Earth’s oceans currently take up almost 50% of our CO2 emissions and absorb over 90% of the heat the other half of the CO2 traps in the atmosphere. Both ocean CO2 and heat absorption will have major consequences for deep-sea ecosystems, like ocean acidification, ocean anoxia, and increasing food scarcity – impacts that take a long time before they reach rock bottom, at several kilometres depth.

But at the same time humans are also doing something else: we’re also dumping hundreds of millions of tonnes of plastic trash into these same oceans. And for deep-sea organisms, that ocean crisis seems to hit home much faster…

The effects of climate change for the deep ocean are dramatic. But long before the marine consequences of our atmospheric CO2 emissions, like ocean acidification (caused by dissolving CO2) and ocean anoxia (caused by rising water temperatures and thermal stratification) have reached rock bottom – another human pollutant has: plastic. Plastic is now found in 100% of organisms in the deepest ocean trench on Earth, creating a huge disturbance of the very delicate deep-ocean food chain. The shown arrow worm has blocked intestines resulting from an ingested blue plastic fibre. Due to the scarce natural food environment benthic ecosystems are especially vulnerable, both to climate change and to plastic pollution. Photograph: plankton scientist Richard Kirby.

The top of the ocean is turbulent, the bottom stable – and paradoxically fragile

The deep ocean is much more fragile than you might think. Life in the deep ocean that is. That is for two main reasons: deep-ocean ecosystems are used to an extremely constant environment – and to very low food concentrations.

To get a sense of how low ‘low’ is, we quote ocean researcher Andrew Sweetman of Heriot-Watt University in Edinburgh in an interview in The Guardian from last February: “The food supply these animals derive energy from in the abyss at 4,000-metre depths is equivalent to a sugar cube per square metre per year.”

Now imagine, when food is that scarce, what the effects of 300 million tonnes of plastic can be, 300 million tonnes of which just a fraction (250 thousand tonnes, 0.08%) is floating as pieces on the surface – the rest fragmenting to the level of individuals fibres and microplastics – and eventually sinking to these deep water layers…

We’ll get to that topic – the impacts of plastic pollution to the deep ocean – in a bit. First back to the climate issue, and to Sweetman.

‘Major impacts of climate change on deep-sea benthic ecosystems’

He is the lead author of a 2017 publication in the journal Elementa – ‘Science of the Anthropocene’ about climate change impacts on deep-sea benthic ecosystems, with benthic referring to the ‘benthic zone’ – an ecological term for the lowest region of a water mass, including the seabed itself and possible sediment layers, and all the interacting organisms (‘benthos’) that live in it.

Above this benthic zone the water column in the world’s oceans is separated into different ecological zones, defined following increasing ocean depth:

From top to bottom first there is the epipelagic zone, that reaches down from the ocean surface to a depth of 200 metres. Sunlight is able to penetrate this entire ocean layer, so there is an abundance of photosynthesising organisms, like floating seaweed and many plankton species. This marine photosynthesis creates a foundation for the entire marine food chain, and as a consequence most of the oceans fish live at this depth too.

Below there is the mesopelagic zone, to a depth of one thousand metres. Here the temperature changes most rapidly with depth (the thermocline). Although some sunlight still penetrates, it is insufficient for photosynthesis. Marine life is still very abundant, also because fish and other species migrate between this zone and the epipelagic zone above. (Some marine species, like Antarctic krill cycle daily between deep ocean depths and surface layers – a good way to separate eating time from being-eaten-time.)

Deep-ocean biodiversity in the Mariana Trench. Marine life may be most abundant in the upper ocean layers, where photosynthesis takes place (and therefore phytoplankton abounds). But also the deep parts of the ocean are full of life, harbouring their own unique biodiversity – even in the deepest trenches, as this BBC Blue Planet photograph from the Mariana Trench, taken at a depth of some 11 kilometres, beautifully illustrates. At this depth the ocean is pitch-black, apart from light that is actually created by luminescent marine species.

The deep ocean: cold, dark, low in oxygen and nutrients – and very stable

From a depth of 1,000 metres, the deep ocean starts, first with the bathyal zone, that extends to 3 or 4 kilometres depth – followed by the abyssal depth, at regions where the oceans stretch far beyond the continental shelves and are that deep, up to 6 kilometres.

