Last week, rivers froze over in Chicago when it got colder than at the North Pole. At the same time, temperatures hit 47℃ in Adelaide during the peak of a heatwave.
Such extreme and unpredictable weather is likely to get worse as ice sheets at both poles continue to melt.
Our research, published today, shows that the combined melting of the Greenland and Antarctic ice sheets is likely to affect the entire global climate system, triggering more variable weather and further melting. Our model predictions suggest that we will see more of the recent extreme weather, both hot and cold, with disruptive effects for agriculture, infrastructure, and human life itself.
We argue that global policy needs urgent review to prevent dangerous consequences.
Accelerated loss of ice
Even though the goal of the Paris Agreement is to keep warming below 2℃ (compared to pre-industrial levels), current government pledges commit us to surface warming of 3-4℃ by 2100. This would cause more melting in the polar regions.
Already, the loss of ice from ice sheets in Antarctica and Greenland, as well as mountain glaciers, is accelerating as a consequence of continued warming of the air and the ocean. With the predicted level of warming, a significant amount of meltwater from polar ice would enter the earth’s oceans.
We have used satellite measurements of recent changes in ice mass and have combined data from both polar regions for the first time. We found that, within a few decades, increased Antarctic melting would form a lens of freshwater on the ocean surface, allowing rising warmer water to spread out and potentially trigger further melting from below.
In the North Atlantic, the influx of meltwater would lead to a significant weakening of deep ocean circulation and affect coastal currents such as the Gulf Stream, which carries warm water from the tropics into the North Atlantic. This would lead to warmer air temperatures in Central America, Eastern Canada and the high Arctic, but colder conditions over northwestern Europe on the other side of the Atlantic.
Recent research suggests that tipping points in parts of the West Antarctic Ice Sheet may have already been passed. This is because most of the ice sheet that covers West Antarctica rests on bedrock far below sea level – in some areas up to 2 kilometres below.
Bringing both poles into one model
It can be a challenge to simulate the whole climate system because computer models of climate are usually global, but models of ice sheets are typically restricted to just Antarctica or just Greenland. For this reason, the most recent Intergovernmental Panel of Climate Change (IPCC) assessment used climate models that excluded ice sheet interactions.
Global government policy has been guided by this assessment since 2013, but our new results show that the inclusion of ice sheet meltwater can significantly affect climate projections. This means we need to update the guidance we provide to policy makers. And because Greenland and Antarctica affect different aspects of the climate system, we need new modelling approaches that look at both ice sheets together.
Seas rise as ice melts on land
Apart from the impact of meltwater on ocean circulation, we have also calculated how ongoing melting of both polar ice caps will contribute to sea level. Melting ice sheets are already raising sea level, and the process has been accelerating in recent years.
Our research is in agreement with another study published today, in terms of the amount that Antarctica might contribute to sea level over the present century. This is good news for two reasons.
First, our predictions are lower than one US modelling group predicted in 2016. Instead of nearly a metre of sea level rise from Antarctica by 2100, we predict only 14-15cm.
Second, the agreement between the two studies and also with previous projections from the IPCC and other modelling groups suggests there is a growing consensus, which provides greater certainty for planners. But the regional pattern of sea level rise is uneven, and islands in the southwest Pacific will most likely experience nearly 1.5 times the amount of sea level rise that will affect New Zealand.
While some countries, including New Zealand, are making progress on developing laws and policies for a transition towards a low-carbon future, globally policy is lagging far behind the science.
The predictions we make in our studies underline the increasingly urgent need to reduce greenhouse gas emissions. It might be hard to see how our own individual actions can save polar ice caps from significant melting. But by making individual choices that are environmentally sustainable, we can persuade politicians and companies of the desire for urgent action to protect the world for future generations.
Sea ice cover in Antarctica shrank rapidly to a record low in late 2016 and has remained well below average. But what’s behind this dramatic melting and low ice cover since?
What happened to Antarctic sea ice in 2016?
