Perhaps the most ominous lesson from the Eemian is that even with CO2 at around 135 ppm lower than today, and worldwide temperatures about the same as modern averages, sea levels were six to ten metres higher than at present. Evidence from the Eemian suggests that about five metres of this sea level rise originated from the partial melting of the Greenland ice sheet, which shrank down to a largish remnant in the north-east of the landmass. The implication is pretty clear: it suggests that today’s temperatures are already high enough to eventually melt the majority of the Greenland ice sheet and deliver a multi-metre rise in sea levels. This seems hard to believe, until you travel to Greenland and witness what was once a frozen wasteland currently undergoing rapid, epochal change.
Greenland’s lakes
Greenland is turning blue. Hundreds of thousands of iridescent blue lakes now dot the fringes of the rapidly melting ice sheet throughout the summer months, and the melt area has been creeping steadily upwards and inwards towards the interior as global heating bites. The direct impacts of this increased thaw can be dramatic – in late July 2012 so much meltwater was sluicing off the south-western portion of the ice sheet that a bridge was washed away on the Watson River, which empties into a fjord just to the south of the town of Kangerlussuaq. Footage shared on YouTube shows a grey-brown torrent resembling a tsunami washing away a tractor and tearing through the bridge and the road leading up to it in just a matter of minutes.
This episode is more than just a revealing anecdote. Meltwater flows in the Watson River have been steadily rising in recent years, and on average are now nearly 50% higher than they were in the 1950s. The tattered, debris-covered edge of the giant ice sheet lies just a few kilometres up the valley from Kangerlussuaq, and most of the hundreds of blue lakes that pepper its surface in summer drain into the Watson River, which is fed by a 12,000-square kilometre catchment across the south-western portion of Greenland. In terms of annual averages, the three largest discharge years on record are 2010, 2012 and 2016, with the largest single discharge peak coming in July 2012, explaining why it was then that the bridge over the Watson at Kangerlussuaq succumbed to the dramatic flood.
The timing of this glacial torrent was no great mystery either. For the first time in living memory unprecedented melting had penetrated right up to the summit of the giant ice sheet, located at 3,216 m in altitude. Summit is classed as a polar desert, an inhospitable place of frigid thin air and snow as dry as dust. On 12 July 2012, however, temperatures edged above freezing at the summit for the first time in recorded history, and the automated weather instruments permanently located there were surrounded by slush. Lower down, on the western side of the ice sheet, climate researchers were forced to rebuild their camp when snow and ice supports melted away. Things were no better in North Greenland where Danish scientists logged six consecutive days of above-freezing temperatures at the surface between 10 and 15 July. It even rained on 11 and 13 July, with melt and rainwater percolating down into what had previously been permanent snow.
When the first satellite data came in about the July 2012 melt event, scientists analysing it thought their instruments must have malfunctioned. Later visuals showed the entire ice sheet coloured deep red, indicating pervasive melt conditions. When the scientists checked their data with output from other satellites, they found that on 8 July the temperature of about 40% of the ice sheet surface was above freezing. On 12 July the figure was 98.6%. Virtually the whole of Greenland was in the melt zone, the first time any of the scientists studying the colossal ice sheet had seen such a thing.
Longer-term data show that all-Greenland melt events are extraordinarily rare. A December 2018 paper concluded that the 2012 melt rates were exceptional and ‘likely also to be unprecedented over the last 6,800–7,800 years’. The same paper stated that there has been a 500% increase in melt intensity at the ice core site in just the past 20 years. The latest data show that the fastest-expanding melt zones are now in the northern portion of Greenland, in what should be the coldest area closest to the North Pole.
Once the domain almost exclusively of ice and snow, Greenland has begun to see more rain, even in the depths of winter. While fresh snow is brilliant white, reflecting most of the sun’s radiation back out to space, thawing slush is less reflective, and the bare ice sheet with snow removed is darker still. All around Greenland the snowline has retreated uphill, exposing more uncovered ice to the full heat of the summer sun. As the fringes of the ice sheet gradually shrink inland, bare rock and glacial debris are revealed, leading to an increase in dust storms. Plants have begun to colonise new areas, and are now coming into leaf a week or two earlier as spring temperatures rise.