Only ocean trenches are deeper still, a depth called the ‘hadal zone’, that locally extends almost 11 kilometres deep below the water surface. The bathyal, abyssal and hadal zone all share complete darkness, low oxygen and nutrient concentrations and a low but very stable water temperature, between 0 to 4 degrees Celsius, depending on whether you measure in the Atlantic, Pacific, Indian or Arctic Ocean (and with abyssal depths colder still than bathyal depths).

Most life forms that live in these deep waters have very slow metabolism to accommodate for the low nutrient levels – and all are adapted to living under immense water pressures, that keep increasing the deeper you go.

Here at Bits of Science we believe in getting the numbers straight. You often hear ‘the oceans absorb 90 percent’ of the heat that’s trapped (by elevated greenhouse gas levels) in the atmosphere. But do they? And does that leave 10 percent for the atmosphere to warm? Well, the numbers are a bit more complicated still. Oceans may absorb as much as 93.4 percent of the heat, but of course land masses (less conductive, but also large) and ice sheets (melting, so very conductive!) also absorb significant amounts, leaving ‘just’ 2.3 percent for the atmosphere – enough however to cause the warming that’s observed by thermometers across the globe, including all the other atmospheric manifestations of climate change, like migrating precipitation bands and increasing extreme weather events. So, if you turn your perspective away from the skies above, imagine the amount of energy that’s actually going into the oceans… Graph by SkepticalScience.com, based on IPCC AR4 data.

Due to their size and stratification, the oceans create very many forms of climate inertia. Not just for the atmosphere, also for their deepest water layers:

Now if we look at the effects of climate change at the oceans we have to get back to their CO2 and heat absorption. Imagine somehow Earth’s geography was different and the oceans did not take up (almost) half our emissions and (more than) 90 percent of the extra atmospheric heat.

Then firstly atmospheric CO2 concentrations would rise twice as fast, and we would now already be somewhere between 500-550 ppm. The subsequent heat build-up in the atmosphere would be twenty times as large. So, we thank a relatively stable – or rather, sturdy – terrestrial climate very much to our oceans.

But these oceans also lull us to sleep. First there is direct thermal inertia (within the definition of Equilibrium Climate Sensitivity) which implies that if we leave atmospheric CO2 concentrations constant, atmospheric temperatures would keep rising by ±0.7 degrees within a couple of decades – as a new equilibrium temperature tries to form, between atmosphere and upper ocean. This time lag and additional warming is called ocean thermal climate inertia.

This same upper ocean is however also still very helpful over a similar time span by continuing to take up CO2 – so if we would stop emitting, the atmospheric concentrations would actually go down – this is ocean CO2 inertia (and again we should be thankful here).

[These two do not even out perfectly, if you combine ocean thermal inertia and ocean CO2 inertia, on a decades timescale, the moment the world stops emitting, there’s some leftover warming in the pipeline.]

The ocean’s still take up a large portion of anthropogenic CO2 emissions (some 40%). Although this uptake could decrease as ocean waters warm and become more saturated, over recent years the net ocean CO2 uptake is actually increasing, another 2017 study explains that was published in the journal Nature. These researches find a inverse relation with decreasing ocean circulation, as this slows down deep-ocean CO2 degassing (remember the CO2 exchange between the oceans and the atmosphere works both directions – there are also continuous emissions to the atmosphere!). Ocean-atmosphere interactions are never easy, and always important…

But then there’s all the processes in the deep ocean, like increased stratification and a possible slowdown of the Meridional Overturning Circulation, that together with melting large ice sheets and albedo changes to the biosphere on land create ‘Earth System Sensitivity’ – a form of thermal climate inertia on a much larger scale, that implies that if we stabilise atmospheric CO2 concentrations around the current level, we may still have 1 or 2 degrees atmospheric warming ‘in the pipeline’.

And then there are positive carbon feedbacks, not just on land, but also very much in the ocean – and these make us question the feasibility of stabilising atmospheric CO2 concentrations, and throw the image of the ocean as our dear eternal CO2 sink overboard:

Firstly warming water rereleases part of the gas-form CO2, while increasing acidification (dissolved CO2, H2CO3) may dissolve calcium carbonate deposits, again releasing CO2. A feared net plankton decrease, or any net decrease in marine biological activity decreases structural CO2 sequestration (as it decreases ‘marine snow’) and eventually, if the heat reaches the deepest, benthic zone of the oceans, locally this may destabilise methane clathrates – something paleoscientists know could happen, by studying mass extinctions in Earth’s geological history.

