
By Tim Radford
Scientists poring over military and satellite imagery have mapped the unimaginable: a network of rivers, streams, ponds, lakes and even a waterfall, flowing over the ice shelf of a continent with an annual mean temperature of more than -50C.
In 1909 Ernest Shackleton and his fellow explorers on their way to the magnetic South Pole found that they had to cross and recross flowing streams and lakes on the Nansen Ice Shelf.
Antarctic Waterways
Now, U.S. scientists report in the journal Nature that they studied photographs taken by military aircraft from 1947 and satellite images from 1973 to identify almost 700 seasonal networks of ponds, channels and braided streams flowing from all sides of the continent, as close as 600km to the South Pole and at altitudes of 1,300 meters.
And they found that such systems carried water for 120km. A second research team reporting a companion study in the same issue of Nature identified one meltwater system with an ocean outflow that ended in a 130-meter wide waterfall, big enough to drain the entire surface melt in a matter of days.
In a world rapidly warming as humans burn ever more fossil fuels, to add ever more greenhouse gases into the atmosphere, researchers expect to observe an increase in the volume of meltwater on the south polar surface. Researchers have predicted the melt rates could double by 2050. What isn't clear is whether this will make the shelf ice around the continent—and shelf ice slows the flow of glaciers from the polar hinterland—any less stable.
"This is not in the future—this is widespread now, and has been for decades," said Jonathan Kingslake, a glaciologist at Columbia University's Lamont-Doherty Earth Observatory, who led the research.
"I think most polar scientists have considered water moving across the surface of Antarctica to be extremely rare. But we found a lot of it, over very large areas."
The big question is: has the level of surface melting increased in the last seven decades? The researchers don't yet have enough information to make a judgment.
"We have no reason to think they have," Dr Kingslake said. "But without further work, we can't tell. Now, looking forward, it will be really important to work out how these systems will change in response to warming, and how this will affect the ice sheets."
Many of the flow systems seem to start in the Antarctic mountains, near outcrops of exposed rock, or in places where fierce winds have scoured snow off the ice beneath. Rocks are dark, the exposed ice is of a blue colour, and during the long days of the Antarctic summer both would absorb more solar energy than white snow or ice. This would be enough to start the melting process.
The Antarctic is already losing ice, as giant floating shelves suddenly fracture and drift north. There is a theory that meltwater could be part of the fissure mechanism, as it seeps deep into the shelves.
Drainage Theory
But the companion study, led by the polar scientist Robin Bell of the Lamont-Doherty Observatory suggests that drainage on the Nansen Ice Shelf might help to keep the ice intact, perhaps by draining away the meltwater in the dramatic waterfall the scientists had identified.
"It could develop this way in other places, or things could just devolve into giant slush puddles," she said. "Ice is dynamic, and complex, and we don't have the data yet."
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By Peter Giger
The speed and scale of the response to COVID-19 by governments, businesses and individuals seems to provide hope that we can react to the climate change crisis in a similarly decisive manner - but history tells us that humans do not react to slow-moving and distant threats.
