Suffering in the heat: the rise in marine heatwaves is harming ocean species



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Recent marine heatwaves have devastated crucial coastal habitats, including kelp forests, seagrass meadows and coral reefs.
Dan Smale, Author provided

Dan Smale, Marine Biological Association and Thomas Wernberg, University of Western Australia

In the midst of a raging heatwave, most people think of the ocean as a nice place to cool down. But heatwaves can strike in the ocean as well as on land. And when they do, marine organisms of all kinds – plankton, seaweed, corals, snails, fish, birds and mammals – also feel the wrath of soaring temperatures.

Our new research, published today in Nature Climate Change, makes abundantly clear the destructive force of marine heatwaves. We compared the effects on ecosystems of eight marine heatwaves from around the world, including four El Niño events (1982-83, 1986-87, 1991-92, 1997-98), three extreme heat events in the Mediterranean Sea (1999, 2003, 2006) and one in Western Australia in 2011. We found that these events can significantly damage the health of corals, kelps and seagrasses.

This is concerning, because these species form the foundation of many ecosystems, from the tropics to polar waters. Thousands of other species – not to mention a wealth of human activities – depend on them.

We identified southeastern Australia, southeast Asia, northwestern Africa, Europe and eastern Canada as the places where marine species are most at risk of extreme heat in the future.




Read more:
Marine heatwaves are getting hotter, lasting longer and doing more damage


Marine heatwaves are defined as periods of five days or more during which ocean temperatures are unusually high, compared with the long-term average for any given place. Just like their counterparts on land, marine heatwaves have been getting more frequent, hotter and longer in recent decades. Globally, there were 54% more heatwave days per year between 1987 and 2016 than in 1925–54.

Although the heatwaves we studied varied widely in their maximum intensity and duration, we found that all of them had negative impacts on a broad range of different types of marine species.

Marine heatwaves in tropical regions have caused widespread coral bleaching.

Humans also depend on these species, either directly or indirectly, because they underpin a wealth of ecological goods and services. For example, many marine ecosystems support commercial and recreational fisheries, contribute to carbon storage and nutrient cycling, offer venues for tourism and recreation, or are culturally or scientifically significant.




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Marine heatwaves have had negative impacts on virtually all these “ecosystem services”. For example, seagrass meadows in the Mediterranean Sea, which store significant amounts of carbon, are harmed by extreme temperatures recorded during marine heatwaves. In the summers of both 2003 and 2006, marine heatwaves led to widespread seagrass deaths.




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Seagrass, protector of shipwrecks and buried treasure


The marine heatwaves off the west coast of Australia in 2011 and northeast America in 2012 led to dramatic changes in the regionally important abalone and lobster fisheries, respectively. Several marine heatwaves associated with El Niño events caused widespread coral bleaching with consequences for biodiversity, fisheries, coastal erosion and tourism.

Mass die-offs of finfish and shellfish have been recorded during marine heatwaves, with major consequences for regional fishing industries.

All evidence suggests that marine heatwaves are linked to human mediated climate change and will continue to intensify with ongoing global warming. The impacts can only be minimised by combining rapid, meaningful reductions in greenhouse emissions with a more adaptable and pragmatic approach to the management of marine ecosystems.The Conversation

Dan Smale, Research Fellow in Marine Ecology, Marine Biological Association and Thomas Wernberg, Associate professor, University of Western Australia

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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A current affair: the movement of ocean waters around Australia



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Where do the ocean waters that wash the Gold Coast come from?
Flickr/LJ Mears , CC BY-NC-SA

Charitha Pattiaratchi, University of Western Australia; Ems Wijeratne, University of Western Australia, and Roger Proctor, University of Tasmania

Many people in Australia will head to the beach this summer and that’ll most likely include a dip or a plunge into the sea. But have you ever wondered where those ocean waters come from, and what influence they may have?

Australia is surrounded by ocean currents that have a strong controlling influence on things such as climate, ecosystems, fish migrations, the transport of ocean debris and on water quality.

We did a study, published in April 2018, that helps to give us a better understanding of those ocean currents.

