Perchè il ghiaccio dell’Antartico sta aumentando ?
Aumenta il riscaldamento del pianeta eppure i ghiacci antartici, in assoluta controtendenza, anche rispetto agli ultimi anni, aumentano. Questo articolo esamina le possibili cause, passando in rassegna fonti e studi fra i più accreditati dal punto di vista scientifico.
What is happening in the Antarctic? Sea ice around the shores of the giant frozen continent has been expanding remorselessly for decades – and all the while Arctic sea ice has been in long term decline.
Sea ice surrounding Antarctica set a new record high extent in 2014, despite average global surface temperatures that, for more than a decade, have been at or near the highest levels since weather measurements began. In September 2014 Antarctic sea ice covered an enormous area, extending out over more than 20 million square kilometres; it covered more of the Southern Ocean than at any time since scientists first began satellite mapping of sea ice in the late 1970s.
America’s National Oceanic and Atmospheric Administration (NOAA) pinned the blame squarely on global warming in an online article on its website which stated “as counter intuitive as expanding winter Antarctic sea ice may appear on a warming planet, it may actually be a manifestation of recent warming”.
According to US space agency NASA, this new Antarctic sea ice record reflects the diversity and complexity of Earth’s environments. The idea is that, just as the temperatures in some regions of the planet are colder than average, even though overall global temperatures are warmer, so it is possible that Antarctic sea ice has been bucking the long-term global trend. And in the Antarctic, where sea ice circles the continent and covers such a large area, it only takes a little extra ice to set a new record.
In this way, according to Claire Parkinson, a senior scientist at NASA’s Goddard Space Flight Center, changes in sea ice coverage are a microcosm of global climate change. Parkinson is quoted in a NASA news release. “Its really not surprising to people in the climate field that not every location on the face of Earth is acting as expected – it would be amazing if everything did,” Parkinson said. “The Antarctic sea ice is one of those areas where things have not gone entirely as expected. So it’s natural for scientists to ask, ‘OK, this isn’t what we expected, now how can we explain it?’”
NASA colleague and polar scientist Joey Comiso points out in a NASA Q&A that geographically, the Antarctic and the Arctic are very different. In the southern hemisphere the sea ice surrounds a continent whereas in the northern hemisphere the sea ice is surrounded by land. This is a fundamental difference that drives different processes and means that the various pressures on Antarctic sea ice are very different from those affecting Arctic sea ice and therefore the trends in the two regions would not be expected to be the same.
Walt Meier, a research scientist at NASA’s Goddard, agrees: “Part of it is just the geography and geometry. With no northern barrier (in Antarctica) around the whole perimeter of the ice, the ice can easily expand if conditions are favorable,” Meier explained in a NASA press release.
Both the Arctic and the Antarctic are responding to climate change, according to glaciologist Ted Scambos, from the US National Snow and Ice Data Center (NSIDC), in an article on NOAA’s website. Both have areas that are warming rapidly but Antarctic sea ice is responding to wind shifts and ocean changes in an unexpected way, “and we’re still trying to fully understand it” he said.
Comiso also believes the atmosphere is a major driving force. Changes in pressure and in wind speeds and in wind direction could cause an increase in ice production and move it to the north. A cooling in sea surface temperature could also cause a more rapid advance at the ice edge.
A warming climate can change weather patterns in this way, according to Meier. Sometimes those weather patterns will bring cooler air to some areas.
Researchers are investigating a number of possible explanations for these changes. One clue, according to NASA’s Parkinson, can be found around the Antarctic Peninsula – a finger of land stretching up toward South America. There, average temperatures are warming, and in the Bellingshausen Sea, just to the west of the peninsula, the sea ice is shrinking.
But beyond the Bellingshausen Sea and past the Amundsen Sea, lies the Ross Sea – where much of the sea ice growth is occurring. Something different is happening here. Data suggests that a low-pressure system centered in the Amundsen Sea could be intensifying or becoming more frequent in the area. This would change the wind patterns with the result that warm air is circulated over the peninsula, shrinking sea ice cover there, while cold air is swept from the deep freeze of the Antarctic continent flowing out over the Ross Sea.