It has long been clear from ice core records that the climate in Greenland can do sudden and strange things. About 11,700 years ago, at the end of a colder period called the Younger Dryas, temperatures shot up by 15°C over a few decades. A similarly dramatic warming, of 9°C in less than 70 years, took place 14,760 years ago during the last ice age. Scientists fear that Greenland may currently be crossing a similar threshold of rapid climate change, one that could lead to the eventual loss over centuries of the entire ice sheet. Mean annual air temperatures are already 3°C higher than they were a few decades ago. But these shifts did not happen gradually: they came about in sudden jumps, of 2°C in 1994 and a further 1.1°C in 2006. This is a rate of change that, if sustained over decades, easily matches the major climate tipping points of the distant past.
And 2019 was another epochal year. Temperatures across Greenland soared to 12°C above the late July average, and the summit of the ice sheet saw another thaw between 30 and 31 July 2019. This time temperatures at the highest point topped the record set in 2012, and stayed above freezing for longer over a two-day period. In response to these rapid changes, some scientists have begun to warn that the projections for 21st-century sea level rise may need to be revised upwards. According to one researcher, the current melt rates weren’t supposed to happen until 2070. In the first Six Degrees I reported that Greenland’s contribution to sea level rise was about 0.3 mm per year. In 2019, according to preliminary estimates, the melt rate may have been as high as 1.5 mm, a fivefold increase. If that’s not a tipping point, I don’t know what is.
Arctic on thin ice
Signs of increasingly rapid global warming are now evident throughout the Arctic, where temperatures are rising at a rate two to three times the global average. In late December 2015 at the North Pole itself temperatures rose close to freezing. Under normal circumstances, air temperatures at the pole would be averaging around -30°C during the frigid polar nights of late December. On this occasion temperatures shot up by 25°C in just one day as a deep low-pressure system wafted in sub-tropical air from the south. Although no one was there to witness it directly, it seems likely that it was even raining at the North Pole for a brief period at the height of the Arctic winter.
This extreme Arctic warming event lasted for 40 days between 29 December 2015 and 6 February 2016, during which the entire polar Arctic above 66°N saw temperatures 4–6°C above average. Scientists studying the event called it ‘unheard of’ and ‘super-extreme’ – but the following December it happened again. ‘A weather buoy about 90 miles south of the North Pole registered a temperature at the melting point of 0 Celsius early Thursday,’ reported the Washington Post, adding: ‘Santa may need water skis instead of a sleigh this year.’ Temperatures throughout a broad area of the Arctic were 20°C or more above normal in November and December. At a time when the Arctic sea ice should have been regrowing due to winter cold, it actually shed another 50,000 square kilometres (19,300 square miles), an event unprecedented during the entire satellite period. The high Arctic temperatures of October and November were ‘off the charts’, said baffled experts.
To test whether the event could have been caused by chance, scientists at the World Weather Attribution network plugged the temperature data into climate models and compared the results with ‘natural’ conditions without global warming. None of them showed Arctic events as hot as 2016 in the simulated world without human carbon emissions.
So what’s going on? Part of the answer comes from the long-term trend in declining Arctic sea ice, which has been shrinking in area by 13% a decade (in its minimum summertime extent) since satellites began measuring it in 1978. That means half the entire Arctic’s stock of sea ice has disappeared since the late 1970s, and what remains has become about 85% thinner. In fact the thinning trend has become so extreme that scientists now struggle to find thick enough ice floes on which to deploy their measuring instruments. All this new open water has transformed the atmospheric dynamics of the Arctic region, allowing warm, moist winds – whose energy would previously have been dissipated by ice – to penetrate right up to the pole itself.
Other researchers have confirmed that the extreme polar heat of 2016 would not have been possible without the extensive disappearance of Arctic sea ice. While 2018’s Arctic ice minimum was only the sixth lowest in the satellite record, the overall trend continues irrevocably downwards, equating to 82,300 km2 lost on average every year, an area four times the size of Wales. Cumulatively this adds up to a vast total: the extent of Arctic summer sea ice lost since the 1980s would be sufficient to cover 40% of the contiguous United States. And the 2019 September sea ice area, at 4.15 million square kilometres, was the second-lowest since the beginning of the satellite era in 1979. Nature reported that 13 of the lowest Arctic sea-ice extents have been recorded in the last 13 years.