So, the oceans create very many forms of climate inertia. First it seems a good thing, but it the long run it may bite us badly – and yes, that’s long beyond a point when we should have agreed on implementing emission reductions, our current timeframe.

Heat equivalent to 40 times current atmospheric warming caught somewhere in the first 2 kilometres of the ocean column

Now let’s speed this up a bit to get to the actual processes. With all that CO2 and all that heat that’s being absorbed (40 times as much as the heat that’s warming the atmosphere!) the oceans are still really, really deep (and of course much denser than our gaseous atmosphere) – and their conveyor belt of interconnected surface-to-bottom-to-surface ocean currents is majestically slow (and possibly even slowing down).

So, the heat of anthropogenic climate change has not reached rock bottom yet. That’s at least what a 2014 publication in Nature Climate Change deduces from ocean thermal expansion calculations, because floating thermometer buoys and direct temperature satellites are very useful if you want to measure the top water layer, but little use if you want to monitor ocean warming below 1 or 2 kilometres depth.

This publication, led by William Llovel of the California Institute of Technology (UCLA) is titled ‘Deep-ocean contribution to sea level and energy budget not detectable over the past decade’ and for now that’s all we need to cite to make the following point: although the oceans (by absorbing both CO2 and heat) decrease the speed of warming of the atmosphere, icesheets and continental land surface about twentyfold, all that trapped heat [an equivalent energy to heat the atmosphere about 40 times to its current +1 degrees level – not the same as warming to +40 degrees because climate sensitivity is logarithmic] is still stuck somewhere afloat halfway in the ocean’s internal stratification.

For those who are doubtful: yes, the upper 2 kilometres of the ocean are already warming – and rapidly so, as the below graph from the same publication illustrates:

The heat will get to the bottom though eventually, the behaviour of water masses is predictable to some extent, knowing they conduct and also knowing there is mixture going on, from the very surface waters to the deep bottom layers of the ocean – especially as we’re sure you’re aware, around ‘deep water formation’ zones off Antarctica and off Greenland in the North Atlantic.

So, the scientific question may be ‘when will there be deep ocean warming and to what extent’, and that was answered in 2017 by the group led by Sweetman.

Well firstly, it’s a gradual process these authors remind us. Here’s another key number: first the oceans absorb some 93 percent of the extra heat that’s currently trapped in the atmosphere by elevated greenhouse gas concentrations. Of this total ocean heat uptake 19 percent has gone to the deep ocean, that is to a depth below 2 kilometres.

And there of course you have a very deep water body left that needs a lot of heat to warm up. But it does already, if at least you measure very carefully: between 0.01 and 0.1 degrees per decade in the abyssal zone, and happening faster at the abyssal seafloor in the North Atlantic, Southern and Arctic Oceans, because that is where MOC turnover occurs, and deep or bottom water is ‘formed’.

Now also a slow trend adds up, the authors in the Elementa publication illustrate: by the end of the century the abyssal ocean will have warmed by 0-1 degrees Celsius, showing a geographical spread per ocean zone, with even some local deep ocean cooling projected in the Atlantic:

Above image shows modelled environmental changes at the deep seafloor in the year 2100. Modelled changes in temperature (°C), dissolved oxygen, pH, and ‘POC flux’ (the flux of particulate organic matter to the seafloor – an important environmental indicator for food supply) that could be seen at the at the deep seafloor by 2100 relative to present-day conditions.

The bathyal zone meanwhile will warm faster, locally reaching temperatures of +4 degrees by the year 2100 – but again under substantial geographical spread.

Apart from warming, there is ocean acidification, deoxygenation – and a huge decrease in deep-ocean food availability

Now if we look at other climate change-related impacts on the deep ocean, we see a similar geographical pattern emerging as for temperature. Not because ocean warming and ocean acidification would be directly correlated [they’re not, unlike ocean warming and ocean deoxygenation – see top right map in the image above] but because both follow the same pattern of ocean currents, and therefore take equally long before they spread out across the deepest ocean layers.