And conditions in some parts of Antarctica were even cooler than usual during the sea ice record breaking winter of 2014. When we think about global warming many people expect temperatures to be increasing everywhere, to increase evenly throughout the globe. But it is more complicated than that because of complex atmospheric circulation. Satellite data indicates that there is some cooling in parts of the Southern Ocean and especially in the Ross Sea region.
Indeed, satellite instruments confirm that conditions have been particularly cold over the Ross Ice Shelf. Compared to seasonal norms, the coldest place in Earth’s atmosphere in June 2014 was over the Ross Ice Shelf, where winter temperatures were as much as 5.4C colder than seasonal norms, according to data released by University of Alabama in Huntsville (UAH).
Research suggests that this exceptionally cold air, and other wind and lower atmospheric pattern changes, could be influenced by the ozone hole higher up in the atmosphere over the south polar region. Computer models indicated that the ozone hole actually caused a deepening of the lows in Western Antarctica, according to Comiso, and this in turn has caused winds to become stronger off the Ross Ice Shelf. This is significant as there is a large area of open water surrounded by sea ice – a polynya; they are typically found right at the edge of ice shelves or glaciers in Antarctica.
There are areas around Antarctica where coastal polynyas form. These polynyas are regarded as ice factories, according to Comiso, and if you have stronger winds it means a stronger rate of ice production. It has been estimated that about one-third of the ice cover around Antarctica originates from these polynya areas. But because of changing atmospheric circulation, the wind conditions may have been more favourable to the formation of more extensive polynyas around Antarctica, Comiso states.
“The winds really play a big role,” according to NASA’s Meier. “They whip around the continent, constantly pushing the thin ice. And if they change direction or get stronger in a more northward direction,” he said, “they push the ice further and grow the extent.”
However, while scientists observed some stronger than normal pressure systems – which act to increase wind strength – during the Antarctic winter that occurred in mid-2014, that element alone is probably not the only reason for the record sea ice extent, according to Meier. Other factors were at work.
Melting land ice on the edges of the Antarctic continent could be leading to more fresh water to enter the sea and this fresh water is at a temperature that is just above freezing. This makes refreezing it into sea ice easier, Parkinson said.
Another possibility is that changes in water circulation patterns, bringing colder waters up to the surface around the Antarctic landmass, could help grow more ice by acting to lower surface water temperatures.
Snowfall could be a factor as well, according to Meier. Snow landing on thin ice can actually push the thin ice below the water, which then allows cold ocean water to seep up through the ice and flood the snow – leading to a slushy mixture that freezes in the cold atmosphere and adds to the thickness of the ice. This new, thicker ice would be more resilient to melting.
“Our models are improving, but they’re far from perfect. One by one, scientists are figuring out that particular variables are more important than we thought years ago, and one by one those variables are getting incorporated into the models,” said NASA’s Parkinson.
For Antarctica, these key variables include the atmospheric and oceanic conditions, as well as the effects of an icy land surface, changing atmospheric chemistry, the ozone hole, months of darkness and more.
There is research to back up some of these ideas. In 2009, a team including NASA’s Comiso reported in the journal Geophysical Research Letters, that scientists from British Antarctic Survey (BAS) and NASA had found that Antarctic sea ice had increased as a result of the ozone hole delaying the impact of greenhouse gas increases on the climate of the continent. Lead author Professor John Turner of BAS aid at the time: “Our results show the complexity of climate change across the Earth. While there is increasing evidence that the loss of sea ice in the Arctic has occurred due to human activity, in the Antarctic human influence through the ozone hole has had the reverse effect and resulted in more ice. Although the ozone hole is in many ways holding back the effects of greenhouse gas increases on the Antarctic, this will not last, as we expect ozone levels to recover by the end of the 21st Century. By then there is likely to be around one third less Antarctic sea ice.”