Because there is now dark-coloured open water during the summer rather than highly reflective snow and ice, solar heat absorption has increased fivefold since the 1980s in the once-frozen seas north of Alaska and Canada, introducing vast quantities of additional energy into the Arctic system. I wrote about the Arctic albedo feedback in the first Six Degrees: ‘While bright white, snow-covered ice reflects more than 80% of the sun’s heat that falls on it, the darker open ocean can absorb up to 95% of incoming solar radiation. Once sea ice begins to melt, in other words, the process quickly becomes self-reinforcing: more ocean surface is revealed, absorbing solar heat, raising temperatures and making it more difficult for the ice to re-form during the next winter.’ This change in albedo (how much sunshine gets absorbed by darker coloured surface versus how much gets reflected from brighter surfaces) has now been measured directly by satellites, and its impact is dramatic. Averaged globally, the heating effect is equivalent to pumping another 200 billion tonnes of CO2 into the atmosphere.
Global warming makes us all responsible for this dramatic on-going transformation of the Arctic, and in a surprisingly direct way. Scientists have found a linear relationship between cumulative CO2 emissions and the September sea ice minimum. It turns out that every metric tonne of CO2 implies a sustained loss of 3 m2 of Arctic ice. Given the disparities in emissions between countries, that means that every American is on average responsible for the loss of nearly 50 square metres of ice each year, while a British person is responsible for just under 20 m2 of annual ice loss. This compares to 22 m2 of disappeared ice for each Chinese, 5 m2 of loss for each Indian but under 1 square metre each for the low fossil fuel-consuming inhabitants of east Africa. For Antarctica the equivalent figures are even more dramatic: each kilogram of released CO2 is responsible for melting two metric tonnes from the much larger ice cap over the South Pole.
This transformation of the Arctic is also having serious effects on the wildlife of the polar region. That most emblematic symbol of global warming – the polar bear – is facing population declines because it depends on the sea ice on which to hunt seals. Less ice means fewer seals can be caught, which translates to fewer cubs able to survive into adulthood, and more hungry, skinny adult bears scavenging on ice-free coastlines during the summer. Body-fat samples recovered from polar bears shot by Inuit hunters – who together typically harvest a couple of hundred bears per year across Canada – show a strong seasonal correlation between the availability of sea ice and the condition of the polar bears. ‘We’re restructuring a whole ecosystem,’ says polar bear ecologist Andrew Derocher. ‘Sea ice is to the Arctic what soil is to the forest. Without sea ice we’ll still have an ecosystem but it won’t include polar bears and many other species.’
It’s not yet too late to save the polar bear. Scientists have determined that dramatic emissions reductions and the stabilisation of CO2 at 450 ppm could still retain enough summer sea ice to prevent the polar bear becoming extinct in the wild. Polar bears are not the only species highly dependent on sea ice; walrus, bearded and ringed seals, bowhead whales, beluga and narwhal all need ice for part or all of their life cycles. And the changing Arctic ecosystem brings other new threats. Those marine mammals that survive the loss of ice face being chopped up by boat propellers thanks to the increase in shipping traversing the newly open Northern Passage. The rapid warming affects the oceans too, with Arctic fish species retreating north towards the pole as Atlantic and Pacific migratory fish move in.
These ecological shifts can have tragic effects. For example, the summer sea ice edge has retreated 400 km further away from the nesting grounds of the black guillemot on Cooper Island, off the north coast of Alaska. The guillemots’ primary prey species, Arctic cod, has retreated with the ice, and the majority of chicks now starve to death. Seabird die-offs have also been recorded thanks to low ice conditions in the Bering Sea. With more moisture in a warmer atmosphere, heavy snow can also be an issue – in the summer of 2018 so much snow was left lying across the high Arctic that scientists documented a near-total collapse in the food web of north-east Greenland. Plants failed to flower until late in the season, while migratory shorebirds had only a few eggs hatching. Those nestlings that did fledge would not have had enough time to grow to full size in time to survive the return southward migration. Some shorebirds even starved to death. There were no Arctic fox cubs, and almost no muskox calves. This was the ‘most complete reproductive failure encountered in the terrestrial ecosystem during more than two decades of monitoring’, scientists reported.
