Shock and thaw
Classified satellite photographs usually involve missile sites or nuclear power installations. But last summer, images of melting ice caps emerged as another category deemed worthy of being kept under lock and key. The pictures, kept from public eyes during George W Bush’s presidency, were among 1,000 images showing the impact of global warming that were declassified by the Obama administration.
Among the most striking was one of solid sea ice around the Alaskan port of Barrow in July 2006, and another, taken a year later in the same area, depicting ice-free waters.
The detailed one-metre-resolution images were unambiguous evidence of the nature of the evaporating ice sheets. They were reinforced by a study of satellite measurements involving NASA’s Goddard Space Flight Center that found that Arctic ice, typically up to about three metres thick, had thinned by 67 centimetres over the past four winters.
‘The trend is very clear and simply astonishing,’ says Dr Ron Kwok, senior researcher for NASA’s Jet Propulsion Laboratory. ‘This is a very clear signal of global warming. We’re seeing a lot more seasonal thin ice rather than perennial ice.’ Between 2005 and 2008, according to NASA, there was a 42 per cent drop in the amount of thick ice. Last year, multi-year ice, which is generally more than 2.5 metres thick, covered about one third of the Arctic Ocean – before 2005, the area covered was more than half the total.
Arctic ice – which, according to the British Antarctic Survey (BAS), has been a permanent feature for at least 100,000 years – is disappearing before our eyes. The total absence of summer ice is, by almost all accounts, now a foregone conclusion in a matter of years.
‘Multi-year ice is now confined to quite a small area near Greenland and Ellesmere Island – perhaps two million square kilometres,’ says Professor Peter Wadhams of the Polar Oceans Physics Group at the University of Cambridge. ‘In 2007, the absolute minimum area of ice in the Arctic was 4.2 million square kilometres. Half of the ice that remains is first-year ice [less than two metres thick and which retreats, thins and melts in summer].’
Grim up north
But it isn’t only the state of ice over the Arctic waters that is causing disquiet. The Greenland ice sheet, which covers the bulk of the world’s largest non-continental island, is also under intense scrutiny. The ice sheet, the WWF points out, holds enough water to raise global sea levels by seven metres if it were to melt.
Observations of the ice sheet from the ground, airborne platforms and satellites show that more ice is being lost to the ocean than is being formed to maintain it. Last year, this mass loss was about 280 gigatonnes per year, and has increased by 20 gigatonnes every year since the 1980s. For comparison, a large city such as Los Angeles uses about one gigatonne of water a year. The WWF calculates that if the ice sheet continues to lose mass at this rate, sea levels will rise worldwide by 31 centimetres from Greenland water sources alone by 2100.
According to the WWF’s Arctic Climate Feedbacks: Global Implications report, published this autumn, the melting on the surface of the Greenland ice sheet in 2007 was by far the most extensive since records began. The area experiencing surface melt was 60 per cent larger than in 1998, when the second-highest level of melting was recorded. In 2007, some areas of the Arctic that were free of surface-water ice were as much as 5°C higher than the long-term average, according to the report.
‘Arctic Ocean waters have been warming in recent years, because declining sea-ice cover allows the water to absorb more heat from the sun,’ notes the report. ‘The Arctic Ocean has also warmed as a result of the influx of warmer water from the Pacific and Atlantic oceans.’ In addition, the World Meteorological Organization (WMO) reports that studies conducted during the winter of 2007–08 showed that the North Pole region was covered only in relatively thin first-year ice in mid-winter for the first time in observational history.
Arctic expeditions have also witnessed this trend. The veteran polar explorer Pen Hadow led the Catlin Arctic Survey (see page 28), which measured the thickness of Arctic ice along a route of more than 400 kilometres this spring. Over the length of the survey, the average thickness of the sea ice was 1.774 metres – far less than he or his scientific team had anticipated.
‘People have predicted that the Arctic ice will completely disappear by 2020,’ says Wadhams. ‘I think they’re wrong, but they’re not far off. The first-year ice will all disappear within a few years, but the area of multi-year ice will take longer to disappear. In summer, within 20–30 years, the multi-year ice will disappear. You will still get big areas of ice in winter, but it will all be first-year ice.’
The State of Polar Research report, published with the support of the WMO, examined a number of studies during the International Polar Year of 2007–08 and concluded that ‘it now appears certain that both the Greenland and the Antarctic ice sheets are losing mass and thus raising sea level, and that the rate of ice loss from Greenland is growing’.
Gone south
At the other end of the planet, public attention has been drawn to a series of spectacular collapses of Antarctic ice shelves, such as Larsen B, which folded in 2002, calving vast icebergs into the Southern Ocean. In the past 30 years, seven floating ice shelves have retreated, and BAS data shows a 3°C warming on the Antarctic Peninsula during the past 50 years, making it one of the most rapidly warming parts of the planet, heating up at a rate nearly five times the mean rate of global warming, according to the Intergovernmental Panel on Climate Change (IPCC).