Surface currents around the Australian continent.
Ems Wijeratne/Charitha Pattiaratchi/Roger Proctor



Read more:
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Go with the flow: Indian Ocean

Our 15 year simulation indicates that water from the Pacific Ocean enters the Indonesian Archipelago through the Mindanao current (north) and Halmahera Sea (south).

It then enters the Indian ocean as the Indonesian Throughflow between many Indonesian Islands, with flow through the Timor Passage being the most dominant.

Most of this water flows west as the South Equatorial Current. Re-circulation of the SEC creates the Eastern Gyre that contributes to the Holloway Current. This in turn feeds the Leeuwin Current – the longest boundary current in the world (Ocean currents that flow adjacent to a coastline are called boundary currents)

The Leeuwin Current is the major boundary current along the west coast and as it moves southward. Indian Ocean water is supplied by the South Indian Counter Current increasing the Leeuwin Current transport by 60%.

The Leeuwin Current turns east at Cape Leeuwin, in Western Australia’s south-west, and continues to Tasmania as the South Australian and Zeehan Currents.

The Leeuwin Current passes the lighthouse at the Cape Leeuwin in WA.
Flickr/Cheng, CC BY-NC-ND

There is a strong seasonal variation in the strength of the boundary currents in the Indian Ocean with a progression southwards of the peak transport along the coast.

The Holloway Current peaks in April/May (coinciding with changes in the monsoon winds), the Leeuwin Current reaches a maximum along the west and south coasts in June and August.




Read more:
Climate change is slowing Atlantic currents that help keep Europe warm


Go with the flow: Pacific Ocean

In the Pacific Ocean, the northern branches of the South Equatorial Current are the main inputs initiating the Hiri Current and East Australian Current.

At around latitude 15 degrees south the currents split in two: southward to form the East Australian Current, and northward to form the Hiri Current which contributes to a clockwise gyre in the Gulf of Papua.

The East Australian Current is the dominant current in the region transporting 33 million cubic metres of water per second southward.

At around 32S, the East Australian Current separates from the coast and 60% of the water flows eastward to New Zealand as the Tasman Front. The remaining 40% flows southward as the East Australian Current extension and contributes to the Tasman Outflow.

The Tasman outflow is the major conduit of water from the Pacific to Indian Ocean and contributes to the Flinders Current, flowing westward from Tasmania and past Cape Leeuwin into the Indian Ocean.

Along the southern continental slope, the Flinders Current appears as an undercurrent beneath the Leeuwin Current and a surface current further offshore. The Flinders Current contributes to the Leeuwin Undercurrent directly as a northward flow, flowing to the north-west of Australia in water depths 300 metres to 800 metres.

Impact of the currents

Understanding ocean circulation is a fundamental tenet of physical oceanography and scientists have been charting the pathways of ocean currents since the American hydrographer Matthew Maury, one of the founders of oceanography, who first charted the Gulf Stream in 1855.

One of the first maps of circulation around Australia was by Halliday (1921) who showed the movement of “warm” and “cold” waters around Australia. Although some of the major features (such as the East Australian Current) were correctly identified, a more fine scale description is now available.

Ocean surface currents around Australia by Halliday 1921.

The unique feature of ocean currents around Australia is that along both east and west coasts they transport warmer water southwards and influence the local climate, particularly air temperature and rainfall, as well as species distribution.




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For example, the south west of Australia is up to 5C warmer in winter and receives more than double the rainfall compared to regions located on similar latitudes along western coastlines of other continents.

Similarly many tropical species of fish are found in the southwest of Australia that hitch a ride on the ocean currents.

The Pacific Ocean is the origin of waters around Australia with a direct link to the east and an indirect link to west.

Ocean water from the Pacific Ocean flows through the Indonesian Archipelago, a region subject to high solar heating and rainfall runoff, creating lower density water. This water, augmented by water from the Indian Ocean, flows around the western and southern coasts, converging along the southern coast of Tasmania.