The ongoing reorganisation of the Arctic system is also beginning to have ricochet effects on weather patterns across the Northern Hemisphere. Record-breaking wildfires in California, for example, have been linked to changes in atmospheric circulation possibly triggered by the disappearance of Arctic sea ice. These changes are one of the factors behind the devastating multi-year drought that has parched much of the south-western United States, making the area tinder-dry and extremely prone to wildfire outbreaks. This drought has also led to declining yields of crops in the United States, thereby fingering faraway Arctic sea ice as a factor in falling food production thousands of kilometres away. One study published in May 2019 suggests an additional relationship between reductions in sea ice in the Hudson Bay and more frequent heatwaves across the southern Plains of the United States. Another points to a ‘regime shift’ towards ice-free conditions taking place in the Barents Sea in 2005 that may have subsequently ‘contributed to the increasingly frequent extreme weather events experienced over Europe in the past decade or so’.
Several studies have also suggested a possible Arctic amplification fingerprint in ‘planetary waves’ – large-scale wave-like meanders in the Northern Hemisphere jet stream that can remain nearly stationary for long periods of time and cause devastating weather extremes, from summer heatwaves to floods or even winter cold events. Although some experts urge caution in assigning blame, multiple studies have linked these waves to events as diverse as the 2003 European heatwave, the Russian heatwave and Pakistan floods of 2010, the 2011 Texas drought and the persistent lack of rainfall in California, evidence that is now supported by climate models. These ‘blocked’ weather patterns have also contributed to record cold extremes in the Eurasian and North American continents in recent years. Warming of the Arctic Ocean may have helped weaken and shift the ‘Polar Vortex’ of extremely cold, high-pressure air, bringing intense winter conditions across Eurasia and North America. The evidence linking this shift with the changing Arctic is increasingly strong: one study showed a clear link between anomalous warmth in the newly ice-free Barents–Kara Sea region of the Arctic Ocean and severe winters in East Asia, and also linked extreme cold in North America to extra heat in the Chukchi Sea north of Alaska. Another group of meteorologists assessed the record-breaking cold and blizzards that hit parts of North America in the winter of 2014/15, and found that reduced Arctic sea ice might have helped drive the ‘anomalous meander of the jet stream’ which led to the extreme cold.
Teasing out the relative contributions of the different factors is incredibly complex, but one thing is clear: the Arctic circulation that has been established for millennia is breaking down, and this must be having impacts further afield as well. As Chris Rapley, professor of climate science at University College London, told the Guardian:
What happens in the Arctic doesn’t stay in the Arctic. By upsetting the energy balance of the planet we are changing the temperature gradient between the equator and the pole. This in turn sets in motion major reorganisations of the flow patterns of the atmosphere and ocean. The consequences are emerging and they are disruptive, and likely to become even more profoundly so.
In the summer of 2019 the Arctic even began to burn. In mid-June more than a hundred wildfires were burning above the Arctic Circle across Alaska, Siberia and Canada. Even Greenland saw wildfires erupting on fenlands scorched by unusual summer heat. But most importantly of all, the Russian wildfires were not just burning trees. They were also smouldering in dried-out peat, much of it formerly immobilised in permafrost. By the end of July it was estimated that the blazes had released more than 120 million tonnes of CO2 – an all-time record, and more than the total annual emissions of Belgium. All this extra carbon can only do one thing: accumulate in the atmosphere and cause more warming. These weren’t just wildfires, they were positive feedbacks, showing that the heating of the Arctic is threatening to run out of control.
Gulf Stream collapse
The winter blizzards of recent years were as nothing compared with the icy onslaught that hit Europe about 12,000 years ago during the Younger Dryas. During this brief return to near ice age conditions, thawing glaciers in the British Isles and other parts of northern Europe suddenly re-advanced. The remains can be seen all around the peaks of Britain’s now ice-free hills, such as the Lake District, the Scottish Highlands and Snowdonia, where tell-tale hummocks of grassed-over moraines testify to the final extent of these long-disappeared small glaciers. The cause of the sudden cold snap is still disputed, but the driver may have been a shift in the strength of the critically important Atlantic current popularly known as the Gulf Stream. Even longer ago, during the depths of the last ice age, abrupt periods of extreme cold – during which armadas of icebergs were launched across the North Atlantic – have also been blamed on fluctuations of this gigantic ocean current.