However, this view has since been challenged by other research published in the Journal of Climate at the end of 2013. Climate scientists Michael Sigmond from the University of Victoria in Canada and John Fyfe of Canadian Centre for Climate Modelling and Analysis used computer models to show that ozone depletion is associated with decreased sea ice extent, not increased sea ice extent, and that ozone recovery acts to mitigate the future sea ice decrease associated with increasing greenhouse gases. It flatly contracdicts the BAS/NASA research.
Other research offers some suggestions from computer models and evidence from satellites that Antarctic sea ice growth in recent years may be due to wind intensification and ocean circulation changes. There is also research evidence to confirm that winds have strengthened over the Southern Ocean.
A paper published in Nature Geoscience by Paul Holland of the BAS and Ron Kwok of the Jet Propulsion Laboratory of the California Institute of Technology presented satellite tracking evidence that “reveals large and statistically significant trends in Antarctic ice drift, which, in most sectors, can be linked to local winds”.
Jinlun Zhang of the University of Washington has used computer models to study the interaction between wind and ice and concluded that changes in winds are resulting in both more compaction within the ice pack and more ridging, causing a thickening of the pack and making it more resistant to summer melt. In simple terms, wind drives ice out to sea, creating open water near the ice-edge that is more likely to freeze.
And winds in the Southern Ocean may be more intense now than they have been for 1,000 years and also pushing further south towards Antarctica, according to research from the Australian National University (ANU). Lead researcher Nerilie Abram, from the ANU Research School of Earth Sciences, said the findings explained the mystery over why Antarctica was not warming as much as the Arctic. “As the westerly winds are getting tighter they’re actually trapping more of the cold air over Antarctica,” Abram explained when the research was published. “This is why Antarctica has bucked the trend.”
One of the challenges faced by scientists is that we do not have good data prior to satellites to compare current conditions with those in the past. Most of what we know about the Antarctic sea ice is what is derived from satellite data which covers a relatively short period since 1979. Studies that made use of ship observations in the 1940s and 1950s are not so reliable but suggest that the ice cover during these time periods may have actually been more extensive than during the satellite period, according to NASA’s Comiso.
And that means that we do not really understand how long term natural changes work in the Antarctic. Indeed, there is also some research evidence to suggest that natural variability has been at work in Antarctica. Ice core analysis by BAS researcher Elizabeth Thomas and a team from the University Cambridge found that climate variability in coastal West Antarctica is strongly driven by sea surface temperatures and atmospheric pressure in the tropical Pacific. The researchers noted that the recent warming trend in Antarctica is similar in magnitude to warming and cooling trends that occurred in the mid-nineteenth and eighteenth centuries in their ice core record, indicating that – in the vicinity of the ice core, at least – the effects of human-induced climate change in recent years have not exceeded natural climate variability over the past 300 years.
These possible explanations are far from proven and, as NSIDC’s Scambos implies, it is clear that climate scientists do not fully understand the processes at work. Slowly the research is coming in but the solution to the puzzle of Antarctic sea ice remains elusive. “There hasn’t been one explanation yet that I’d say has become a consensus, where people say, ‘We’ve nailed it, this is why it’s happening,’” said NASA’s Parkinson.
Indeed, the IPCC admitted as much in its latest scientific report published in September 2013 where it stated that there is “low confidence in the scientific understanding of the observed increase in Antarctic sea ice extent since 1979”. The IPCC explained that “the shortness of the observed record and differences in simulated and observed variability preclude an assessment of whether or not the observed increase since 1979 is inconsistent with internal variability”. This is a long-winded way of saying that they do not understand what is happening.
It is clear that climate science can not yet completely explain what is happening in the Antarctic. There are many credible theories but so far no completely satisfactory comprehensive explanation has emerged. It remains a mystery.
However, whether the sea ice changes are due to global warming or natural variability, one stark fact remains unarguable: Antarctic sea ice extent has been growing consistently for many years – and it set another record in 2014.