Ocean acidification happens fastest in the Arctic, but eventually acidification will spread out across similar lines as (relative) ocean warming – and by the year 2100 in bathyal depths (up to 3 kilometres deep) a pH reduction of 0.29 to 0.37 is expected – with largely unknown ecological effects.

Deoxygenation, a decrease of the ocean oxygen content, leading to ocean anoxia (‘dead zones’) is an important consequence of climate change. It is caused directly by warming waters, but in the deep ocean it is also increased where rapid surface ocean warming disrupts turnover currents, as climate change could lead to increased thermal stratification of the oceans – decreasing oxygen transportation from the photosynthetic upper ocean. In bathyal depths the oxygen content could go down by several percents and that is yet another factor that may tip a fragile ecosystem balance.

Decreasing marine snow. Or, hang on a second – there’s plenty! It’s sadly not digestible though…

Now the last important environmental parameter that the research group of Sweetman focused on is ‘particulate organic matter’ or POC – as it’s carbon that measured, and not just at any depth, but the POC flux to the actual benthic zone, so the bottom of the oceans, where all of life depends on what ecologists sometimes refer to as ‘marine snow’ – dead biomass from biologically active upper ocean layers, that’s slowly settling for a seaman’s grave, to re-enter the (deep ocean) marine food chain.

The POC flux, as it’s centred on the element of carbon, is of course of huge significance to Earth’s carbon cycle and therefore the entire climate system. But before we would be concerned about yet another positive climate feedback in form of this declining flux of carbon to the seafloor, let’s not forget that the seafloor is an ecosystem, or rather a tapestry of ecosystems, that stretches across some 70 percent of Earth’s surface, the blue planet, at this multiple-kilometre ocean depth: the benthic ecosystems.

Here many organisms live that humans have not yet discovered – and these have probably lived there for much longer than us, that one great ape that accidentally fell out of a savanna tree, kick-starting a cascade of events until, well, you’re here, reading this article – and consequently our planet is entering the Anthropocene.

The benthic ecosystems are fragile, because they are so stable. For thousands, possibly millions of years climatic changes in (parts of) the atmosphere can blow by largely unnoticed, and although temperatures are very low and so is oxygen, as long as these values don’t fluctuate, the organisms are perfectly well adapted – at very low metabolic rates and simply swallowing food particles when these do float by or suddenly appear on the seafloor right in front of where you are crawling to (if you are a sea cucumber or a brittle star). It’s the ‘simpler times’ that some humans too seem to long for.

But what happens when suddenly this food supply of slowly settling biomass from all the ocean layers above decreases by 40 to 55 percent? Well, we can’t measure into the future, but that is the projection for the Indian Ocean at least by the end of this century, and also elsewhere in Earth’s oceans the POC flux is set to decline dramatically as a consequence of climate change, Sweetman and his 21 other marine scientist co-authors warn.

The ecological effects of climate change on the benthic ecosystems. Deep-ocean marine biodiversity depends on maintaining environmetal stability, stability of temperature, oxygen – and food supply.

All in all it is very clear that anthropogenic climate change will have major effects on the Earth’s oceans, possibly effects that are no less dramatic than the ones we’ll experience on land – albeit it after a somewhat longer delay, due to the various forms of ocean climate inertia we discussed.

Although, what is a century right? We are talking about the year 2100 – and looking at ecological changes that will define Earth’s entire fossil record for millions of years to come.

But what is a century if you compare to the present?

That’s why we want to finish this piece by comparing the marine climate crisis to another marine ecological crisis – one that’s only recently managing to receive broad public attention, perhaps partly because it is so banal: plastic. Our plastic waste – the plastic waste of 7.5 billion human inhabitants, that through endless indirect routes including washing machines, sewer systems and all the world’s rivers, is dumped in these same oceans.

Graphs of escalating environmental problems often have this shape. You’re looking at global plastic production, with the red line showing discarded plastic – a large part of which ends up in waterways and ultimately the oceans. What is different if you compare to other environmental problems is the timescale: all this is happening between 1950 and 2050 – we’re creating a plastic planet in only a century’s time.