Antarctic sea ice is frozen seawater, usually less than a few metres thick. It differs from ice shelves, which are formed by glaciers, float in the sea, and are up to a kilometre thick.
Sea ice cover in Antarctica is crucial to the global climate and marine ecosystems and satellites have been monitoring it since the late 1970s. In contrast to the Arctic, sea ice around Antarctica had been slowly expanding (see figure below).
However, in late 2016 Antarctic sea ice dramatically and rapidly melted reaching a record low. This piqued the interest of climate scientists because such large, unexpected and rapid changes are rare. Sea ice coverage is still well below average now.
We wanted to know what caused this unprecedented decline of Antarctic sea ice and what changes in the system have sustained those declines. We also wanted to know if this was a temporary shift or the beginning of a longer-term decline, as predicted by climate models. Finally, we wanted to know whether human-induced climate change contributed to these record lows.
Hunting for clues
Sea ice cover around Antarctica varies a lot from one year or decade to the next. In fact, Antarctic sea ice cover had reached a record high as recently as 2014.
That provided a clue. As year-to-year and decade-to-decade sea ice cover varies so much, this can mask longer-term melting of sea ice due to anthropogenic warming.
The next clue was in records broken far away from Antarctica. In the spring of 2016 sea surface temperatures and rainfall in the tropical eastern Indian Ocean were at record highs. This was in association with a strongly negative Indian Ocean Dipole (IOD) event, which brought warmer waters to the northwest of Australia.
While IOD events influence rainfall in south-eastern Australia, we found (using both statistical analysis and climate model experiments) that it promoted a pattern in the winds over the Southern Ocean that was particularly conducive to decreasing sea ice.
These surface winds blowing from the north not only pushed the sea ice back towards the Antarctic continent, they were also warmer, helping to melt the sea ice.
These northerly winds almost perfectly matched the main regions where sea ice declined.
Though previous studies had linked this wind pattern to the sea ice decline, our studies are the first to argue for the dominant role of the tropical eastern Indian Ocean in driving it.
But this wasn’t the only factor.
Later in 2016 the typical westerly winds that surround Antarctica weakened to record lows. This caused the ocean surface to warm up, promoting less sea ice cover.
The weaker winds started at the top of the atmosphere over Antarctica, in the region known as the stratospheric polar vortex. We think this sequential occurrence of tropical and then stratospheric influences contributed to the record declines in 2016.
Taken together, the evidence we present supports the idea that the rapid Antarctic sea ice decline in late 2016 was largely due to natural climate variability.
The current state of Antarctic sea ice
Since then, sea ice has remained mostly well below average in association with warmer upper ocean temperatures around Antarctica.
We argue these are the product of stronger than normal westerly winds in the previous 15 or so years around Antarctica, driven again from the tropics. These stronger westerlies induced a response in the ocean, with warmer subsurface water moving towards the surface over time.
The combination of record tropical sea surface temperatures and weakened westerly winds in 2016 warmed the entire upper 600m of water in most regions of the Southern Ocean around Antarctica. These warmer ocean temperatures have maintained the reduced extent of sea ice.
Antarctic sea ice extent is seeing a record low start to the New Year. It suggests the initial rapid decline seen in late 2016 was not an isolated event and, when combined with the decadal-timescale warming of the upper Southern Ocean, could mean reduced sea ice extent for some time.
We argue what we are seeing so far can be understood in terms of natural variability superimposed on a long-term human-induced warming signal.
This is because the rainfall and ocean temperature records seen in the tropical eastern Indian Ocean that led to the initial sea ice decline in 2016 likely have some climate change contribution.
This warming and the recovery of the Antarctic ozone hole may also impact the surface wind patterns over coming decades.
Such changes could be driving climate change effects that are starting to emerge in the Antarctic region. However the limited data record and large variability indicate it’s still too early to tell.
We would like to acknowledge the role of our co-authors S Abhik, Cecilia M Bitz, Christine TY Chung, Alice DuVivier, Harry H Hendon, Marika M Holland, Eun-Pa Lim, LuAnne Thompson, Peter van Rensch and Dongxia Yang in contributing to the research discussed in this article.