Yet despite this formidably solid evidence that Antarctica is warming up, the picture is less clear-cut than in the Arctic. ‘There’s a lot of uncertainty about what’s happening with the Antarctic ice sheet,’ says Dr John King, science leader for the BAS climate research programme. ‘It’s hard to nail it down. The consensus at the moment is that it’s just about in balance – not contributing to sea level rises one way or another.’
Unlike in the Arctic, there has been a small but statistically significant increase in the overall extent of Antarctic sea ice. However, there are strong geographical variations at a regional level. Sea ice cover has declined substantially in the seas to the west of the Antarctic Peninsula but has increased in other parts of the Antarctic. According to the BAS, the majority of stations in East Antarctica, including the two long-term records from the high plateau of East Antarctica (Amundsen-Scott South Pole and Vostok stations), show no statistically significant warming or cooling trends.
The ice sheet covering East Antarctica is stable, says the BAS, because it lies on rock that is above sea level and is thought unlikely to collapse; West Antarctica is less stable, because it sits on rock below sea level. ‘The West Antarctic ice sheet looks the most vulnerable – essentially because it’s in direct contact with the ocean,’ says King.
The Amundsen Sea Embayment, one of the three major ice drainage basins of the West Antarctic ice sheet, is also causing concern. The bedrock beneath the ice is a long way below sea level, and the ice is only kept in place because it’s thick enough to rest on the bed. Thinning of the ice around the coast could lead to glacier acceleration and further thinning of the ice sheet. Essentially, the ice sheet may be unstable, according to the BAS, and recently identified patterns of thinning could be a precursor to wholesale loss of the Amundsen Sea Embayment.
Explaining loss
For both poles, there is still a vigorous debate about just how much of this ice reduction is directly attributed to dynamic forces, such as ice redistribution (ice being crushed together, reducing the extent but not necessarily the volume of ice) or ice export (ice being pushed out of the Arctic Ocean), and how much is the direct result of thermodynamic forces (melting ice).
The BAS says that a total of 25,000 square kilometres of ice shelf has been lost from around the Antarctic Peninsula since the 1950s. The volume of this lost ice is the equivalent of the UK’s domestic water requirements for about 1,000 years. The concern is that the Larsen B ice shelf appeared to have been stable for the past 10,000 years before its collapse, suggesting that recent warm temperatures are exceptional within the context of this period – and making it unlikely that its loss can
be explained by natural variability alone.
There’s considerably less debate over whether or not warming is taking place, according to Kwok. ‘The thickness of ice in the Arctic has declined over multiple decades – it’s not a blip of a few years. All the evidence suggests this is a global effect caused by warming, and the poles are more sensitive than other areas of the planet.’
Models project a warming of a few degrees Celsius over much of continental Antarctica in the coming decades. However, according to King, because mean temperatures over most of the continent are well below freezing, even this warming won’t greatly increase loss of ice from the continent through melting. In fact, the opposite may happen: increases in snowfall resulting from a warmer atmosphere (which can hold more water vapour) may actually thicken the Antarctic ice sheets.
Warming is also predicted in and over the oceans surrounding Antarctica. Where warmer ocean waters come into contact with the continental ice sheets, the loss of ice from the continent will be accelerated. As a result, the BAS’s best bet, heavily qualified, is that sea ice cover may decline by around a quarter.
‘If it gets warmer, the atmosphere will hold more moisture and that means more snow, which will increase the size of the ice sheet and offset any rise in sea level,’ says King. ‘But a warmer climate also leads to greater ablation [reduction] of the ice sheets. It’s a question of which of these effects will win out. Increasingly, it seems that melting of the ice sheet is going to win, and it’s likely that Antarctica will contribute to sea-level rises over the next century.’
More to learn
Greater understanding of the underlying geology of the Antarctic is leading to a re-evaluation of scenarios for warming. ‘The paradigm is changing when it comes to understanding Antarctica,’ says Dr Vladimir Ryabinin, senior scientific officer of the WMO’s World Climate Research Programme. ‘People used to imagine it was more or less a solid block of ice, but now we know there are even rivers running underneath its surface.
‘Taking into account the fact that some of the land below ice is also below sea level, that makes [the ice sheets] more vulnerable to the warming waters of the ocean, and that may lead to increased melting and hence an increased rate of sea-level rise. I think we will see some surprises in what the ice sheets will do, and melting may happen more easily than we thought before.’
What is also clear is that although most of the science points in only one direction, even the world’s experts are uncertain as to just how things may unfold. ‘There’s an awful lot of activity going on in both the south and north polar regions,’ says King. ‘The more we find out, the more we realise we don’t know. These systems are
fantastically complex, more so than we ever imagined. A lot more work needs to be done.’