So next time you head for a dip in the coastal waters around Australian, spare a thought for where that water has come from and where it may be going next.The Conversation

Time for a plunge in the water at Bondi Beach, NSW.
Flickr/Roderick Eime, CC BY-ND

Charitha Pattiaratchi, Professor of Coastal Oceanography, University of Western Australia; Ems Wijeratne, Assistant Professor, UWA Oceans Institute, University of Western Australia, and Roger Proctor, Director, Australian Ocean Data Network, University of Tasmania

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Antarctic seas host a surprising mix of lifeforms – and now we can map them



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In contrast to common perceptions, Antarctic seafloor communities are highly diverse. This image shows a deep East Antarctic reef with plenty of corals, sponges and brittlestars. Can you spot the octopus?
Australian Antarctic Division

Jan Jansen, University of Tasmania; Craig Johnson, University of Tasmania, and Nicole Hill, University of Tasmania

What sort of life do you associate with Antarctica? Penguins? Seals? Whales?

Actually, life in Antarctic waters is much broader than this, and surprisingly diverse. Hidden under the cover of sea-ice for most of the year, and living in cold water near the seafloor, are thousands of unique and colourful species.

A diverse seafloor community living under the ice near Casey station in East Antarctica.

Our research has generated new techniques to map where these species live, and predict how this might change in the future.

Biodiversity is nature’s most valuable resource, and mapping how it is distributed is a crucial step in conserving life and ecosystems in Antarctica.




Read more:
Explainer: what is biodiversity and why does it matter?


Surprises on the seafloor

The ocean surrounding the Antarctic continent is an unusual place. Here, water temperatures reach below freezing-point, and the ocean is covered in ice for most of the year.

While commonly known for its massive icebergs and iconic penguins, Antarctica’s best-kept secret lies on the seafloor far below the ocean surface. In this remote and isolated environment, a unique and diverse community of animals has evolved, half of which aren’t found anywhere else on the planet.

These solitary sea squirts stand up to half a metre tall at 220m depth in the dark, cold waters of East-Antarctica. Images such as this one were taken with cameras towed behind the Australian Icebreaker Aurora Australis.
Australian Antarctic Division

Colourful corals and sponges cover the seafloor, where rocks provide hard substrate for attachment. These creatures filter the water for microscopic algae that sink from the ocean surface during the highly productive summer season between December and March.

In turn, these habitat-forming animals provide the structure for all sorts of mobile animals, such as featherstars, seastars, crustaceans, sea spiders and giant isopods (marine equivalents of “slaters” or “woodlice”).

The Antarctic seafloor is also home to a unique group of fish that have evolved proteins to stop their blood from freezing.

Most Antarctic fish have evolved ‘anti-freeze blood’ allowing them to survive in water temperature below zero degrees C.
Australian Antarctic Division

Mapping biodiversity is hard

Biodiversity is a term that describes the variety of all life forms on Earth. The unprecedented rate of biodiversity loss is one of the biggest challenges of our time. And despite its remoteness, Antarctica’s biodiversity is not protected from human impact through climate change, pollution and fisheries.




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Although scientists have broadly known about Antarctica’s unique marine biodiversity for some time, we still lack knowledge of where each species lives and where important hotspots of biodiversity are located. This is an issue because it hinders us from understanding how the ecosystem functions – and makes it hard to assess potential threats.

Why don’t we know more about the distribution of Antarctic marine species? Primarily, because sampling at the seafloor a few thousand metres below the surface is difficult and expensive, and the Antarctic continental shelf is vast and remote. It usually takes the Australian Icebreaker Aurora Australis ten days to reach the icy continent.

A selection of the diverse and colourful species found on the Antarctic seafloor.
Huw Griffiths/British Antarctic Survey

To make the most of the sparse and patchy biological data that we do have, in our research we take advantage of the fact that species usually have a set of preferred environmental conditions. We use the species’ relationship with their environment to build statistical models that predict where species are most likely to occur.

This allows us to map their distribution in places where we have no biological samples and only environmental data. Critically, until now important environmental factors that influence the distribution of seafloor species have been missing.




Read more:
Antarctica has lost 3 trillion tonnes of ice in 25 years. Time is running out for the frozen continent


Using predictions to make a map

In a recent study, we were able to predictively map how much food from the ocean-surface was available for consumption by corals, sponges and other suspension feeders at the seafloor.