Julie Arblaster, Associate Professor, Monash University; Gerald A Meehl, Senior scientist, National Center for Atmospheric Research , and Guomin Wang, Research scientist, Australian Bureau of Meteorology
The Antarctic Circumpolar Current, or ACC, is the strongest ocean current on our planet. It extends from the sea surface to the bottom of the ocean, and encircles Antarctica.
It is vital for Earth’s health because it keeps Antarctica cool and frozen. It is also changing as the world’s climate warms. Scientists like us are studying the current to find out how it might affect the future of Antarctica’s ice sheets, and the world’s sea levels.
The ACC carries an estimated 165 million to 182 million cubic metres of water every second (a unit also called a “Sverdrup”) from west to east, more than 100 times the flow of all the rivers on Earth. It provides the main connection between the Indian, Pacific and Atlantic Oceans.
The tightest geographical constriction through which the current flows is Drake Passage, where only 800 km separates South America from Antarctica. While elsewhere the ACC appears to have a broad domain, it must also navigate steep undersea mountains that constrain its path and steer it north and south across the Southern Ocean.
What is the Antarctic Circumpolar Current?
A satellite view over Antarctica reveals a frozen continent surrounded by icy waters. Moving northward, away from Antarctica, the water temperatures rise slowly at first and then rapidly across a sharp gradient. It is the ACC that maintains this boundary.
The ACC is created by the combined effects of strong westerly winds across the Southern Ocean, and the big change in surface temperatures between the Equator and the poles.
Ocean density increases as water gets colder and as it gets more salty. The warm, salty surface waters of the subtropics are much lighter than the cold, fresher waters close to Antarctica. We can imagine that the depth of constant density levels slopes up towards Antarctica.
The westerly winds make this slope steeper, and the ACC rides eastward along it, faster where the slope is steeper, and weaker where it’s flatter.
Fronts and bottom water
In the ACC there are sharp changes in water density known as fronts. The Subantarctic Front to the north and Polar Front further south are the two main fronts of the ACC (the black lines in the images). Both are known to split into two or three branches in some parts of the Southern Ocean, and merge together in other parts.
Scientists can figure out the density and speed of the current by measuring the ocean’s height, using altimeters. For instance, denser waters sit lower and lighter waters stand taller, and differences between the height of the sea surface give the speed of the current.
The path of the ACC is a meandering one, because of the steering effect of the sea floor, and also because of instabilities in the current.
The ACC also plays a part in the meridional (or global) overturning circulation, which brings deep waters formed in the North Atlantic southward into the Southern Ocean. Once there it becomes known as Circumpolar Deep Water, and is carried around Antarctica by the ACC. It slowly rises toward the surface south of the Polar Front.
Once it surfaces, some of the water flows northward again and sinks north of the Subarctic Front. The remaining part flows toward Antarctica where it is transformed into the densest water in the ocean, sinking to the sea floor and flowing northward in the abyss as Antarctic Bottom Water. These pathways are the main way that the oceans absorb heat and carbon dioxide and sequester it in the deep ocean.
The ACC is not immune to climate change. The Southern Ocean has warmed and freshened in the upper 2,000 m. Rapid warming and freshening has also been found in the Antarctic Bottom Water, the deepest layer of the ocean.
Waters south of the Polar Front are becoming fresher due to increased rainfall there, and waters to the north of the Polar Front are becoming saltier due to increased evaporation. These changes are caused by human activity, primarily through adding greenhouse gases to the atmosphere, and depletion of the ozone layer. The ozone hole is now recovering but greenhouse gases continue to rise globally.
Winds have strengthened by about 40% over the Southern Ocean over the past 40 years. Surprisingly, this has not translated into an increase in the strength of the ACC. Instead there has been an increase in eddies that move heat towards the pole, particularly in hotspots such as Drake Passage, Kerguelen Plateau, and between Tasmania and New Zealand.
We have observed much change already. The question now is how this increased transfer of heat across the ACC will impact the stability of the Antarctic ice sheet, and consequently the rate of global sea-level rise.
