, reducing the inflow of warm water. Maurice Huguenin, CC BY-SA
We call this water mass “warm”, but that’s relative – it’s only 1–2°C above freezing, and the heat only warms the water on the continental shelf by about 0.5°C. But that’s enough to begin melting ice shelves, which are at or below freezing point.
As you’d expect, the longer the warm water stays on the shelf and the hotter it is, the more melting occurs.
During La Niña, the opposite occurs and the ice rebounds. Winds along the coast strengthen, pushing more cold surface water onto the continental shelf and preventing warm water from flowing under the ice shelves.
If this trend continues, as climate projections suggest, we can expect warming around West Antarctica to get even stronger during El Niño events, accelerating ice shelf melting and speeding up sea level rise.
More frequent and stronger El Niño events could also push us closer to a tipping point in the West Antarctic ice sheet, after which accelerated melting and mass loss could become self-perpetuating. That means the ice wouldn’t melt and reform but begin to steadily melt.
More bad news? Unfortunately, yes. The only way to stop the worst from happening is to get to net zero carbon emissions as quickly as humanly possible.