The science behind linking food-particles from the ocean surface to the biodiversity of Antarctic seafloor fauna. Satellites (1) can detect the amount of algae at the ocean-surface. Algae-production is particularly high in ice-free areas (2) compared to under the sea-ice (3). Algae sink from the surface (4) and reach the seafloor. Where ocean-currents are high (5), many corals feed from the suspended particles. In areas with slow currents (6), particles settle onto the seafloor and feed deposit-feeding animals such as seacucumbers.
Jansen et al. (2018), Nature Ecology & Evolution 2, 71-80.

Although biological samples are still scarce, this allowed us to map the distribution of seafloor biodiversity in a region in East Antarctica with high accuracy.

Further, estimates of how and where the supply of food increased after the tip of a massive glacier broke off and changed ocean conditions in the region allowed us to predict where abundances of habitat forming fauna such as corals and sponges will increase in the future.

Colourful and diverse communities are also found living in shallow waters.
Australian Antarctic Division

Antarctica is one of the few regions where the total biomass of seafloor animals is likely to increase in the future. Retreating ice-shelves increase the amount of suitable habitat available and allow more food to reach the seafloor.

For the first time in history, we now have the information, computational power and research capacity to map the distribution of life on the entire continental shelf around Antarctica, identify previously unknown hotspots of biodiversity, and assess how the unique biodiversity of the Antarctic will change into the future.


The Conversation


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Jan Jansen, Quantitative Marine Ecologist, University of Tasmania; Craig Johnson, Professor, University of Tasmania, and Nicole Hill, Research fellow, University of Tasmania

This article was originally published on The Conversation. Read the original article.

Ocean waves and lack of sea ice can trigger Antarctic ice shelves to disintegrate


Luke Bennetts, University of Adelaide; Rob Massom, and Vernon Squire

Large waves after the loss of sea ice can trigger Antarctic ice shelf disintegration over a period of just days, according to our new research.

With other research also published today in Nature showing that the rate of annual ice loss from the vulnerable Antarctic Peninsula has quadrupled since 1992, our study of catastrophic ice shelf collapses during that time shows how the lack of a protective buffer of sea ice can leave ice shelves, already weakened by climate warming, wide open to attack by waves.




Read more:
Antarctica has lost 3 trillion tonnes of ice in 25 years. Time is running out for the frozen continent


Antarctica is covered by an ice sheet that is several kilometres thick in places. It covers an area of 14 million square kilometres – roughly twice the size of Australia. This ice sheet holds more than 90% of the world’s ice, which is enough to raise global mean sea level by 57 metres.

As snow falls and compacts on the ice sheet, the sheet thickens and flows out towards the coast, and then onto the ocean surface. The resulting “ice shelves” (and glacier tongues) buttress three-quarters of the Antarctic coastline. Ice shelves act as a crucial braking system for fast-flowing glaciers on the land, and thus moderate the ice sheet’s contribution to sea-level rise.

In the southern summer of 2002, scientists monitoring the Antarctic Peninsula (the northernmost part of mainland Antarctica) by satellite witnessed a dramatic ice shelf disintegration that was stunning in its abruptness and scale. In just 35 days, 3,250 square km of the Larsen B Ice Shelf (twice the size of Queensland’s Fraser Island) shattered, releasing an estimated 720 billion tonnes of icebergs into the Weddell Sea.

This wasn’t the first such recorded event. In January 1995, roughly 1,500 square km of the nearby Larsen A Ice Shelf suddenly disintegrated after several decades of warming and years of gradual retreat. To the southwest, the Wilkins Ice Shelf suffered a series of strikingly similar disintegration events in 1998, 2008 and 2009 — not only in summer but also in two of the Southern Hemisphere’s coldest months, May and July.

These sudden, large-scale fracturing events removed features that had been stable for centuries – up to 11,500 years in the case of Larsen B. While ice shelf disintegrations don’t directly raise sea level (because the ice shelves are already floating), the removal of shelf ice allows the glaciers behind them to accelerate their discharge of land-based ice into the ocean – and this does raise sea levels. Previous research has shown that the removal of Larsen B caused its tributary glaciers to flow eight times faster in the year following its disintegration.