Helen Phillips, Senior Research Fellow, Institute for Marine and Antarctic Studies, University of Tasmania; Benoit Legresy, , CSIRO, and Nathan Bindoff, Professor of Physical Oceanography, Institute for Marine and Antarctic Studies, University of Tasmania
Antarctica is owned by no one, but there are plenty of countries interested in this frozen island continent at the bottom of the Earth.
While there are some regulations on who can do what there, scientific research has no definition in Antarctic law. So any research by a country conducted in or about Antarctica can be interpreted as legitimate Antarctic science.
There are 30 countries – including Australia – operating bases and ships, and flying aircraft to and from runways across the continent.
It is not surprising there is significant interest in who is doing what, where – especially if countries ramp up their investment in Antarctic infrastructure with new stations, ships or runways.
Their actions might raise eyebrows and fuel speculation. But the freedom of countries to behave autonomously is guided by the laws that apply to this sovereign-neutral continent.
Treaties and signatories
There are 12 original signatories to the 1959 Antarctic Treaty, including Australia, and they do not have to prove their commitment to the treaty since they wrote the rules.
Another 41 countries have signed on since 1959, and they do need to prove commitment.
Non-signatory countries, such as Iran or Indonesia, are freed from many of these legal obligations.
Until such time as the Antarctic Treaty has been designated customary international law applicable to all states by a high authority (such as the International Court of Justice), non-signatories can essentially do what they like in Antarctica.
The appliance of science
Autonomous freedom of activity by signatory countries is legitimised through the fact that science is the currency of credibility in Antarctica. This is important for two reasons:
- scientific research has legal priority
- new signatories can become decision-makers when they do science.
The “freedom of scientific investigation” is preserved in Article II of the Antarctic Treaty. It directs that signatories to the treaty can conduct scientific research of any kind anywhere in the Antarctic, without anybody else’s permission.
Further, the treaty outlines the process for new signatories (that is, other than the original 12) to achieve Consultative Party (decision-making) status.
Decisions are made by consensus (that is, everyone agrees or there is no formal objection). So every country’s “vote” counts and new countries aspire to gain a seat at the table to further their national agendas.
They become Consultative Parties by conducting “substantial scientific research activity” (Article IX.2) and when this has been accomplished to the satisfaction of the other decision-makers, they will be accepted.
Demonstrating interest in Antarctic science was initially interpreted as building a base or dispatching an expedition (Article IX.2). But after the adoption of the environmental protocol to the treaty in 1991, this was re-interpreted.
Parties were encouraged (but not legally bound) to consider piggy-backing on existing national scientific expeditions of other countries, and to share stations and other resources such as ships and aircraft where possible.
Currently there is only one jointly operated scientific base – Concordia, occupied by both France and Italy. The Novolazarevskaya airfield is a joint operation coordinated by Russia.
This encouragement was designed to reduce the potential for expansion of the footprint of human activities.
In 2017 the Consultative Parties adopted revised guidelines for how to become a decision maker. These outline new rules on a concept that has never been articulated publicly in an Antarctic forum before – evaluating the quality of scientific research.
This could put the brakes on the rapid addition of new signatories to the table.
There are limits
Although there is freedom to conduct science anywhere in Antarctica, what any country cannot do is lay claim to territory on the basis of its research efforts.
The treaty expressly excludes new claims or the extension of existing claims. Signatories that conduct research, and support those endeavours by building a base and infrastructure such as an airstrip, cannot use those actions as a basis of a claim while the treaty is in force.
Seven countries claim Antarctic territory: Argentina, Australia, Chile, France, New Zealand, Norway and the United Kingdom. Two others – the United States and the Russian Federation – have reserved their rights to claim any or all of Antarctica in the future.
These paper claims are acknowledged by Article IV of the treaty. But its artful craftsmanship prevents conflict over the claims and reservations during the life of the Treaty – which incidentally has neither an expiry nor a future review date.