Last July a study led by industrial ecologist Roland Geyer of the University of California, Santa Barbara appeared in the journal Science Advances titled ‘Production, use, and fate of all plastics ever made’ – an ambitious attempt to quantify the plastic problem, including a projection of future trends, as shown in the graph above.

This study estimates that since the year 1950 humans have produced 9.1 billion tonnes of plastic, 7 billion tonnes of which are no longer in use – of which 5.5 billion tonnes is littered on land and in waterways. Because so much of this plastic litter ultimately ends up in the oceans, often as very tiny pieces, just one century after the start of mass production, by the year 2050, it is estimated that ocean plastic will outweigh the total weight of all the fish in these same oceans.

Think about it.

If current trends continue by 2050 there will be more plastic than fish by weight in all of Earth’s oceans, the World Economic Form warned in a special report about ocean plastic, published earlier this year.

And what’s possibly worse: whenever you catch a fish – you’ll catch both, because the fish will have eaten some of that plastic. It will never leave the food chain again, together with all the poisonous chemical substances it contains.

To develop a sense of the geographical scale of plastic pollution a special team led by marine ecologist Alan Jamieson of Newcastle University, a specialist in deep-ocean environments who had earlier researched organic pollution in the world’s most remote ocean habitats, the deep ocean trenches of the Pacific.

This team of researchers is uniquely equipped because they have developed special ‘deep-sea landers’ that can freefall to the ocean floor, even in trenches over 10 kilometres deep, and carry out various sampling in the benthic zone. (The same team cooperated with the crew of BBC’s Blue Planet documentary, to create the beautiful camera shots of the deepest ocean trenches, showing their unique biodiversity.)

Last week the team published their results with a Newcastle University press release, presenting findings after studying the 6 deepest places on Earth, spanning the entire Pacific Ocean – which is, as we’re sure you are aware, the world’s largest, almost a hemisphere wide in its widest part and ranging from the Arctic Bering Sea in the North to the Southern Ocean around Antarctica in the South.

The trenches researched for plastic contamination are the Mariana, Japan, Izu-Bonin, Peru-Chile, New Hebrides and Kermadec trenches.

Now as we’ve learned from the climate studies we discussed above, marine life in these deepest ocean parts is fully dependent on ‘marine snow’ – the fallout of biomass from the upper ocean, as a food source for the entire deep-ocean food chain. Now food is already naturally very scarce in the deep ocean and under the consequences of anthropogenic climate change the marine snow to these benthic ecosystems is set to decrease even further.

This means that life forms that live at these great depths will have to swallow anything that floats by, not aware that these days not all of the marine snow is of biologic origin, unaware that this new substance called plastic is both poisonous and indigestible.

As so much of the ocean plastic is ultimately shredded in the waves to microsize particles before they lose buoyancy, increasing amounts of single plastic fibers and microplastic particles sink to the deep ocean, where they are swallowed by various organisms.

These amphipods were collected from the bottom of the world’s deepest place, the Mariana Trench, at almost 11 kilometres depth. In 100% of the sampled organisms in this extremely remote environment the researchers found plastics.

Blue man-made fibre, collected from the stomach of an amphipod (one of the crustaceans shown above) from the Mariana Trench.

The shock-result of the Newcastle team is that in the crustaceans and other life forms that they dug up from these deepest, remotest places they found omnipresent plastic pollution, ranging from 50 percent of all organisms in the New Hebrides Trench to a staggering 100 percent in the organisms that were brought to the surface from the Mariana Trench, the deepest place on Earth.

And ‘plastics’ is a very broad mixture of related man-made substances, all of which are found in the deep trenches we read: “The fragments identified include semi-synthetic cellulosic fibres, such as Rayon, Lyocell and Ramie, which are all microfibres used in products such as textiles, to Nylon, polyethylene, polyamide, or unidentified polyvinyls closely resembling polyvinyl alcohol or polyvinylchloride – PVA and PVC.”

As the first (New Hebrides) is on the southern hemisphere and the second (Mariana Trench) on the northern, we were wondering, is there (like atmospheric warming) a hemisphere difference, with the northern hemisphere being more affected?

On the surface of the ocean, plastic is caught up in wind-driven currents. Part of the North Pacific Gyre is notoriously referred to as ‘the plastic soup’ – but sadly the problem is far more widespread. Image: CBC.