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Cold and calculating: what the two different types of ice do to sea levels


The ocean around ice shelves is typically covered by a very different (but equally important) type of ice, called sea ice. This is formed from frozen seawater and is generally no more than a few metres thick. But it stretches far out into the ocean, doubling the area of the Antarctic ice cap when at its maximum extent in winter, and varying in extent throughout the year.

The response of Antarctic sea ice to climate change and variability is complex, and differs between regions. Around the Antarctic Peninsula, in the Bellingshausen and northwestern Weddell seas, it has clearly declined in extent and annual duration since satellite monitoring began in 1979, at a similar rate to the Arctic’s rapidly receding sea ice.

The Southern Ocean is also host to the largest waves on the planet, and these waves are becoming more extreme. Our new study focuses on “long-period” swell waves (with swells that last up to about 20 seconds). These are generated by distant storms and carry huge amounts of energy across the oceans, and can potentially flex the vulnerable outer margins of ice shelves.

The earliest whalers and polar pioneers knew that sea ice can damp these waves — Sir Ernest Shackleton reported it in his iconic book South!. Sea ice thus acts as a “buffer” that protects the Antarctic coastline, and its ice shelves, from destructive ocean swells.

Strikingly, all five of the sudden major ice shelf disintegrations listed above happened during periods when sea ice was abnormally low or even absent in these regions. This means that intense swell waves crashed directly onto the vulnerable ice shelf fronts.

The straw that broke the camel’s back

The Antarctic Peninsula has experienced particularly strong climate warming (roughly 0.5℃ per decade since the late 1940s), which has caused intense surface melting on its ice shelves and exacerbated their structural weaknesses such as fractures. These destabilising processes are the underlying drivers of ice shelf collapse. But they do not explain why the observed disintegrations were so abrupt.

Our new study suggests that the trigger mechanism was swell waves flexing and working weaknesses at the shelf fronts in the absence of sea ice, to the point where they calved away the shelf fronts in the form of long, thin “sliver-bergs”. The removal of these “keystone blocks” in turn led to the catastrophic breakup of the ice shelf interior, which was weakened by years of melt.

Our research thus underlines the complex and interdependent nature of the various types of Antarctic ice – particularly the important role of sea ice in forming a protective “buffer” for shelf ice. While much of the focus so far has been on the possibility of ice shelves melting from below as the sea beneath them warms, our research suggests an important role for sea ice and ocean swells too.

The edge of an ice shelf off the Antarctic Peninsula, with floating sea ice beyond (to the left in this image).
NASA/Maria Jose Vinas

In July 2017 an immense iceberg broke away from the Larsen C Ice Shelf, just south of Larsen B, prompting fears that it could disintegrate like its neighbours.

Our research suggests that four key factors will determine whether it does: extensive flooding and fracturing across the ice shelf; reduced sea ice coverage offshore; extensive fracturing of the ice shelf front; and calving of sliver-bergs.




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Don’t worry about the huge Antarctic iceberg – worry about the glaciers behind it


If temperatures continue to rise around the Antarctic, ice shelves will become weaker and sea ice less extensive, which would imply an increased likelihood of future disintegrations.

However, the picture is not that clear-cut, as not all remaining ice shelves are likely to respond in the same way to sea ice loss and swell wave impacts. Their response will also depend on their glaciological characteristics, physical setting, and the degree and nature of surface flooding. Some ice shelves may well be capable of surviving prolonged absences of sea ice.

The ConversationIrrespective of these differences, we need to include sea ice and ocean waves in our models of ice sheet behaviour. This will be a key step towards better forecasting the fate of Antarctica’s remaining ice shelves, and how much our seas will rise in response to projected climate change over coming decades. In parallel, our new findings underline the need to better understand and model the mechanisms responsible for recent sea ice trends around Antarctica, to enable prediction of likely future change in the exposure of ice shelves to ocean swells.