Because the Article II freedoms permit research to be undertaken anywhere on the continent, the borders delineating claims become irrelevant to all but the claimant.
A party has an option of recognising a claim, or not, and does not need anyone’s permission to build a station or send an expedition. This means that the claimants have very limited capacity to exercise sovereignty in their territory. This effectively reduces their power to that of jurisdiction only over their own nationals.
The sting in the tail is that conducting substantial scientific research activity in Antarctica – including the building of support infrastructure – is the pathway new states must take to achieve decision-making status.
This is only constrained by the legal requirement to undertake an environmental impact assessment of any activity prior to its commencement.
Irrespective of whether the activity’s proponent complies with best practice environmental evaluation, under the rules, no other party can veto that activity.
Essentially, any country – whether a party to the treaty or not – can do whatever they like in Antarctica.
Ever since the ancient Greeks speculated a continent must exist in the south polar regions to balance those in the north, Antarctica has been popularly described as remote and extreme. Over the past two centuries, these factors have combined to create, in the human psyche, an almost mythical land – an idea reinforced by tales of heroism and adventure from the Edwardian golden age of “heroic exploration” and pioneers such as Robert Falcon Scott, Roald Amundsen and Ernest Shackleton.
Recent research, however, is casting new light on the importance of the southernmost continent, overturning centuries of misunderstanding and highlighting the role of Antarctica in how our planet works and the role it may play in a future, warmer world.
What was once thought to be a largely unchanging mass of snow and ice is anything but. Antarctica holds a staggering amount of water. The three ice sheets that cover the continent contain around 70% of our planet’s fresh water, all of which we now know to be vulnerable to warming air and oceans. If all the ice sheets were to melt, Antarctica would raise global sea levels by at least 56m.
Where, when, and how quickly they might melt is a major focus of research. No one is suggesting all the ice sheets will melt over the next century but, given their size, even small losses could have global repercussions. Possible scenarios are deeply concerning: in addition to rising sea levels, meltwater would slow down the world’s ocean circulation, while shifting wind belts may affect the climate in the southern hemisphere.
In 2014, NASA reported that several major Antarctic ice streams, which hold enough water to trigger the equivalent of a one-and-a-half metre sea level rise, are now irreversibly in retreat. With more than 150m people exposed to the threat of sea level rise and sea levels now rising at a faster rate globally than any time in the past 3,000 years, these are sobering statistics for island nations and coastal cities worldwide.
An immediate and acute threat
Recent storm surges following hurricanes have demonstrated that rising sea levels are a future threat for densely populated regions such as Florida and New York. Meanwhile the threat for low-lying islands in areas such as the Pacific is immediate and acute.
Multiple factors mean that the vulnerability to global sea level rise is geographically variable and unequal, while there are also regional differences in the extremity of sea level rise itself. At present, the consensus of the IPPC 2013 report suggests a rise of between 40 and 80cm over the next century, with Antarctica only contributing around 5cm of this. Recent projections, however, suggest that Antarctic contributions may be up to ten times higher.
Studies also suggest that in a world 1.5-2°C warmer than today we will be locked into millennia of irreversible sea level rise, due to the slow response time of the Antarctic ice sheets to atmospheric and ocean warming.
We may already be living in such a world. Recent evidence shows global temperatures are close to 1.5°C warmer than pre-industrial times and, after the COP23 meeting in Bonn in November, it is apparent that keeping temperature rise within 2°C is unlikely.
So we now need to reconsider future sea level projections given the potential global impact from Antarctica. Given that 93% of the heat from anthropogenic global warming has gone into the ocean, and these warming ocean waters are now meeting the floating margins of the Antarctic ice sheet, the potential for rapid ice sheet melt in a 2°C world is high.
In polar regions, surface temperatures are projected to rise twice as fast as the global average, due to a phenomenon known as polar amplification. However, there is still hope to avoid this sword of Damocles, as studies suggest that a major reduction in greenhouse gases over the next decade would mean that irreversible sea level rise could be avoided. It is therefore crucial to reduce CO₂ levels now for the benefit of future generations, or adapt to a world in which more of our shorelines are significantly redrawn.
