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By: Matthew England, Scientia Professor and Deputy Director of the ARC Australian Centre for Excellence in Antarctic Science (ACEAS), UNSW Sydney; Adele Morrison Research Fellow, Australian National University; Andy Hogg Professor, Australian National University; Qian Li Massachusetts Institute of Technology (MIT); Steve Rintoul CSIRO Fellow, CSIRO.

 Republished in full from The Conversation under a Creative Commons license. See the original article: March 3, 2023

Off the coast of Antarctica, trillions of tonnes of cold salty water sink to great depths. As the water sinks, it drives the deepest flows of the “overturning” circulation – a network of strong currents spanning the world’s oceans. The overturning circulation carries heat, carbon, oxygen and nutrients around the globe, and fundamentally influences climate, sea level and the productivity of marine ecosystems.

But there are worrying signs these currents are slowing down. They may even collapse. If this happens, it would deprive the deep ocean of oxygen, limit the return of nutrients back to the sea surface, and potentially cause further melt back of ice as water near the ice shelves warms in response. There would be major global ramifications for ocean ecosystems, climate, and sea-level rise.

Conveyer overturning schematic showing the pathways of flow in the upper, deep and bottom layers of the ocean.

Our new research, published today in the journal Nature, uses new ocean model projections to look at changes in the deep ocean out to the year 2050. Our projections show a slowing of the Antarctic overturning circulation and deep ocean warming over the next few decades. Physical measurements confirm these changes are already well underway.

Climate change is to blame. As Antarctica melts, more freshwater flows into the oceans. This disrupts the sinking of cold, salty, oxygen-rich water to the bottom of the ocean. From there this water normally spreads northwards to ventilate the far reaches of the deep Indian, Pacific and Atlantic Oceans. But that could all come to an end soon. In our lifetimes.

The authors explain the results of their landmark paper: Is the Southern Ocean about to have its own ‘Day After Tomorrow’ moment? Fact-checked by experts, the video explains how these changes would profoundly alter the ocean’s overturning of heat, freshwater, oxygen, carbon and nutrients, with impacts felt throughout the global ocean for centuries to come.

Why does this matter?

As part of this overturning, about 250 trillion tonnes of icy cold Antarctic surface water sinks to the ocean abyss each year. The sinking near Antarctica is balanced by upwelling at other latitudes. The resulting overturning circulation carries oxygen to the deep ocean and eventually returns nutrients to the sea surface, where they are available to support marine life.

If the Antarctic overturning slows down, nutrient-rich seawater will build up on the seafloor, five kilometres below the surface. These nutrients will be lost to marine ecosystems at or near the surface, damaging fisheries.

Changes in the overturning circulation could also mean more heat gets to the ice, particularly around West Antarctica, the area with the greatest rate of ice mass loss over the past few decades. This would accelerate global sea-level rise.

An overturning slowdown would also reduce the ocean’s ability to take up carbon dioxide, leaving more greenhouse gas emissions in the atmosphere. And more greenhouse gases means more warming, making matters worse.

Meltwater-induced weakening of the Antarctic overturning circulation could also shift tropical rainfall bands around a thousand kilometres to the north.

Put simply, a slowing or collapse of the overturning circulation would change our climate and marine environment in profound and potentially irreversible ways.

Signs of worrying change

The remote reaches of the oceans that surround Antarctica are some of the toughest regions to plan and undertake field campaigns. Voyages are long, weather can be brutal, and sea ice limits access for much of the year.

This means there are few measurements to track how the Antarctic margin is changing. But where sufficient data exist, we can see clear signs of increased transport of warm waters toward Antarctica, which in turn causes ice melt at key locations.

Indeed, the signs of melting around the edges of Antarctica are very clear, with increasingly large volumes of freshwater flowing into the ocean and making nearby waters less salty and therefore less dense. And that’s all that’s needed to slow the overturning circulation. Denser water sinks, lighter water does not.

How did we find this out?

Apart from sparse measurements, incomplete models have limited our understanding of ocean circulation around Antarctica.

For example, the latest set of global coupled model projections analysed by the Intergovernmental Panel on Climate Change exhibit biases in the region. This limits the ability of these models in projecting the future fate of the Antarctic overturning circulation.

To explore future changes, we took a high resolution global ocean model that realistically represents the formation and sinking of dense water near Antarctica.

We ran three different experiments, one where conditions remained unchanged from the 1990s; a second forced by projected changes in temperature and wind; and a third run also including projected changes in meltwater from Antarctica and Greenland.

In this way we could separate the effects of changes in winds and warming, from changes due to ice melt.

