Antarctica froze millions of years before the Arctic

How Antarctica freeze before the  Arctic?               

East Antarctica hosts the largest ice sheet on Earth, containing enough water to raise global sea levels by 52 metres, were it to fully melt. Yet it has puzzled scientists for decades how and why this ice sheet formed. Scientists may have solved the mystery of why Antarctica froze millions of years before the Arctic. Slow-moving waves deep inside Earth gradually lifted East Antarctica, building mountains and a high plateau where snow and ice could survive even in a much warmer world. Once glaciers took hold, their bright surfaces reflected sunlight and helped cool the region further. In fact, there are two interlinked mysteries. First, Antarctica became covered in ice around 34 million years ago, a period known as the Eocene-Oligocene transition, while the Arctic region stayed largely ice-free for another 25 million years or so. CO2 levels in the atmosphere were falling dramatically at the time, and played an important role in falling temperatures. But if that was the sole factor behind the transition, both poles should have cooled together. They didn’t. This means that something else was probably giving Antarctica a head start. The second mystery is that sea-surface temperatures in the Southern Ocean remained unexpectedly warm for 10 million years or so after the East Antarctic Ice Sheet formed. This is not what we’d expect to see if the ice sheet had formed purely in response to global cooling, in which case the surrounding oceans should have cooled considerably too. A new study points to an answer buried deep below the ice sheets: Antarctica’s mountains, and the slow motion geological forces which built them. Antarctic ice meets the rocky coastline. Researchers traced landscape features from the two-kilometer-high coastal escarpment of Dronning Maud Land to the subglacial Gamburtsev Mountains, buried beneath 1–3 km of ice. Some of the important factors are as follows:-

When Africa and Antarctica separated during the Jurassic period, it sent slow disturbances deep into Earth's mantle.

These underground waves stripped material from the base of the continental crust, making the land more buoyant and lifting East Antarctica.

The uplift raised the interior terrain, including the Gamburtsev Mountains, high enough for snow to survive year-round instead of melting each summer. Higher altitudes have much colder air temperatures, triggering the initial glaciation.

As glaciers and snow grew on the high peaks, their bright white surfaces reflected more sunlight, cooling the local and global environment further.

Colder air held less water vapor, reducing the greenhouse effect over the region and allowing temperatures to drop even lower until ice spread to the coast.

Northern Hemisphere landmasses lacked this high-altitude geological boost, meaning the Arctic required another 25 million years of dropping global CO2 levels before major ice sheets could form.

Scientists have found a new explanation for why Antarctica became covered in ice millions of years before the Arctic. The international study addresses a long-standing climate mystery: how Antarctica could develop a massive ice sheet at a time when Earth was around 5ºC warmer than it is today. The findings point to the gradual formation of high terrain in East Antarctica. As an escarpment, plateau and mountain system rose, it created elevations cold enough for snow and ice to persist and build up over time. The process began after Antarctica and Africa started separating during the Jurassic Period, 201-143 million years ago. Powerful forces deep inside Earth gradually lifted much of East Antarctica over more than 100 million years, helping set the stage for ice sheet formation about 34 million years ago. The study was led by researchers at the University of Southampton, working with scientists from Durham University, GFZ Helmholtz Centre for Geosciences in Germany, the University of Potsdam in Germany, Utrecht University in the Netherlands, and the University of Florence in Italy. Lead author Thomas Gernon, Professor of Earth Science at the University of Southampton, explained: "Antarctica's land surface was gradually lifted to the point where ice could gain a permanent foothold, even while the surrounding polar oceans as well as global temperatures remained surprisingly warm." The East Antarctic Ice Sheet is now the largest ice sheet on Earth. It contains enough frozen water to raise global sea levels by around 52 meters if it were to melt completely.

Story begins around 170 million years ago, when Antarctica and Africa were last joined together as part of the supercontinent Gondwana. Their split sent Antarctica on a trajectory toward the South Pole, and this massive rupture also set off a chain of events far below the surface.  Africa and Antarctica broke apart during the Jurassic period, around 170 million years ago. When continents break apart, hot material from Earth’s mantle wells up beneath them, cools and then sinks. This swirling motion destabilises the base of the neighbouring continent, triggering a series of lava lamp-like instabilities which remove chunks of its deep roots, one by one. These disturbances, called “mantle waves”, sweep below continents over millions of years, travelling more than 1,000 km's as they ripple through the hot, sticky rock beneath the landmass. In two Nature papers, multiple independent lines of evidence that all pointed to the same conclusion: mantle waves can trigger diamond-bearing volcanic eruptions, violent explosions that rocket magma from the deep roots of the continents, more than 150 km's below the surface. Those mantle waves can generate unexplained pulses of uplifting land far from the rift zones where the continent originally broke. Using computer models that simulate how landscapes evolve over tens of millions of years, we have now traced the effect these waves could have had in East Antarctica. Near the coast, the rifting formed a towering cliff-like feature, called an escarpment, more than two km's high.