A Game of Jenga
<p>Think of it as a game of Jenga and the planet's climate system as the tower. For generations, we have been slowly removing blocks. But at some point, we will remove a pivotal block, such as the collapse of one of the major global ocean circulation systems, for example the Atlantic Meridional Overturning Circulation (AMOC), that will cause all or part of the global climate system to fall into a planetary emergency.</p><p>But worse still, it could cause runaway damage: Where the tipping points form a domino-like cascade, where breaching one triggers breaches of others, creating an unstoppable shift to a radically and swiftly changing climate.</p><p>One of the most concerning tipping points is mass methane release. Methane can be found in deep freeze storage within permafrost and at the bottom of the deepest oceans in the form of methane hydrates. But rising sea and air temperatures are beginning to thaw these stores of methane.</p><p>This would release a powerful greenhouse gas into the atmosphere, 30-times more potent than carbon dioxide as a global warming agent. This would drastically increase temperatures and rush us towards the breach of other tipping points.</p><p>This could include the acceleration of ice thaw on all three of the globe's large, land-based ice sheets – Greenland, West Antarctica and the Wilkes Basin in East Antarctica. The potential collapse of the West Antarctic ice sheet is seen as a key tipping point, as its loss could eventually <a href="https://science.sciencemag.org/content/324/5929/901" target="_blank">raise global sea levels by 3.3 meters</a> with important regional variations.</p><p>More than that, we would be on the irreversible path to full land-ice melt, causing sea levels to rise by up to 30 meters, roughly at the rate of two meters per century, or maybe faster. Just look at the raised beaches around the world, at the last high stand of global sea level, at the end of the Pleistocene period around 120,0000 years ago, to see the evidence of such a warm world, which was just 2°C warmer than the present day.</p>Cutting Off Circulation
<p>As well as devastating low-lying and coastal areas around the world, melting polar ice could set off another tipping point: a disablement to the AMOC.</p><p>This circulation system drives a northward flow of warm, salty water on the upper layers of the ocean from the tropics to the northeast Atlantic region, and a southward flow of cold water deep in the ocean.</p><p>The ocean conveyor belt has a major effect on the climate, seasonal cycles and temperature in western and northern Europe. It means the region is warmer than other areas of similar latitude.</p><p>But melting ice from the Greenland ice sheet could threaten the AMOC system. It would dilute the salty sea water in the north Atlantic, making the water lighter and less able or unable to sink. This would slow the engine that drives this ocean circulation.</p><p><a href="https://www.carbonbrief.org/atlantic-conveyor-belt-has-slowed-15-per-cent-since-mid-twentieth-century" target="_blank">Recent research</a> suggests the AMOC has already weakened by around 15% since the middle of the 20th century. If this continues, it could have a major impact on the climate of the northern hemisphere, but particularly Europe. It may even lead to the <a href="https://ore.exeter.ac.uk/repository/handle/10871/39731?show=full" target="_blank" rel="noopener noreferrer">cessation of arable farming</a> in the UK, for instance.</p><p>It may also reduce rainfall over the Amazon basin, impact the monsoon systems in Asia and, by bringing warm waters into the Southern Ocean, further destabilize ice in Antarctica and accelerate global sea level rise.</p>The Atlantic Meridional Overturning Circulation has a major effect on the climate. Praetorius (2018)
Is it Time to Declare a Climate Emergency?
<p>At what stage, and at what rise in global temperatures, will these tipping points be reached? No one is entirely sure. It may take centuries, millennia or it could be imminent.</p><p>But as COVID-19 taught us, we need to prepare for the expected. We were aware of the risk of a pandemic. We also knew that we were not sufficiently prepared. But we didn't act in a meaningful manner. Thankfully, we have been able to fast-track the production of vaccines to combat COVID-19. But there is no vaccine for climate change once we have passed these tipping points.</p><p><a href="https://www.weforum.org/reports/the-global-risks-report-2021" target="_blank">We need to act now on our climate</a>. Act like these tipping points are imminent. And stop thinking of climate change as a slow-moving, long-term threat that enables us to kick the problem down the road and let future generations deal with it. We must take immediate action to reduce global warming and fulfill our commitments to the <a href="https://www.ipcc.ch/sr15/" target="_blank" rel="noopener noreferrer">Paris Agreement</a>, and build resilience with these tipping points in mind.</p><p>We need to plan now to mitigate greenhouse gas emissions, but we also need to plan for the impacts, such as the ability to feed everyone on the planet, develop plans to manage flood risk, as well as manage the social and geopolitical impacts of human migrations that will be a consequence of fight or flight decisions.</p><p>Breaching these tipping points would be cataclysmic and potentially far more devastating than COVID-19. Some may not enjoy hearing these messages, or consider them to be in the realm of science fiction. But if it injects a sense of urgency to make us respond to climate change like we have done to the pandemic, then we must talk more about what has happened before and will happen again.</p><p>Otherwise we will continue playing Jenga with our planet. And ultimately, there will only be one loser – us.</p>By John R. Platt
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