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  • Unavoidable rise in West Antarctic Ice Sheet melting.

    Unavoidable rise in West Antarctic Ice Sheet melting.

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    Newswise — Scientists ran simulations on the UK’s national supercomputer to investigate ocean-driven melting of the West Antarctic Ice Sheet: how much is unavoidable and must be adapted to, and how much melting the international community still has control over through reduction of greenhouse gas emissions.

    Taking into account climate variability like El Niño, they found no significant difference between mid-range emissions scenarios and the most ambitious targets of the 2015 Paris Agreement. Even under a best-case scenario of 1.5°C global temperature rise, melting will increase three times faster than during the 20th century.

    The West Antarctic Ice Sheet is losing ice and is Antarctica’s largest contributor to sea-level rise. Previous modelling finds this loss could be driven by warming of the Southern Ocean, particularly the Amundsen Sea region. Collectively the West Antarctic Ice Sheet contains enough ice to raise global mean sea-level by up to five metres.

    Around the world millions of people live near the coast and these communities will be greatly impacted by sea level rise. A better understanding of the future changes will allow policymakers to plan ahead and adapt more readily.

    Lead author Dr Kaitlin Naughten, a researcher at the British Antarctic Survey says:

    “It looks like we’ve lost control of melting of the West Antarctic Ice Sheet. If we wanted to preserve it in its historical state, we would have needed action on climate change decades ago. The bright side is that by recognising this situation in advance, the world will have more time to adapt to the sea level rise that’s coming. If you need to abandon or substantially re-engineer a coastal region, having 50 years lead time is going to make all the difference.”

    The team simulated four future scenarios of the 21st century, plus one historical scenario of the 20th century. The future scenarios either stabilised global temperature rise at the targets set out by the Paris Agreement, 1.5°C and 2°C, or followed standard scenarios for medium and high carbon emissions.

    All scenarios resulted in significant and widespread future warming of the Amundsen Sea and increased melting of its ice-shelves. The three lower-range scenarios followed nearly identical pathways over the 21st century. Even under the best-case scenario, warming of the Amundsen Sea sped up by about a factor of three, and melting of the floating ice shelves which stabilise the inland glaciers followed, though it did begin to flatten by the end of the century.

    The worst-case scenario had more ice shelf melting than the others, but only after 2045. The authors heed that this high fossil fuel scenario, where emissions increase rapidly, is considered unlikely to occur.

    This study presents sobering future projections of Amundsen Sea ice-shelf melting but does not undermine the importance of mitigation in limiting the impacts of climate change.

    Naughten cautions: “We must not stop working to reduce our dependence on fossil fuels. What we do now will help to slow the rate of sea level rise in the long term. The slower the sea level changes, the easier it will be for governments and society to adapt to, even if it can’t be stopped.”

    Unavoidable future increase in West Antarctic ice-shelf melting over the 21st century by Kaitlin Naughten (BAS), Paul Holland (BAS), Jan De Rydt (Northumbria) is published in the journal Nature Climate Change.

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  • Antarctic fur seals face new threat.

    Antarctic fur seals face new threat.

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    Newswise — Antarctic fur seals that were hunted to near extinction have recovered but now face dangerous decline because of a lack of food, new research suggests.

    The study of fur seals, almost all of which live on the sub-Antarctic islands of South Georgia, shows that the modern-day population peaked in 2009 at about 3.5 million – a healthy number, although significantly less than previously estimated. But a more detailed count of animals living on a particular South Georgia island called Bird Island also shows the seals are finding it harder to find krill – a small shrimp-like crustacean – with numbers crashing in the last decade.

    The research is carried out by scientists at the British Antarctic Survey (BAS) and colleagues and is published in the journal Global Change Biology.

    Jaume Forcada, a BAS scientist who led the new study and is lead author on the paper, says:

    “We found both good and bad news about the fur seals. The population has recovered very impressively throughout the twentieth century when seal hunting was banned. But twenty-first century changes to the abundance of krill in the Southern Ocean are now threatening these iconic animals all over again.” 

    Prized and hunted for their pelts in the 1700s and 1800s, by the early 1900s there were too few animals left to hunt commercially. Combined with stronger conservation protections, plenty of food and fast breeding, the population boomed and recovered much more quickly than other previously hunted species in the region, including humpback whales. By the year 2000, South Georgia was reportedly home to between 4.5 million and 6.2 million fur seals. However, re-examination of this data and the methods used suggests that these booming population numbers were not accurate – offering a further challenge to scientists now trying to understand the current decline.

    Forcada continues: “Our new results show this was a massive overestimation. That matters because the fur seal population size is used to judge the overall health of the species and the wider Antarctic ecosystems. And it turns out that neither were as robust as people thought.”

    Counting seals is harder than it may appear. Surveys typically assess numbers at seal breeding beaches, but these figures are skewed because most male fur seals at South Georgia don’t usually breed until they are ten-years old, and then only for two to three years. That means some 80% of the male population is missing from surveys, and how researchers account for this can overstate the overall count.  

    The new estimate of 3.5 million animals comes from several week-long helicopter surveys of South Georgia from 2007 to 2009 and improved population assessment methods.

    Fur seal populations at Bird Island, a well-studied location in the northwest of South Georgia, showed among the fastest rebounds over the last century or so. But the new survey identified a worrying reversal: numbers peaked in 2009 and have declined by 7% each year since. That takes the island’s current population down to a level not seen since the 1970s, when the population was still recovering.

    Scientists looked for evidence of krill fishing pressure on fur seal population dynamics, but this was not found to be significant. However, initial analysis of climate data shows that that rapidly rising sea temperatures in the region correlate with the seal population decline – pointing to a loss of krill as the most likely cause. 

    “Krill can make up to 80% or more of the diet of fur seals at South Georgia, so they experience catastrophic declines in the number of pups produced and survival of individuals when environmental conditions remove the krill from their immediate foraging areas,” Forcada says.

    This sensitivity of fur seals to krill availability, along with the existence of longer-term population data, is what makes them an important known indicator for the ecosystems around Bird Island. More detailed research is needed to establish why krill around Bird Island is now less available, and how widespread the change could be across the rest of the Southern Ocean.

    “If the pressure on the fur seals at Bird Island also applies to the greater South Georgia population there could be an ongoing decline there as well. So even though there were three and a half million of them there, the fast decline at Bird Island tells us they could be in trouble,” Forcada says.

    Ninety years of change, from commercial extinction to recovery, range expansion and decline for Antarctic fur seals at South Georgia, by Jaume Forcada, Joseph I. Hoffman, Olivier Gimenez, Iain J. Staniland, Pete Bucktrout, Andrew G. Wood is published in Global Change Biology on Monday 16 October.

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