The findings were striking. The model projects the overturning circulation around Antarctica will slow by more than 40% over the next three decades, driven almost entirely by pulses of meltwater.

Abyssal ocean warming driven by Antarctic overturning slowdown, Credit: Matthew England and Qian Li.

Over the same period, our modelling also predicts a 20% weakening of the famous North Atlantic overturning circulation which keeps Europe’s climate mild. Both changes would dramatically reduce the renewal and overturning of the ocean interior.

We’ve long known the North Atlantic overturning currents are vulnerable, with observations suggesting a slowdown is already well underway, and projections of a tipping point coming soon. Our results suggest Antarctica looks poised to match its northern hemisphere counterpart – and then some.

What next?

Much of the abyssal ocean has warmed in recent decades, with the most rapid trends detected near Antarctica, in a pattern very similar to our model simulations.

Our projections extend out only to 2050. Beyond 2050, in the absence of strong emissions reductions, the climate will continue to warm and the ice sheets will continue to melt. If so, we anticipate the Southern Ocean overturning will continue to slow to the end of the century and beyond.

The projected slowdown of Antarctic overturning is a direct response to input of freshwater from melting ice. Meltwater flows are directly linked to how much the planet warms, which in turn depends on the greenhouse gases we emit.

Our study shows continuing ice melt will not only raise sea-levels, but also change the massive overturning circulation currents which can drive further ice melt and hence more sea level rise, and damage climate and ecosystems worldwide. It’s yet another reason to address the
climate crisis – and fast.

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By: Professor of Energy and Climate Change, University of Manchester. Republished in full from The Conversation under a Creative Commons license. See the original article: March 25, 2023

The Intergovernmental Panel on Climate Change’s (IPCC) synthesis report recently landed with an authoritative thump, giving voice to hundreds of scientists endeavouring to understand the unfolding calamity of global heating. What’s changed since the last one in 2014? Well, we’ve dumped an additional third of a trillion tonnes of CO₂ into the atmosphere, primarily from burning fossil fuels. While world leaders promised to cut global emissions, they have presided over a 5% rise.

The new report evokes a mild sense of urgency, calling on governments to mobilise finance to accelerate the uptake of green technology. But its conclusions are far removed from a direct interpretation of the IPCC’s own carbon budgets (the total amount of CO₂ scientists estimate can be put into the atmosphere for a given temperature rise).

The report claims that, to maintain a 50:50 chance of warming not exceeding 1.5°C above pre-industrial levels, CO₂ emissions must be cut to “net-zero” by the “early 2050s”. Yet, updating the IPCC’s estimate of the 1.5°C carbon budget, from 2020 to 2023, and then drawing a straight line down from today’s total emissions to the point where all carbon emissions must cease, and without exceeding this budget, gives a zero CO₂ date of 2040.

Graph with lines
 A full description of the above chart is available here.
 
If emissions stay at their current levels, we will exhaust the 50% chance of 1.5°C in 9 years. If we begin to immediately cut emissions following the blue line, then to stay within the carbon budget for 50:50 chance of not exceeding 1.5°C we need zero global emissions by 2040. The vertical axis represents how much carbon is emitted each year – note the pandemic-related blip in 2020.
 

Given it will take a few years to organise the necessary political structures and technical deployment, the date for eliminating all CO₂ emissions to remain within 1.5°C of warming comes closer still, to around the mid-2030s. This is a strikingly different level of urgency to that evoked by the IPCC’s “early 2050s”. Similar smoke and mirrors lie behind the “early 2070s” timeline the IPCC conjures for limiting global heating to 2°C.

IPCC science embeds colonial attitudes

For over two decades, the IPCC’s work on cutting emissions (what experts call “mitigation”) has been dominated by a particular group of modellers who use huge computer models to simulate what may happen to emissions under different assumptions, primarily related to price and technology. I’ve raised concerns before about how this select cadre, almost entirely based in wealthy, high-emitting nations, has undermined the necessary scale of emission reductions.

In 2023, I can no longer tiptoe around the sensibilities of those overseeing this bias. In my view, they have been as damaging to the agenda of cutting emissions as Exxon was in misleading the public about climate science. The IPCC’s mitigation report in 2022 did include a chapter on “demand, services and social aspects” as a repository for alternative voices, but these were reduced to an inaudible whisper in the latest report’s influential summary for policymakers.

The specialist modelling groups (referred to as Integrated Assessment Modelling, or IAMs) have successfully crowded out competing voices, reducing the task of mitigation to price-induced shifts in technology – some of the most important of which, like so-called “negative emissions technologies”, are barely out of the laboratory.

