Science & technology | Out of sight, out of mind
科学与技术眼不见心不烦

Antarctica, Earth’s largest refrigerator, is defrosting
南极洲，地球上最大的冰箱，正在解冻

The world must pay more attention to its southern pole
世界必须更加关注其南极

Video: British Antarctic Survey
视频：英国南极调查局

Mar 27th 2024 27 3月 2024

The deadliest hurricane ever to hit America made landfall over Galveston, Texas, a barrier island in the Gulf of Mexico, on September 8th 1900. Terrified inhabitants watched a 4.5-metre-high wall of water approach their shores and tear through their homes. More than 8,000 people died. In the aftermath, a massive concrete seawall was built to keep future storm surges at bay. The engineers thought making it 5 metres tall would be enough.
1900 年 9 月 8 日，有史以来最致命的飓风袭击了美国，在德克萨斯州加尔维斯顿登陆，这是墨西哥湾的一个屏障岛。惊恐的居民眼睁睁地看着一堵4.5米高的水墙接近他们的海岸，撕裂了他们的家园。超过8,000人死亡。之后，建造了一条巨大的混凝土海堤，以防止未来的风暴潮。工程师们认为让它高5米就足够了。

And it was, for 120 years. But even as the wall was being built, human-caused climate change was getting under way, piling new heat and energy into the global climate system which would push the seas up shorelines around the world. Today, the US Army Corps of Engineers has a $57bn plan to build a new barrier, dubbed “Ike Dike”, to protect Galveston as well as the Houston region and large petrochemical facilities that sit behind the island from ever bigger and more powerful storm surges. It could be the largest civil-engineering project in American history. Where will the water that it is designed to hold back come from? Much of it will hail from Earth’s southernmost continent more than 10,000km away, home to the world’s largest ice mass by a factor of ten: Antarctica.
120 年来一直如此。但是，即使在修建隔离墙的同时，人为造成的气候变化也正在发生，将新的热量和能量堆积到全球气候系统中，这将推动世界各地的海岸线上升。今天，美国陆军工程兵团（US Army Corps of Engineers）计划耗资570亿美元建造一座名为“艾克堤坝”（Ike Dike）的新屏障，以保护加尔维斯顿以及休斯顿地区和位于该岛后面的大型石化设施免受更大、更强大的风暴潮的影响。这可能是美国历史上最大的土木工程项目。它被设计用来阻挡的水从哪里来？其中大部分将来自地球最南端的大陆，距离地球超过10,000公里，那里拥有世界上最大的冰块：南极洲。

When it comes to polar climate impacts, the Arctic receives most of the world’s attention. For years, scientists have warned of dramatic changes there being the canary in the climate coal mine. The blanket of summer ice that has persisted on the northernmost ocean for millennia steadily dwindled to almost nothing but it wasn’t until Siberia, land of permanently frozen soil, caught fire in 2019 that anyone outside the (alarmed) climate community and (delighted) shipping industry paid much attention.
在极地气候影响方面，北极受到世界大部分关注。多年来，科学家们一直警告说，气候煤矿中的金丝雀将发生巨大变化。在最北端的海洋上持续了数千年的夏季冰层稳步减少到几乎为零，但直到 2019 年永久冻土之地西伯利亚着火，（惊慌失措的）气候社区和（高兴的）航运业之外的任何人都非常关注。

The southern pole, by contrast, has been neglected in the climate narrative. That is due in part to its remoteness and in part to an early scientific miscalculation. While the Arctic was melting rapidly, Antarctica looked relatively stable. Not only that, climate models suggested it would see more snowfall in a warming world, causing its ice pack to grow, not shrink.
相比之下，南极在气候叙事中被忽视了。这部分是由于它的遥远性，部分是由于早期的科学误判。当北极迅速融化时，南极洲看起来相对稳定。不仅如此，气候模型还表明，在变暖的世界中，它将看到更多的降雪，导致其冰层扩大，而不是缩小。

The models, it turns out, were wrong. A build-up of jaw-dropping events and extremes in recent years has shown that Antarctica is undergoing massive changes on land, sea and in the atmosphere above. As a result, a new portrait of the continent is emerging which has, so far, received little attention. Polar scientists are warning of a regime shift.
事实证明，这些模型是错误的。近年来，令人瞠目结舌的事件和极端事件的积累表明，南极洲的陆地、海洋和大气层正在发生巨大变化。因此，非洲大陆的新图景正在出现，但迄今为止，这种图景很少受到关注。极地科学家警告说，政权将发生更迭。