Hundreds of km's inland, the mantle wave stripped away rock deep beneath the continent. Like a hot air balloon rising after dropping its ballast, the land above slowly lifted, creating a vast plateau and triggering a wave of erosion across the landscape. The uplift didn’t stop there. It kept migrating inland, taking roughly 100 million years to reach the Gamburtsev mountains, over 1,500km from the coast. This range is now buried under 3km or more of ice. How East Antarctica’s landscape changed over a period of 125 million years up to 34 million years ago, the point at which a continent-wide ice sheet first formed. Elevation matters enormously for ice. Air temperature drops by roughly 1°C for every 100 metres of elevation gained, so even a modest additional uplift can tip a mountain range from losing its snow each summer to keeping it year round. Until around 50 million years ago, most of the Gamburtsev mountains sat below 1.5km, too low for much snow to survive the summer. But our models show that from around this time, the wave of uplift reached the mountain region and pushed much of the range above 2km. At this elevation, snow and ice could persist and start building up. According to  calculations, by around 45 million years ago, enough of East Antarctica’s landscape had crossed this threshold for mountain glaciers to take hold and begin spreading. According to another strand of  analysis, the ice sheet started to form at precisely this time. By the point of continental glaciation, the global temperatures had fallen from a high of around 30°C 50 million years ago, to closer to 20°C. Once glaciers formed on the highlands, two feedback loops took over. First, ice and snow reflect far more sunlight than bare rock, so as the ice sheet grew, it cooled the surrounding region further. Our modelling suggests this alone lowered global temperatures by around 1°C.

Second, as the air over Antarctica cooled, it held less water vapour, which is a powerful greenhouse gas. Drier air meant a weaker insulating blanket over the region, allowing temperatures to fall further still. Together, these feedback loops let the ice sheet expand from its mountain strongholds down to the coast, eventually merging into the single ice sheet we see today. Crucially, the global cooling of roughly 1°C was not enough to freeze the Arctic, as northern hemisphere landmasses didn’t have the elevation to cross this threshold. It would take another 25 million years or so, and much lower CO₂ levels and global temperatures, before major ice sheets could build up there too. The temperature change that came from ice sheet formation was not enough to make temperatures plummet in polar oceans around Antarctica either, reconciling both mysteries surrounding the origin of its ice sheet. Mantle waves were recently identified by Prof Gernon's team. They travel beneath continents after tectonic plates begin to separate and have previously been linked to the eruption of diamond volcanoes and mysterious phases of uplift within continents. As these slow-moving waves passed beneath East Antarctica, they helped raise a vast plateau topped by the Gamburtsev Mountains. The recent discovery of mantle waves gave researchers a mechanism which could explain how Antarctica reached elevations high enough to begin freezing. Dr. Thea Hincks, Senior Research Fellow at the University of Southampton who co-led the study, said: "We found that our models can realistically capture the evolution of the two-kilometer-high coastal escarpment, elevated plateau and inland mountains, eventually seeding the East Antarctic Ice Sheet."

The results may also explain why the two polar regions followed very different paths. Antarctica became heavily glaciated around 34 million years ago, while large ice sheets in the Northern Hemisphere did not form until approximately the past five million years. Falling levels of carbon dioxide (CO2) in the atmosphere are widely considered an important trigger for Antarctic glaciation. However, the earliest Antarctic ice sheets began developing while the global climate was still relatively mild. Prof Gernon explained: "If falling levels of CO2 acted alone, you would expect the poles to respond more symmetrically. Instead, Antarctica gained a major head start because geological processes had raised land to higher elevations, making it colder." Even relatively modest changes in mountain elevation can determine whether snow disappears during summer or remains long enough to accumulate from one year to the next. Dr. Guy Paxman, Royal Society University Research Fellow at Durham University and study co-author, explained: "Topography is fundamentally important for glaciation. Air temperatures can drop by up to 1ºC for every 100 meters of altitude gained." Once the ice sheet began expanding, additional climate feedbacks strengthened the cooling.

Dr. Philip Goodwin, climate physicist at the University of Southampton and study co-author, added: "As the ice sheet expanded, its bright surface reflected more sunlight back into space, cooling the region further." The researchers estimate that this process, known as the 'ice-albedo effect', reduced global temperatures by about 1ºC. Even that additional cooling was not enough to create major ice sheets in the Northern Hemisphere. Arctic landmasses remained largely free of ice because they were generally at lower elevations. As Antarctica cooled, another feedback began to reinforce the temperature decline. Colder air can hold less water vapor, which normally acts like an insulating blanket around Earth. As the atmosphere became drier, that insulating effect weakened and temperatures dropped further. Together, these feedbacks allowed the Antarctic ice sheet to spread from the mountains across the continent, eventually reaching the coast," added Dr. Goodwin. The findings could reshape how scientists think about the origins of major ice ages. Rather than climate alone determining when ice sheets form, geological forces deep inside Earth may first have to prepare the landscape by raising it to elevations where permanent ice can survive.

These findings reveal that the Earth's interior preconditions landscapes to glaciation, determining when and where major climate transitions like the glaciation of Antarctica become possible," explained Prof Gernon. "That's incredibly important for understanding Earth's ancient ice ages as well as future tipping points in the climate system." This work shows how geology sets the stage for ice ages. The height of the land determines whether a given climate is cold enough to grow ice. This concept is important for other climate events in Earth’s past. If deep Earth processes can condition a landscape for ice long before the climate cools enough for ice sheets to form, they may too have contributed to earlier ice ages. Understanding the growth of past ice sheets can also give us clues about the future. This study shows that the conditions required for a continental ice sheet to form are extraordinarily specific, and took geological timescales to assemble. When ice sheets melt, however, they disappear much faster than they formed. And once lost, they cannot simply grow back. The research was made possible by the support of the WoodNext Foundation, a fund of a donor-advised fund program.