The IPCC offers many “scenarios” of future low-carbon energy systems and how we might get there from here. But as the work of academic Tejal Kanitkar and others has made clear, not only do these scenarios prefer speculative technology tomorrow over deeply challenging policies today (effectively a greenwashed business-as-usual), they also systematically embed colonial attitudes towards “developing nations”.

With few if any exceptions, they maintain current levels of inequality between developed and developing nations, with several scenarios actually increasing the levels of inequality. Granted, many IAM modellers strive to work objectively, but they do so within deeply subjective boundaries established and preserved by those leading such groups.

Four people in formal attire converse on a stage behind a
March 2023’s synthesis report capped eight years of research. IISD/ENB/Anastasia Rodopoulou

 

What happened to equity?

If we step outside the rarefied realm of IAM scenarios that leading climate scientist Johan Rockström describes as “academic gymnastics that have nothing to do with reality”, it’s clear that not exceeding 1.5°C or 2°C will require fundamental changes to most facets of modern life.

Starting now, to not exceed 1.5°C of warming requires 11% year-on-year cuts in emissions, falling to nearer 5% for 2°C. However, these global average rates ignore the core concept of equity, central to all UN climate negotiations, which gives “developing country parties” a little longer to decarbonise.

Include equity and most “developed” nations need to reach zero CO₂ emissions between 2030 and 2035, with developing nations following suit up to a decade later. Any delay will shrink these timelines still further.

Most IAM models ignore and often even exacerbate the obscene inequality in energy use and emissions, both within nations and between individuals. As the International Energy Agency recently reported, the top 10% of emitters accounted for nearly half of global CO₂ emissions from energy use in 2021, compared with 0.2% for the bottom 10%. More disturbingly, the greenhouse gas emissions of the top 1% are 1.5 times those of the bottom half of the world’s population.

So where does this leave us? In wealthier nations, any hope of arresting global heating at 1.5 or 2°C demands a technical revolution on the scale of the post-war Marshall Plan. Rather than relying on technologies such as direct air capture of CO₂ to mature in the near future, countries like the UK must rapidly deploy tried-and-tested technologies.

Retrofit housing stock, shift from mass ownership of combustion-engine cars to expanded zero-carbon public transport, electrify industries, build new homes to Passivhaus standard, roll-out a zero-carbon energy supply and, crucially, phase out fossil fuel production.

Three decades of complacency has meant technology on its own cannot now cut emissions fast enough. A second, accompanying phase, must be the rapid reduction of energy and material consumption.

Given deep inequalities, this, and deploying zero-carbon infrastructure, is only possible by re-allocating society’s productive capacity away from enabling the private luxury of a few and austerity for everyone else, and towards wider public prosperity and private sufficiency.

For most people, tackling climate change will bring multiple benefits, from affordable housing to secure employment. But for those few of us who have disproportionately benefited from the status quo, it means a profound reduction in how much energy we use and stuff we accumulate.

The question now is, will we high-consuming few make (voluntarily or by force) the fundamental changes needed for decarbonisation in a timely and organised manner? Or will we fight to maintain our privileges and let the rapidly changing climate do it, chaotically and brutally, for us?

Sea levels the last time CO2 in the atmosphere as at 400ppm. We are currently at 415ppm.

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The latest State of the Cryosphere pulls no punches, so it’s worth reproducing the Forward here. Meanwhile, the Reserve Bank is vastly underestimating the residential mortgage exposure to flooding risks:

“We asked banks to identify the value of their mortgage exposures that would be affected by permanent sea level rises of 20 centimetres, 50 centimetres and 1 metre in the most severe case. This range of sea level rise is consistent with climate change modelling out from 2040 to 2100.”

A sea level rise of just 1 metre (the banks’ ‘most severe case’) by 2100 is entirely inconsistent with real world events now unfolding. COP27 was an abysmal failure. Emissions are increasing instead of decreasing. Greenland is decanting into the ocean, along with most of our mountain glaciers, and even parts of the East Antarctic Ice Sheet, which was assumed to be stable for thousands of years, are showing signs of destablising (Video 2), while…

‘Sections of the West Antarctic Ice Sheet may collapse even without further emissions, causing more than 4 meters of additional sea-level rise…’ – State of the Cryosphere 2022 (p8)

 

State of the Cryosphere Forward
Pam Pearson Director, ICCI

Far too much human suffering occurred in 2022 due to rapid loss of ice in the cryosphere, Earth’s snow and ice regions, from human-caused global warming.

Most occurred in places far from ice and snow: coastal sea-level rise from ice sheet loss; water shortages well downstream from shrinking glaciers and snowpack; floods, avalanches, wildfires and extreme weather events.