We decided to ask lead researcher Alan Jamieson a couple of questions, to clarify the situation for us:

Q (Rolf): You write that you find 100% plastic contamination at the bottom of the Mariana Trench and 50% in the New Hebrides Trench. Am I right to think there is a hemisphere difference (as plastics on surface would follow trade winds, therefore stay either North of South of the equator)?

A (Alan): Actually no, these values were the lowest and highest, we included Japan, Izu-Bonin, Kermadec and Peru-Chile trenches and these were all ranging from 70 to 80%, so we can’t say anything about hemisphere differences.

Q: Which current system in the Pacific would you say is most contaminated? And how diffuse is the source of the problem? (Mariana plastic – would these come from Asia predominately, or from North America?)

A: Very hard to say due to mass gyres and so on, the salient is that they are there.

Q: Do plastics sink to the bottom of trenches, or are there actually upwelling & downwelling currents at play?

A: There is very little, if any at all upward current, so once something has reached negative buoyancy it will simply sink.

That’s bad news, because it implies the plastic already down in the trenches is there to stay, while of course more will follow, slowly accumulating in the benthic ecosystems of the world’s oceans.

But we asked for another reason, to find out if deep-ocean plastic follows the same transportation routes as anthropogenic climate change. We know ocean acidification, just like heat and deoxygenation, use the thermohaline circulation or Meridional Overturning Circulation, the famous ‘conveyor belt’ of interconnected ocean currents, to eventually reach the bottom waters of the deep ocean.

Plastic does not. Or at least, plastic has a much more independent means of transportation: plastic simply falls down at the point where it loses its buoyancy. And that’s a big difference for the benthic ecosystems, because it means the plastic crisis hits home as we speak, while deep-ocean climate change effects are set to escalate over the course of the century or even beyond…

So, we come to the inevitable, impossible question of this piece:

What’s worse, plastic pollution, or climate change?

In this article we’re talking about the deepest and most remote parts of Earth’s ocean, to discover no place is left untouched by plastic pollution. What we don’t write about here is the situation on the surface, populated coastlines, islands, coral reefs, inland seas – where of course it’s worse still.

We’ve all seen those images. A seagull, an albatross, their tiny hatchlings, a turtle – even giant whales, starved to death, because their stomachs were filled with huge amounts of indigestible plastic. It’s an image of death, on a global scale that points to one source: plastics.

If we compare these crises to the oceans, plastic pollution and climate change, then let us acknowledge that indeed ocean plastic is a new crisis, not just in public awareness, but also in its development.

Lest we forget, the climate crisis has been in the making ever since the onset of the Industrial Revolution in the 18^th century when large-scale coal burning first took off, later followed by other fossil fuels as the energy demand of the global economy kept growing, as it still is today. We are increasingly feeling its consequences, but that’s nothing compared to what future generations will have to deal with, because climate change is an inert process – and as explained in this article, that’s very much because of its complex interactions with the oceans.

Plastics however, have only been mass-produced since the 1950s – and have since managed to contaminate every place on Earth.

This is not to say that the plastic crisis is worse than the climate crisis, it is not – the disturbance of climate change is going much deeper, melting entire ice sheets, raising global sea levels by 29 to 55 metres, forcing entire biomes to migrate and eventually also touching the deep ocean with unimaginable consequences of acidification, anoxia, food declines and melting methane clathrates – even the system of ocean currents could change, come to a grind.

The plastic crisis is faster evolving though, and in that sense equally urgent, if Earth’s ecological health is your concern (as it is ours).

Comparing these two environmental crises serves another purpose, to show that both climate change and plastic pollution are not problems, but symptoms – symptoms of another, underlying problem: us. Humans. Our inability to live sustainably with the planet that gave us our lives and still feeds us today, our non-circular global economy; simply dumping our waste in increasing amounts, be it greenhouse gases to the atmosphere, plastics to the oceans – or any other by-product of our modern civilisation that we still fail to recycle within our own ecosystem.

Welcome to the Anthropocene.

Let’s show some character and rewrite this unfolding ugly piece of Earth’s geological history – and create a better picture of us, that one great ape that started this unfortunate cascade. You have means, wherever you live.

© Rolf Schuttenhelm | www.bitsofscience.org