November 2009
Among the most striking was one of solid sea ice around the Alaskan port of Barrow in July 2006, and another, taken a year later in the same area, depicting ice-free waters.
The detailed one-metre-resolution images were unambiguous evidence of the nature of the evaporating ice sheets. They were reinforced by a study of satellite measurements involving NASA’s Goddard Space Flight Center that found that Arctic ice, typically up to about three metres thick, had thinned by 67 centimetres over the past four winters.
‘The trend is very clear and simply astonishing,’ says Dr Ron Kwok, senior researcher for NASA’s Jet Propulsion Laboratory. ‘This is a very clear signal of global warming. We’re seeing a lot more seasonal thin ice rather than perennial ice.’ Between 2005 and 2008, according to NASA, there was a 42 per cent drop in the amount of thick ice. Last year, multi-year ice, which is generally more than 2.5 metres thick, covered about one third of the Arctic Ocean – before 2005, the area covered was more than half the total.
Arctic ice – which, according to the British Antarctic Survey (BAS), has been a permanent feature for at least 100,000 years – is disappearing before our eyes. The total absence of summer ice is, by almost all accounts, now a foregone conclusion in a matter of years.
‘Multi-year ice is now confined to quite a small area near Greenland and Ellesmere Island – perhaps two million square kilometres,’ says Professor Peter Wadhams of the Polar Oceans Physics Group at the University of Cambridge. ‘In 2007, the absolute minimum area of ice in the Arctic was 4.2 million square kilometres. Half of the ice that remains is first-year ice [less than two metres thick and which retreats, thins and melts in summer].’
Grim up north
But it isn’t only the state of ice over the Arctic waters that is causing disquiet. The Greenland ice sheet, which covers the bulk of the world’s largest non-continental island, is also under intense scrutiny. The ice sheet, the WWF points out, holds enough water to raise global sea levels by seven metres if it were to melt.
Observations of the ice sheet from the ground, airborne platforms and satellites show that more ice is being lost to the ocean than is being formed to maintain it. Last year, this mass loss was about 280 gigatonnes per year, and has increased by 20 gigatonnes every year since the 1980s. For comparison, a large city such as Los Angeles uses about one gigatonne of water a year. The WWF calculates that if the ice sheet continues to lose mass at this rate, sea levels will rise worldwide by 31 centimetres from Greenland water sources alone by 2100.
According to the WWF’s Arctic Climate Feedbacks: Global Implications report, published this autumn, the melting on the surface of the Greenland ice sheet in 2007 was by far the most extensive since records began. The area experiencing surface melt was 60 per cent larger than in 1998, when the second-highest level of melting was recorded. In 2007, some areas of the Arctic that were free of surface-water ice were as much as 5°C higher than the long-term average, according to the report.
‘Arctic Ocean waters have been warming in recent years, because declining sea-ice cover allows the water to absorb more heat from the sun,’ notes the report. ‘The Arctic Ocean has also warmed as a result of the influx of warmer water from the Pacific and Atlantic oceans.’ In addition, the World Meteorological Organization (WMO) reports that studies conducted during the winter of 2007–08 showed that the North Pole region was covered only in relatively thin first-year ice in mid-winter for the first time in observational history.
Arctic expeditions have also witnessed this trend. The veteran polar explorer Pen Hadow led the Catlin Arctic Survey (see page 28), which measured the thickness of Arctic ice along a route of more than 400 kilometres this spring. Over the length of the survey, the average thickness of the sea ice was 1.774 metres – far less than he or his scientific team had anticipated.
‘People have predicted that the Arctic ice will completely disappear by 2020,’ says Wadhams. ‘I think they’re wrong, but they’re not far off. The first-year ice will all disappear within a few years, but the area of multi-year ice will take longer to disappear. In summer, within 20–30 years, the multi-year ice will disappear. You will still get big areas of ice in winter, but it will all be first-year ice.’
The State of Polar Research report, published with the support of the WMO, examined a number of studies during the International Polar Year of 2007–08 and concluded that ‘it now appears certain that both the Greenland and the Antarctic ice sheets are losing mass and thus raising sea level, and that the rate of ice loss from Greenland is growing’.
Gone south
At the other end of the planet, public attention has been drawn to a series of spectacular collapses of Antarctic ice shelves, such as Larsen B, which folded in 2002, calving vast icebergs into the Southern Ocean. In the past 30 years, seven floating ice shelves have retreated, and BAS data shows a 3°C warming on the Antarctic Peninsula during the past 50 years, making it one of the most rapidly warming parts of the planet, heating up at a rate nearly five times the mean rate of global warming, according to the Intergovernmental Panel on Climate Change (IPCC).