Terra incognita

This matters: despite appearances, Antarctica is not isolated from the rest of the world. For millennia, the continent has acted like a massive refrigerator for Earth. It sits at the centre of a vast ocean, which is full of krill and fish and is a huge sink for carbon dioxide. Its ice sheets store the largest volume of water on Earth, keeping it out of the seas. As that ice is reduced to liquid, the water will disproportionately raise sea levels in the northern hemisphere, where most humans live.
这很重要：尽管看起来南极洲与世界其他地区并不孤立。几千年来，这片大陆就像地球的一台巨大的冰箱。它坐落在一片广阔的海洋的中心，那里到处都是磷虾和鱼类，是一个巨大的二氧化碳汇。它的冰盖储存了地球上最大的水量，使其远离海洋。当冰被还原成液态时，水将不成比例地提高大多数人居住的北半球的海平面。

“When I first started working in Antarctic glaciology [30 years ago], it was about adventure and discovery,” says Martin Siegert, director of the Grantham Institute for Climate Change at Imperial College London. “It still is about those things but it has got really serious really quickly.”
“当我（30年前）第一次开始从事南极冰川学工作时，它是关于冒险和发现的，”伦敦帝国理工学院格兰瑟姆气候变化研究所所长马丁·西格特说。“它仍然是关于这些事情的，但它很快就变得非常严重。

Alarm bells rang loud and clear in the second half of 2023. They began with a second consecutive summer where the expanse of sea ice floating around the continent hit an all-time low—10% lower than it was in 2022, itself a record-setter. Then, from May onwards and as the region headed into its winter, researchers watched with increasing alarm as the ice struggled to recover. By July, the extent of sea ice was 2.5m square kilometres smaller than the recent average (see chart 1). Ella Gilbert and Caroline Holmes, researchers with the British Antarctic Survey, wondered: “Antarctica is missing a chunk of sea ice bigger than Greenland—what’s going on?”
2023年下半年，警钟敲响。他们从连续第二个夏天开始，漂浮在非洲大陆周围的海冰创下了历史新低——比 2022 年低 10%，这本身就是一个记录。然后，从5月开始，随着该地区进入冬季，研究人员越来越警惕地看到冰层难以恢复。到7月，海冰的面积比最近的平均水平小了2.5m平方公里（见图1）。英国南极调查局的研究人员艾拉·吉尔伯特（Ella Gilbert）和卡罗琳·霍姆斯（Caroline Holmes）想知道：“南极洲缺少一大块比格陵兰岛还大的海冰——这是怎么回事？

On thin ice 如履薄冰

What is going on could be a long-term shift in the state of Antarctic ice. In an analysis published in the Journal of Climate earlier this month, Will Hobbs of the University of Tasmania in Australia, and colleagues, looked at 44 years of satellite records and found evidence of a regime shift in the seasonal waxing and waning patterns of the sea ice, starting 15 years ago. Since then the ice has been characterised by greater swings and variability. In a related analysis, Ariaan Purich at Monash University in Australia and Dr Hobbs’s co-author Edward Doddridge, also at the University of Tasmania, showed that on average the extent of sea ice briefly increased around 2007 before abruptly entering a “new low-extent state” in 2016. Data for the Southern Ocean suggests that warmer water due to climate change could be partially to blame. The last two years of remarkable sea-ice lows accentuate the pattern.
正在发生的事情可能是南极冰层状态的长期变化。在本月早些时候发表在《气候杂志》上的一项分析中，澳大利亚塔斯马尼亚大学的威尔·霍布斯（Will Hobbs）及其同事研究了44年的卫星记录，并发现了从15年前开始的海冰季节性起伏模式发生政权转变的证据。从那时起，冰的特点是更大的波动和变化。在一项相关分析中，澳大利亚莫纳什大学的Ariaan Purich和Hobbs博士的合著者Edward Doddridge也来自塔斯马尼亚大学，他们表明，平均而言，海冰的范围在2007年左右短暂增加，然后在2016年突然进入“新的低范围状态”。南大洋的数据表明，气候变化导致的海水变暖可能是部分原因。过去两年显著的海冰低点加剧了这种模式。

“If Antarctica is starting to behave like the Arctic and losing sea ice, that is a major concern,” says Dr Siegert. It would suggest a profound shift for a region that has for millennia helped to keep the rest of Earth cooler than it would otherwise be.