Today, the ‘Atlantic Meridional Overturning Circulation’ (AMOC, of which the true Gulf Steam is but a small component) continues to have an outsized impact on Europe’s climate. It is thanks to the AMOC that north-western Europe is roughly 6°C warmer than equivalent maritime latitudes on the Pacific coast of North America. This mighty current carries sub-tropical surface water equivalent in volume to the flow of all the world’s rivers north via the Gulf Stream to the British Isles and Scandinavia. The AMOC is so big that it even has its own unit, the Sverdrup (Sv), equivalent to one million cubic metres of water moving at one metre per second. The AMOC has an average strength of around 17 Sv in the North Atlantic; compare this with the flow of the Amazon – the world’s biggest river – at a paltry 0.2 Sv. It transports oceanic heat of 0.9 petawatts, equivalent to the output of half a million or so nuclear power stations from the tropics to the higher latitudes. For this reason alone, the Northern Hemisphere as a whole is warmer than the Southern Hemisphere, because ocean heat is transported from the south across the equator and into the North Atlantic. A fictionalised AMOC shutdown was the scenario depicted in the 2004 film The Day After Tomorrow; while the giant tsunami hitting New York might have been stretching things a little, it is widely agreed that a total AMOC collapse has the potential to plunge Europe back into decades of sub-zero winters and destabilise weather across the globe.
Whether the AMOC is beginning to weaken – and if so, whether human global warming is to blame – is currently the focus of intense debate. The driving forces behind the AMOC are the areas of very salty, colder water that form offshore from Greenland, Iceland, Scandinavia and Arctic Canada during the winter. This water has a higher density than warmer, fresher surface water and consequently sinks to the ocean floor, where it begins a long journey south to restart the process, like a gigantic heat conveyor belt. Although there is substantial variability year to year, recent data from ocean sensors shows that the strength of the AMOC has already slowed by 15% since the mid-20th century, making it weaker now than probably at any point in the last 1,500 years.
Studies based on data gathered by ocean arrays found that the AMOC’s strength declined by 0.6 Sv per year – that’s three River Amazons’-worth – between 2007 to 2011, and that this weaker state still persists today. It is possible that this is just part of a longer natural fluctuation, as one later study suggests. However, concerns have been growing that the weakening of the AMOC current is linked with the huge quantities of fresh water that are now sluicing off the thawing Greenland ice sheet every summer. This meltwater, combined with increased rainfall, is diluting the waters of the north Atlantic, reducing its salinity. Less saline water is less dense and heavy, and therefore does not sink into the depths and drive the movement of the current.
Climate models also disagree about whether and how the AMOC will collapse, making this issue one of the classic ‘known unknowns’ in the climate debate, a potential shift that could have huge impacts through global weather destabilisation. Whether the current weakening trend will continue is highly uncertain. But one thing we do know: AMOC collapse has happened before, and it had drastic impacts on the climate of the entire Northern Hemisphere. Any prolonged shutdown would have equally disastrous effects today.
Antarctic icebergs
They called it B-46. This was not a code name for a new bomber or some other piece of military hardware, but something potentially much more significant. B-46 was huge, 300 km2 in area – a gigantic iceberg spawned in November 2018 by one of Antarctica’s biggest glaciers, Pine Island Glacier (PIG). And B-46 was not even a record, being substantially smaller than the more than 500 km2 monster that floated away from the same Antarctic glacier in 2015. PIG, the mothership for these mega-bergs, is one of the largest ice streams draining the West Antarctic Ice Sheet (WAIS) and has consequently been the focus of intense study in recent years. Something big is happening to PIG: from 1974 to 2010 it accelerated by 75% to a forward speed of 4 km per year. At this speed an observer standing on land at the edge of the glacier would almost be able to see the massive stream of ice grind by with the naked eye. Its rate of annual ice loss in the same period increased from 6 billion tonnes per year to 46 billion tonnes, an increase of a whopping 750%. The accelerating rate of melt now makes PIG the largest single glaciological contributor to sea level rise in the world. This one glacier now adds a tenth of a millimetre each year to the height of the global oceans.