Luke Bennetts, Lecturer in applied mathematics, University of Adelaide; Rob Massom, Leader, Sea Ice Group, Antarctica & the Global System program, Australian Antarctic Division and Antarctic Climate and Ecosystems CRC, and Vernon Squire, Deputy Vice-Chancellor Academic, Professor of Applied Mathematics

This article was originally published on The Conversation. Read the original article.

Lava in Hawai’i is reaching the ocean, creating new land but also corrosive acid mist


Dave McGarvie, The Open University and Ian Skilling, The University of South Wales

There is something special and awe-inspiring about watching new land form. This is what is now happening in Hawai’i as its Kīlauea volcano erupts. Lava is reaching the ocean and building land while producing spectacular plumes of steam. These eruptions are hugely important for the creation of new land. But they are also dangerous. Where the lava meets the ocean, corrosive acid mist is produced and glass particles are shattered and flung into the air. Volcanic explosions can also hurl lava blocks hundreds of metres and produce waves of scalding hot water.

At Kīlauea, lava is erupting from a line of vents on the volcano’s flanks, and is moving downslope to the edge of the island, where it enters the ocean. This is a process that has been witnessed many times at Hawai’i and other volcanic islands. And it is through many thousands of such eruptions that volcanic islands like Hawai’i form.

The new lava being added to Hawai’i by this latest Kīlauea eruption replaces older land that is being lost by erosion, and so prolongs the island’s lifespan. In contrast, older islands to the north-west have no active volcanoes, so they are being eroded by the ocean and will eventually disappear beneath the waves. The opposite is happening to the south-east of Hawai’i, where an underwater volcano (Lōʻihi Seamount) is building the foundations of what will eventually become the next volcanic island in this area.

How lava gets to the ocean at Hawai’i

The lava erupting from the current Kīlauea vents has a temperature of roughly 1150 degrees °C, and has a journey of between 4.5km and 5.5km to reach the ocean. As this lava moves swiftly in channels, it loses little heat and so it can enter the ocean at a temperature of over 1000 degrees°C.

When lava meets the sea, new land is formed.
EPA

What happens when lava meets the ocean?

We are witnessing one of the most spectacular sights in nature – billowing white plumes of steam (technically water droplets) as hot lava boils seawater. Although these billowing steam clouds appear harmless, they are dangerous because they contain small glass shards (fragmented lava) and acid mist (from seawater). This acid mist known as “laze” (lava haze) can be hot and corrosive. If anyone goes to near it, they can experience breathing difficulties and irritation of their eyes and skin.

Apart from the laze, the entry of lava into the ocean is usually a gentle process, and when steam is free to expand and move away, there are no violent steam-driven explosions.

But a hidden danger lurks beneath the ocean. The lava entering the sea breaks up into blobs (known as pillows), angular blocks, and smaller fragments of glass that form a steep slope beneath the water. This is called a lava delta.

A newly formed lava delta is an unstable beast, and it can collapse without warning. This can trap water within the hot rock, leading to violent steam-driven explosions that can hurl metre-sized blocks up to 250 metres. Explosions occur because when the water turns to steam it suddenly expands to around 1,700 times its original volume. Waves of scalding water can also injure people who are too close. People have died and been seriously injured during lava delta collapses

So, the ocean entry points where lava and seawater meet are doubly dangerous, and anyone in the area should pay careful attention to official advice on staying away from them.

Pillow Lavas form underneath the ocean.
National Oceanic & Atmospheric Adminstration (NOAA)

What more can we learn from these eruptions?

Once lava deltas have cooled and become stable they represent new land. Studies have revealed that lava deltas have distinctive features, and this has enabled volcanologists to recognise lava deltas in older rocks.

Remarkable examples of lava deltas have been discovered near the top of extinct volcanoes (called tuyas) in Iceland and Antarctica. These deltas can only form in water and the only plausible source of this water in this case is melted ice. This means that these volcanoes had melted water-filled holes up to 1.5km deep in ice sheets, which is an astonishing feat. In fact, these lava deltas are the only remaining evidence of long-vanished ice sheets.