Less sea ice means more water is exposed to the sky. Because water is darker than ice, it absorbs more heat. This accelerates warming, a phenomenon known as the ice-albedo feedback. In the Arctic, this positive feedback loop is well-established and explains why the region is warming up four times faster than the global average. Recently, Antarctica has begun warming at twice the global average, a sign that it is also starting to become a driver of global warming rather than acting as a buffer. “We are deeply, deeply concerned about that,” says Dr Siegert.

In an effort to raise the alarm, Dr Siegert and 13 other polar experts have compiled a list of anomalous events that have been observed both on the Antarctic continent and in the surrounding Southern Ocean, from sea-ice lows to marine heatwaves (19 between 2002 and 2018), and record numbers of melt ponds on top of the continental ice sheets. The most extreme heatwave ever recorded on Earth, for example, happened in Antarctica in March 2022. Temperatures rose by 30 to 40°C above the norm for that time of year across large parts of East Antarctica. The Conger ice shelf, spanning an area 1.5 times the size of New York City and already in a critical state, collapsed within days.

Scientists are still trying to work out how these gobsmacking extremes are related to the continual fluctuations they see in the oceans and atmosphere, and the extent to which climate change is the ultimate culprit for each event or trend. Yet there have been signs of long-term climate-related change in Antarctica since about the turn of the 21st century, when several massive ice shelves collapsed.

Ice shelves are floating slabs of ice that form where glaciers on land flow out to sea. In Antarctica there are 15 large ones, each of which hugs a different part of the coastline. In January 1995, the Larsen A ice shelf, which covered 1,500 square kilometres, disintegrated. Seven years later, dramatic satellite images showed the neighbouring Larsen B ice shelf splinter in a matter of weeks. Some 3,250 square kilometres of ice were lost, an area the size of Rhode Island.

Despite the scale of these events—and studies that had warned, decades earlier, that this part of Antarctica was particularly vulnerable to warming and had helped push up sea levels by several metres during hotter times 120,000 years ago—the Larsen A and B collapses did not raise urgent worries about a long-term shift in Antarctic climate. At the time, computer models were not so good at simulating how polar ice would respond to a warming world. In December 1995, a year after Larsen A collapsed, the Intergovernmental Panel on Climate Change (IPCC) declared in its second major assessment of climate change: “Little change in the extent of the Greenland and Antarctic ice sheets is expected over the next 50–100 years.”

The intervening years have revealed just how wrong that prediction was. Antarctica’s ice sheets are nearly 1.5 times as vast as the United States and so thick (up to 5km) that their smooth surface hides a mountain range as large and nearly as high as the European Alps. They hold 90% of the world’s land ice. A 2019 report of the IPCC noted that, contrary to its earlier assessment, the rate at which they were losing mass had rapidly increased since 2002 (see chart 2). Water and ice flowing out of Greenland’s and Antarctica’s ice sheets will soon be the biggest driver of sea level rise, most of which was previously caused by seawater warming up and therefore expanding and taking up more room.

Several factors are to blame. Collapsing ice shelves, made more fragile from below by warming waters and jostled by more turbulent seas, have hurried things along. Ice sheets, which sit on the Antarctic continent, are connected to the ocean by glaciers that slowly flow towards the water. Ice shelves, which float at the edge of the continent, act like corks that buttress the glaciers behind them. Break the seal and the glaciers flow more quickly. Breakdown at the continent’s edges also allows warmer ocean waters to creep up under the ice margins, seeping into cracks in the ice and between the ice and the bedrock. “That’s a problem because when the oceans get involved, they can melt the ice ten times more quickly than atmospheric heat can,” says Dr Siegert.

The greatest and most immediate concern is for the West Antarctic ice sheet, which is losing ice much faster than its East Antarctic counterpart (see map). The good news is it holds less than a tenth of the water sitting on East Antarctica. The bad news is that despite appearances it does not sit on solid land but on a frozen landscape made up of islands separated by deep basins. As the ice margins move back towards the continent, ocean water is able to flow into this frozen landscape, exposing more and more of the ice’s underbelly to melting from below.