All were caused in whole or in part by loss of the cryosphere’s stabilizing influence on the global climate system. Our global ice stores are receding at rates unthinkable just a decade ago, whether held in ice sheets, land glaciers, permafrost or sea ice.

This State of the Cryosphere Report 2022: Growing Losses, Global Impacts especially raises up the IPCC Sixth Assessment conclusion that complete loss of Arctic sea ice in summer is now inevitable, even with the very lowest emissions pathways that peak temperatures at 1.6°C. This finding is a terminal diagnosis for that ecosystem and its essential role reflecting sunlight as the “Earth’s refrigerator,” something sea ice scientists have been warning for decades would come with continued high emissions. No one seems to have listened.

But the impending loss of Arctic summer sea ice it is not the only sign of growing cryosphere collapse. This year also saw March rains on East Antarctica, with temperatures 40°C above normal; a spike in Greenland surface melt for the first time ever in September; loss of over 5% of glacier ice in the Alps over a single summer; and the first documented rise in methane release due to global warming from a permafrost monitoring site. It also saw greater shell damage in parts of the Arctic Ocean, a clear sign of acidification; and an apparent crash in snow crab populations likely tied to warming waters.

Research published this year also shows growing convergence between ice sheet models and what paleo-climatologists (those who study Earth history going back millions of years) have warned for decades: that ice loss and irreversible sea-level rise may be occurring faster, and at lower temperatures than previously forecast: even from the massive East Antarctic Ice Sheet, with temperatures above 1.8°C. Nearly all these impacts are irreversible on human timescales. It might take only decades to melt a glacier, but centuries to thousands of years of lower temperatures to grow it back.

The message from the global cryosphere in 2022 is that we are already well into the risk zone for irreversible damage from continued loss of the planet’s global ice stores. Nevertheless, the 2022 UNFCCC Synthesis Report on reported pledges (NDCs) showed not the 45% decrease in emissions needed by 2030 to keep 1.5°C within reach, but a 10% increase by 2030.

Our planet’s melting ice pays no attention to climate pledges and NDCs. It responds only to the level of CO2 and warming in the atmosphere. In 2022, the year-on-year increase of about 2.5 ppm continued unabated, as it has for the past two decades. Until this CO2 rise slows, halts and begins to decrease, the ice will continue to respond as it always has: to the only number that really matters.

Antarctica image: James Eades

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By: Maurice Huguenin, PhD Candidate, UNSW Sydney; Matthew England, Scientia Professor and Deputy Director of the ARC Australian Centre for Excellence in Antarctic Science (ACEAS), UNSW Sydney, and Ryan Holmes, Research fellow, University of Sydney

Republished in full from The Conversation under a Creative Commons license. See the original article: September 7, 2022

Over the last 50 years, the oceans have been working in overdrive to slow global warming, absorbing about 40% of our carbon dioxide emissions, and over 90% of the excess heat trapped in the atmosphere.

But as our research published today in Nature Communications has found, some oceans work harder than others.

We used a computational global ocean circulation model to examine exactly how ocean warming has played out over the last 50 years. And we found the Southern Ocean has dominated the global absorption of heat. In fact, Southern Ocean heat uptake accounts for almost all the planet’s ocean warming, thereby controlling the rate of climate change.

This Southern Ocean warming and its associated impacts are effectively irreversible on human time scales, because it takes millennia for heat trapped deep in the ocean to be released back into the atmosphere.

This means changes happening now will be felt for generations to come – and those changes are only set to get worse, unless we can stop carbon dioxide emissions and achieve net zero.


It’s important yet difficult to measure ocean heating

Ocean warming buffers the worst impacts of climate change, but it’s not without cost. Sea levels are rising because heat causes water to expand and ice to melt. Marine ecosystems are experiencing unprecedented heat stress, and the frequency and intensity of extreme weather events is changing.

Yet, we still don’t know enough about exactly when, where and how ocean warming occurs. This is because of three factors.

First, temperature changes at the ocean surface and in the atmosphere just above track each other closely. This makes it difficult to know exactly where excess heat is entering the ocean.

Second, we don’t have measurements tracking temperatures over all of the ocean. In particular, we have very sparse observations in the deep ocean, in remote locations around Antarctica and under sea ice.

Last, the observations we do have don’t go back very far in time. Reliable data from deeper than 700 metres depth is virtually non-existent prior to the 1990s, apart from observations along specific research cruise tracks.

Earth’s heat inventory since 1960 (ZJ = 10²¹ J). Credit: von Schuckmann et al. (2020).