This is both a scientific and societal issue. We have choices: technological innovations are providing new ways to reduce CO₂ emissions, and offer the reality of a low-carbon future. This may help minimise sea level rise from Antarctica and make mitigation a viable possibility.
Given what rising sea levels could mean for human societies across the world, we must maintain our longstanding view of Antarctica as the most remote and isolated continent.
Chris Fogwill, Professor of Glaciology and Palaeoclimatology, Keele University; Chris Turney, Professor of Earth Sciences and Climate Change, UNSW, and Zoe Robinson, Reader in Physical Geography and Sustainability/Director of Education for Sustainability, Keele University
The lush moss beds that grow near East Antarctica’s coast are among the only plants that can withstand life on the frozen continent. But our new research shows that these slow-growing plants are changing at a far faster rate than anticipated.
We began monitoring plant ecosystems 18 years ago, near Australia’s Casey Station in the Windmill Islands, East Antarctica.
As we report in Nature Climate Change today, within just 13 years we observed significant changes in the composition and health of these moss beds, due to the drying effects of weather changes prompted by damage to the ozone layer.
Living on the edge
Visitors to Antarctica expect to see a stark landscape of white and blue: ice, water, and sky. But in some places summer brings a surprisingly verdant green, as lush mosses emerge from under their winter snow blanket.
Because it contains the best moss beds on continental Antarctica, Casey Station is dubbed the Daintree of the Antarctic. Individual plants have been growing here for at least 100 years; fertilised by ancient penguin poo.
Antarctic mosses are extremophiles, the only plants that can survive the continent’s frigid winters. They live in a frozen desert where life-sustaining water is mostly locked up as ice, and they grow at a glacial pace – typically just 1 mm a year.
These mosses are home to tardigrades and other organisms, all of which survive harsh conditions by drying out and becoming dormant. When meltwater is available, mosses soak it up like a sponge and spring back to life.
The short summer growing season runs from December to March. Day temperatures finally rise above freezing, providing water from melting snow. Overnight temperatures drop below zero and mosses refreeze. Harsh, drying winds reach speeds of 200 km per hour. This is life on the edge.
When we first began monitoring the moss beds, they were dominated by Schistidium antarctici, a species found only in Antarctica. These areas were typically submerged through most of the summer, favouring the water-loving Schistidium. But as the area dries, two hardy, global species have encroached on Schistidium’s turf.
Like tree rings, mosses preserve a record of past climate in their shoots. From this we found nearly half of the mosses showed evidence of drying.
Healthy green moss has turned red or grey, indicating that plants are under stress and dying. This is due to the area drying because of colder summers and stronger winds. This increased desertification of East Antarctica is caused by both climate change and ozone depletion.
Since the 1970s, man-made substances have thinned Earth’s protective sunscreen, the ozone layer, creating a hole that appears directly over Antarctica during the southern spring (September–November). This has dramatically affected the southern hemisphere’s climate. Westerly winds have moved closer to Antarctica and strengthened, shielding much of continental East Antarctica from global warming.
Our study shows that these effects are contributing to drying of East Antarctica, which is in turn altering plant communities and affecting the health of some native plant species. East Antarctica’s mosses can be viewed as sentinels for a rapidly drying coastal climate.
But there is good news. The ozone layer is slowly recovering as pollutants are phased out thanks to the 1987 Montreal Protocol. What is likely to happen to Antarctic coastal climates when ozone levels recover fully by the middle of this century?
Antarctic ice-free areas are currently less than 1% of the continent but are predicted to expand over the coming century. Our research suggests that this may isolate moss beds from snow banks, which are their water reservoirs. Ironically, increased ice melt may be bad news for some Antarctic mosses.
East Antarctica is drying – first at the hands of ozone depletion, and then by climate change. How its native mosses fare in the future depends on how we control greenhouse gas emissions. But with decisive action and continued monitoring, we can hopefully preserve these fascinating ecosystems for the future.