All the ice we cannot see

West Antarctica’s Thwaites glacier, which is the size of Great Britain, currently drives around 4% of global annual sea-level rise. It is melting rapidly, retreating hundreds of metres per year (new data published in February shows that this began, unnoticed, in the 1940s). Its upstream bedrock dips downwards towards the middle of the West Antarctic ice sheet, bottoming out around 2.5km below sea level. As a result Thwaites acts as a gateway that can either hold in or release a much larger mass of ice. “If Thwaites goes it’s pretty much the whole of the West Antarctic ice sheet that follows,” says Dr Siegert. Perhaps appropriately, it has been given the moniker “doomsday glacier”.

The most obvious impacts of all this melting ice, beyond the Antarctic Circle, will be on global sea levels. Antarctica’s ice sheets hold enough water to raise them by a staggering 60 metres, though such a catastrophic rise would take many centuries to occur. Much smaller changes, however, could still have a sizeable impact on coastal regions around the world.

The extra water from melting ice will not be distributed equally. Ice slipping off land and into the oceans decreases the gravitational pull of that landmass. Each mass of ice on Earth has its own gravitational fingerprint and its own associated geographical pattern of sea-level rise. Ice lost from Greenland’s ice sheet, for example, decreases sea levels on the nearby shores of north-western Europe and eastern Canada, and raises them around South America. Losses of ice from West Antarctica push water up around the coastlines of North America, Australia and Oceania.

Greenland has been losing ice since the early 2000s, but losses from West Antarctica have been increasing since the early 2010s and are likely to continue on this path. This means that future sea level rise will have disproportionate consequences for the northern hemisphere (including the Houston area).

The consequences of defrosting Antarctica extend beyond rising seas. The Southern Ocean that surrounds it is the largest sink for carbon dioxide and heat on the planet, soaking up roughly 12% of annual carbon-dioxide emissions from human sources. Large-scale currents around Antarctica redistribute carbon, heat and nutrients to the rest of the world’s oceans. The Southern Ocean’s value as a carbon sink was recently estimated by Natalie Stoeckl, an economist at the University of Tasmania, to be between $28bn and $720bn per year depending on the assumed cost of carbon, with a best estimate of $180bn, in the current climate. How the rate of carbon draw-down will change as the climate warms up even further is not yet known.

Already, though, there are signs that these ocean currents, known as the Antarctic overturning circulation, are slowing as more fresh water from melting ice flows off Antarctica. Fresh water is less dense than salty ocean water, so it floats at the surface. That puts the brakes on an underwater “waterfall” in which trillions of tonnes of water, rich in nutrients and gases, tumbles into the ocean’s depths when it reaches Antarctica’s icy shores.

Research led by Matthew England at the University of New South Wales in Sydney, Australia, showed last year that the Antarctic overturning circulation has slowed by 30% in 30 years, suggesting the vital carbon dioxide sink is being disrupted. The change is likely to have knock-on effects for fisheries north of the Antarctic circle, where a large share of wildlife relies on nutrients transported there from Antarctica’s coastline. Populations of Antarctic krill, which is turned into food for people or feed for aquaculture, will probably also be affected.

Matt King, director of the Australian Centre for Excellence in Antarctic Science, recently warned that one of Antarctica’s nearest neighbours, Australia, is woefully unprepared for the consequences that Antarctica’s changing regime could have on its economy. Studies suggest what happens at the southern pole could be linked to Australia’s weather patterns, temperature and rainfall. “Some of the studies are partially contradictory,” notes Dr King. That only emphasises how much work remains to be done to truly understand what lies ahead.

And that is the crux of it. For too long, too few have worried about Earth’s remotest continent and its largest refrigerator, while attention focused on the fires that burned, literally and figuratively, on more populated shores. Antarctica was variously portrayed as an explorer’s playground, a utopian land of scientific co-operation or a poster child for environmental protectionism. The exceptionalism is misguided, as the people of Galveston are finding out.■

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Antarctica, Earth’s largest refrigerator, is defrosting南极洲，地球上最大的冰箱，正在解冻

The world must pay more attention to its southern pole世界必须更加关注其南极

Terra incognita

On thin ice 如履薄冰

All the ice we cannot see

Antarctica, Earth’s largest refrigerator, is defrosting
南极洲，地球上最大的冰箱，正在解冻

The world must pay more attention to its southern pole
世界必须更加关注其南极