Our modelling approach

To work out the intricacies of how ocean warming has played out, we first ran an ocean model with atmospheric conditions perpetually stuck in the 1960s, prior to any significant human-caused climate change.

Then, we separately allowed each ocean basin to move forward in time and experience climate change, while all other basins were held back to experience the climate of the 1960s.

We also separated out the effects of atmospheric warming from surface wind-driven changes to see how much each factor contributes to the observed ocean warming.

By taking this modelling approach, we could isolate that the Southern Ocean is the most important absorber of this heat, despite only covering about 15% of the total ocean’s surface area.

In fact, the Southern Ocean alone could account for virtually all global ocean heat uptake, with the Pacific and Atlantic basins losing any heat gained back into the atmosphere.

One significant ecological impact of strong Southern Ocean warming is on Antarctic krill. When ocean warming occurs beyond temperatures they can tolerate, the krill’s habitat contracts and they move even further south to cooler waters.

As krill is a key component of the food web, this will also change the distribution and population of larger predators, such as commercially viable tooth and ice fish. It will also further increase stress for penguins and whales already under threat today.

So why is the Southern Ocean absorbing so much heat?

This largely comes down to the geographic set-up of the region, with strong westerly winds surrounding Antarctica exerting their influence over an ocean that’s uninterrupted by land masses.

This means the Southern Ocean winds blow over a vast distance, continuously bringing masses of cold water to the surface. The cold water is pushed northward, readily absorbing vast quantities of heat from the warmer atmosphere, before the excess heat is pumped into the ocean’s interior around 45-55°S (a latitude band just south of Tasmania, New Zealand, and the southern regions of South America).

This warming uptake is facilitated by both the warmer atmosphere caused by our greenhouse gas emissions, as well as wind-driven circulation which is important for getting heat into the ocean interior.

And when we combine the warming and wind effects only over the Southern Ocean, with the remaining oceans held back to the climate of the 1960s, we can explain almost all of the global ocean heat uptake.

But that’s not to say the other ocean basins aren’t warming. They are, it’s just that the heat they gain locally from the atmosphere cannot account for this warming. Instead, the massive heat uptake in the Southern Ocean is what has driven changes in total ocean heat content worldwide over the past half century.

We have much to learn

While this discovery sheds new light on the Southern Ocean as a key driver of global ocean warming, we still have a lot to learn, particularly about ocean warming beyond the 50 years we highlight in our study. All future projections, including even the most optimistic scenarios, predict an even warmer ocean in future.

And if the Southern Ocean continues to account for the vast majority of ocean heat uptake until 2100, we might see its heat content increase by as much as seven times more than what we have already seen up to today.

This will have enormous impacts around the globe: including further disturbances to the Southern Ocean food web, rapid melting of Antarctic ice shelves, and changes in the ocean conveyor belt.

To capture all of these changes, it’s vital we continue and expand our observations taken in the Southern Ocean.

One of the most important new data streams will be new ocean floats that can measure deeper ocean temperatures, as well as small temperature sensors on elephant seals, which give us essential data of oceanic conditions in winter under Antarctic sea ice.

Even more important is the recognition that the less carbon dioxide we emit, the less ocean change we will lock in. This will ultimately limit the disruption of livelihoods for the billions of people living near the coast worldwide.The Conversation

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“In the Arctic, the end of the northern-hemisphere summer brings with it the close of the melt season. It is also the traditional moment for Carbon Brief to publish its annual health check of the Greenland ice sheet, written by scientists at the Danish Meteorological Institute.

This year’s review finds that “taking into account surface melting, breaking off of icebergs and frictional effects under glaciers, the Greenland ice sheet lost 84Gt of ice over the 12 months from September 2021 to August 2022”.

This makes 2022 the “26th year in a row where Greenland lost ice overall”, the authors note, adding that “Greenland last saw an annual net gain of ice in 1996”.

The end of the summer also means the arrival of this year’s Arctic sea ice summer minimum. Provisional numbers from the US National Snow and Ice Data Center (NSIDC) – announced late yesterday – suggest that 2022 is tied with 2017 and 2018 as the 10th smallest on record.
While one scientist described this summer to Carbon Brief as “uneventful” for Arctic sea ice, the data also highlights the long-term impacts of warming, with the last 16 years seeing the 16 lowest minimum extents in the satellite record. – Carbon Brief

What does this mean for Aotearoa?

  • Sea levels are rising faster than anticipated.
  • Less ice means darker ocean waters absorb even more warmth, which in turn leads to more warming and less future sea ice, in a feedback effect.
  • This in turn is changing how ocean currents behave, and this reaches all the way down to Aotearoa and has an impact on oceanic ecosystems.