The ConversationIt is a privilege to see these incredible scenes of lava meeting the ocean. The ongoing eruptions form part of the natural process that enables beautiful volcano islands like Hawai’i to exist. But the creation of new land here can also bring danger to those who get too close, whether it be collapsing lava deltas or acid mist.

Dave McGarvie, School of Physical Sciences, The Open University and Ian Skilling, Senior Lecturer (Volcanology), The University of South Wales

This article was originally published on The Conversation. Read the original article.

Climate change is slowing Atlantic currents that help keep Europe warm



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Natalie Renier/Woods Hole Oceanographic Institution, Author provided

Peter T. Spooner, UCL

The ocean currents that help warm the Atlantic coasts of Europe and North America have significantly slowed since the 1800s and are at their weakest in 1600 years, according to new research my colleagues and I have conducted. As we’ve set out in a new study in Nature, the weakening of this ocean circulation system may have begun naturally but is probably being continued by climate change related to greenhouse gas emissions.

This circulation is a key player in the Earth’s climate system and a large or abrupt slowdown could have global repercussions. It could cause sea levels on the US east coast to rise, alter European weather patterns or rain patterns more globally, and hurt marine wildlife.

We know that at the end of the last major ice age, rapid fluctuations in the circulation led to extreme climate shifts on a global scale. An exaggerated (but terrifying) example of such a sudden event was portrayed in the 2004 blockbuster film The Day After Tomorrow.

The recent weakening we have found was likely driven by warming in the north Atlantic and the addition of freshwater from increased rainfall and melting ice. It has been predicted many times but, until now, just how much weakening has already occurred has largely remained a mystery. The extent of the changes we have discovered comes as a surprise to many, including myself, and points to significant changes in the future.

The circulation system in question is known as the “Atlantic Meridional Overturning Circulation” (AMOC). The AMOC is like a giant conveyor belt of water. It transports warm, salty water to the north Atlantic where it gets very cold and sinks. Once in the deep ocean the water flows back southwards and then all around the world’s oceans. This conveyor belt is one of the most important transporters of heat in the climate system and includes the Gulf Stream, known for keeping western Europe warm.

Climate models have consistently predicted that the AMOC will slow down due to greenhouse gas warming and associated changes in the water cycle. Because of these predictions – and the possibility of abrupt climate changes – scientists have monitored the AMOC since 2004 with instruments strung out across the Atlantic at key locations. But to really test the model predictions and work out how climate change is affecting the conveyor we have needed much longer records.

Looking for patterns

To create these records, our research group – led by University College London’s Dr David Thornalley – used the idea that a change in the AMOC has a unique pattern of impact on the ocean. When the AMOC gets weaker, the north-eastern Atlantic Ocean cools and parts of the western Atlantic get warmer by a specific amount. We can look for this pattern in past records of ocean temperature to trace what the circulation was like in the past.

Another study in the same issue of Nature, led by researchers at the University of Potsdam in Germany, used historical observations of temperature to check the fingerprint. They found that the AMOC had reduced in strength by around 15% since 1950, pointing to the role of human-made greenhouse gas emissions as the primary cause.

In our paper, which also forms part of the EU ATLAS project, we found the same fingerprint. But instead of using historical observations we used our expertise in past climate research to go back much further in time. We did this by combining known records of the remains of tiny marine creatures found in deep-sea mud. Temperature can be worked out by looking at the amounts of different species and the chemical compositions of their skeletons.

We were also able to directly measure the past deep ocean current speeds by looking at the mud itself. Larger grains of mud imply faster currents, while smaller grains mean the currents were weaker. Both techniques point to a weakening of the AMOC since about 1850, again by about 15% to 20%. Importantly, the modern weakening is very different to anything seen over the last 1,600 years, pointing to a combination of natural and human drivers.

The difference in timing of the start of the AMOC weakening in the two studies will require more scientific attention. Despite this difference, both of the new studies raise important questions regarding whether climate models simulate the historical changes in ocean circulation, and whether we need to revisit some of our future projections.

The ConversationHowever, each additional long record makes it easier to evaluate how well the models simulate this key element of the climate system. In fact, evaluating models against these long records may be a crucial step if we hope to accurately predict possible extreme AMOC events and their climate impacts.