Yet despite this formidably solid evidence that Antarctica is warming up, the picture is less clear-cut than in the Arctic. ‘There’s a lot of uncertainty about what’s happening with the Antarctic ice sheet,’ says Dr John King, science leader for the BAS climate research programme. ‘It’s hard to nail it down. The consensus at the moment is that it’s just about in balance – not contributing to sea level rises one way or another.’
Unlike in the Arctic, there has been a small but statistically significant increase in the overall extent of Antarctic sea ice. However, there are strong geographical variations at a regional level. Sea ice cover has declined substantially in the seas to the west of the Antarctic Peninsula but has increased in other parts of the Antarctic. According to the BAS, the majority of stations in East Antarctica, including the two long-term records from the high plateau of East Antarctica (Amundsen-Scott South Pole and Vostok stations), show no statistically significant warming or cooling trends.
The ice sheet covering East Antarctica is stable, says the BAS, because it lies on rock that is above sea level and is thought unlikely to collapse; West Antarctica is less stable, because it sits on rock below sea level. ‘The West Antarctic ice sheet looks the most vulnerable – essentially because it’s in direct contact with the ocean,’ says King.
The Amundsen Sea Embayment, one of the three major ice drainage basins of the West Antarctic ice sheet, is also causing concern. The bedrock beneath the ice is a long way below sea level, and the ice is only kept in place because it’s thick enough to rest on the bed. Thinning of the ice around the coast could lead to glacier acceleration and further thinning of the ice sheet. Essentially, the ice sheet may be unstable, according to the BAS, and recently identified patterns of thinning could be a precursor to wholesale loss of the Amundsen Sea Embayment.
Explaining loss
For both poles, there is still a vigorous debate about just how much of this ice reduction is directly attributed to dynamic forces, such as ice redistribution (ice being crushed together, reducing the extent but not necessarily the volume of ice) or ice export (ice being pushed out of the Arctic Ocean), and how much is the direct result of thermodynamic forces (melting ice).
The BAS says that a total of 25,000 square kilometres of ice shelf has been lost from around the Antarctic Peninsula since the 1950s. The volume of this lost ice is the equivalent of the UK’s domestic water requirements for about 1,000 years. The concern is that the Larsen B ice shelf appeared to have been stable for the past 10,000 years before its collapse, suggesting that recent warm temperatures are exceptional within the context of this period – and making it unlikely that its loss can
be explained by natural variability alone.
There’s considerably less debate over whether or not warming is taking place, according to Kwok. ‘The thickness of ice in the Arctic has declined over multiple decades – it’s not a blip of a few years. All the evidence suggests this is a global effect caused by warming, and the poles are more sensitive than other areas of the planet.’
Models project a warming of a few degrees Celsius over much of continental Antarctica in the coming decades. However, according to King, because mean temperatures over most of the continent are well below freezing, even this warming won’t greatly increase loss of ice from the continent through melting. In fact, the opposite may happen: increases in snowfall resulting from a warmer atmosphere (which can hold more water vapour) may actually thicken the Antarctic ice sheets.
Warming is also predicted in and over the oceans surrounding Antarctica. Where warmer ocean waters come into contact with the continental ice sheets, the loss of ice from the continent will be accelerated. As a result, the BAS’s best bet, heavily qualified, is that sea ice cover may decline by around a quarter.
‘If it gets warmer, the atmosphere will hold more moisture and that means more snow, which will increase the size of the ice sheet and offset any rise in sea level,’ says King. ‘But a warmer climate also leads to greater ablation [reduction] of the ice sheets. It’s a question of which of these effects will win out. Increasingly, it seems that melting of the ice sheet is going to win, and it’s likely that Antarctica will contribute to sea-level rises over the next century.’
More to learn
Greater understanding of the underlying geology of the Antarctic is leading to a re-evaluation of scenarios for warming. ‘The paradigm is changing when it comes to understanding Antarctica,’ says Dr Vladimir Ryabinin, senior scientific officer of the WMO’s World Climate Research Programme. ‘People used to imagine it was more or less a solid block of ice, but now we know there are even rivers running underneath its surface.
‘Taking into account the fact that some of the land below ice is also below sea level, that makes [the ice sheets] more vulnerable to the warming waters of the ocean, and that may lead to increased melting and hence an increased rate of sea-level rise. I think we will see some surprises in what the ice sheets will do, and melting may happen more easily than we thought before.’
What is also clear is that although most of the science points in only one direction, even the world’s experts are uncertain as to just how things may unfold. ‘There’s an awful lot of activity going on in both the south and north polar regions,’ says King. ‘The more we find out, the more we realise we don’t know. These systems are
fantastically complex, more so than we ever imagined. A lot more work needs to be done.’
November 2009
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