Icebergs cool southern hemisphere

The cooling effect of increased iceberg formation could lead to temporary postponement of the climate

Large icebergs like this can help cool the South Ocean - temporarily delaying warming. © Elizabeth Crapo / NOAA
Read out

Like ice cubes in a glass: Climate change not only melts the Antarctic glaciers, they could also calve more icebergs. However, this would even slow down regional warming - albeit temporarily, as a study revealed. Because the icy rafts cool the ocean during their north drift and thus delay the warming of the southern hemisphere. This could give cities such as Buenos Aires and Cape Town a climate delay, as the researchers report in the journal "Nature Climate Change".

As global warming progresses, glaciers are melting, especially in the West Antarctic. As a result, more and more meltwater flows into the Southern Ocean and promotes sea-level rise. At the same time, large icebergs are increasingly breaking away from the glaciers and ice shelves, for example at the Larsen C Ice Shelf or near the Brunt Ice Shelf in the near future. The icebergs then drift on four main roads slowly into more northerly climes, where they slowly melt in contact with warmer marine areas.

Effect of icebergs so far barely explored

The problem, however, is that while the consequences of increased meltwater inflow into the sea have been relatively well researched, this is not yet true for iceberg production. In many common simulations, the effect of icebergs does not even exist - and this can be significant, as explained by Fabian Schloesser of the University of Hawaii in Manoa and his team.

"Icebergs can drift long distances before they completely melt away, " the researchers explain. "As a result, they produce spatially and temporally varying meltwater patterns that can extend over the entire Southern Ocean." Like ice cubes in a glass, the thawing icebergs locally provide a cooling effect and even counteract the increased warming and stratification of the sea due to direct glacier thawing.

Measurable cooling of the sea surface

But how strong is this iceberg effect? To find out, Schloesser and his team carried out climate simulations with varying amounts of direct meltwater and iceberg production and compared their effects on sea conditions and temperatures. They used a modest climate scenario (RCP 4.5) and one that assumes an almost unchecked warming (RCP 8.5)

The result: assuming a substantial proportion of icebergs formed, they actually counteract the effect of direct meltwater. Instead of making the sea surface warmer and thus stabilizing the stratification, the icebergs cool the sea surface. Because they only melt when they reach warmer marine regions der and the transition to a liquid state requires heat energy that removes the ice from the surrounding water.

Delay of heating for up to 50 years

"This surface cooling is the strongest in the regions where the defrosting of the icebergs concentrated, " the researchers report. Along the main routes, where the icebergs drift northward, the local cooling effect can even reach up to six degrees. According to the models, this is most pronounced in the northern Ross Sea and in the Scotia Sea between Tierra del Fuego and the West Antarctic Peninsula.

However, this means that if more icebergs are created as a result of climate change, they could even temporarily offset or at least slow down the warming in parts of the southern hemisphere. "Depending on how fast the Antarctic ice sheet shatters, the iceberg effect could delay future warming in cities like Buenos Aires and Cape Town by ten to fifty years ", says co-author Axel Timmermann of the Institute for Basic Research in Korea.

The warm end comes anyway

However, from time to time the positive effect of the icebergs is unfortunately not. Because they also give off meltwater, which in the long run causes sea levels to rise. In the hottest scenario, the melting of icebergs alone would increase sea levels by 80 centimeters over the course of the 21st century, the researchers report. In addition: If the warming continues, the glaciers are at some point so decimated that instead of icebergs only melt water is created.

"Our results demonstrate that it is important for future climate and sea-level models to assume a realistic coupling of ice sheet decay with iceberg production, " said Schloesser and his team. Now you want to explore this link more closely. (Nature Climate Change, 2019; doi: 10.1038 / s41558-019-0546-1)

Source: Institute for Basic Science

- Nadja Podbregar