Peter T. Spooner, Research Associate in Paleoceanography, UCL

This article was originally published on The Conversation. Read the original article.

Ocean heat waves and weaker winds will keep Australia warm for a while yet


Jonathan Pollock, Australian Bureau of Meteorology; Andrew B. Watkins, Australian Bureau of Meteorology, and Catherine Ganter, Australian Bureau of Meteorology

The Australian Bureau of Meteorology’s latest climate outlook, issued today, suggests the above-average warmth of April is likely to extend into May, and for parts of the south, potentially into winter.

The outlooks for May temperatures show that both days and nights are likely to be warmer than average for much of Australia. Only northeast Queensland is likely to miss out on warmer temperatures, with no strong push there towards warmer or cooler conditions.

The unseasonable warmth, which has broken records in Adelaide and Sydney, appears to be driven by high ocean temperatures, and weaker westerly winds and much lower than average soil moisture across southern Australia.


Bureau of Meteorology

The rainfall outlook for May is mixed, but generally shows no strong shift towards a wetter or drier month for most of Australia.




Read more:
Winter heatwaves are nice … as extreme weather events go


By June the tendency for warmer than normal days may start to wane. This easing of the outlook for above average temperatures as we head into winter is reflected in the full May-July outlook, with only some parts of southern Australia likely to be warmer than average. Southern parts of Western Australia and South Australia have a moderate chance of warmer than average daytime temperatures, with stronger odds over southern Victoria.

The full May to July outlook shows a more balanced picture, with southern Australia more likely to experience higher than average temperatures.
Bureau of Meteorology

Odds don’t favour a strong push towards a particularly wet or dry three months for much of Australia, apart from some areas in the far southeast.

What’s behind the warmth?

The El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) are two of Australia’s major climate drivers. ENSO is currently in a neutral phase, meaning its neither El Niño nor La Niña. Our outlooks suggest it is likely to stay neutral leading into winter.




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Explainer: El Niño and La Niña


The IOD is also neutral, and most models suggest it will remain so over the coming months.

But given it is harder to forecast ENSO and the IOD in autumn compared to other times of the year, climatologists will be monitoring Indian and Pacific Ocean temperature patterns closely as we edge towards winter.

With near-average temperature patterns in the tropical oceans to our east and west, there is no strong shift in the outlook towards widespread wetter or drier conditions for Australia.

Rainfall during May is expected to remain essentially average.
Bureau of Meteorology

However, for temperatures it’s a little different. Sure ENSO and the IOD are playing a minor role right now, but other factors are coming into play.




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The BOM outlook for the weather over the next three months is ‘neutral’ – here’s what that really means


Ocean heat waves

Ocean temperatures in the Tasman Sea and around New Zealand are much warmer than average – in fact at record levels in the past few months – and are expected to remain warm over the coming months. These warm sea temperatures are associated with a large area of lower than usual air pressure to Australia’s east, which is likely to weaken the westerly winds that normally bring cooler air to southern Australia in autumn and winter.

Another factor in the current and forecast warmth is the very much below average soil moisture across southern Australia. With little moisture available to evaporate and cool the air, and the soils themselves not able to store as much heat, the air above the ground heats more rapidly in the daytime.


Bureau of Meteorology

In addition to our natural climate drivers, Australian climate patterns are being influenced by the long-term trend in global air and ocean temperatures. Winter maximum temperatures have increased by 1℃ over the past century, with three of the top five warmest winters in the past 108 years occurring since 2009. Oceans around Australia have warmed by slightly more, with four of our top five warmest years since 2010.

The ConversationSo while the normal big two drivers of our climate remain benign, it would actually be wrong to assume there will be a quick return to more average temperatures. The outlook released today suggests we may have to wait at least another month until service returns to normal for much of the country.

Jonathan Pollock, Climatologist, Australian Bureau of Meteorology; Andrew B. Watkins, Manager of Long-range Forecast Services, Australian Bureau of Meteorology, and Catherine Ganter, Senior Climatologist, Australian Bureau of Meteorology

This article was originally published on The Conversation. Read the original article.