Climate change dampens oceanic carbon sink

Oceans absorb a significant proportion of the CO2 emitted by humans from the atmosphere. Therefore, oceans have a significant effect on the amount of greenhouse gases in the air and, consequently, on the climate. But the service that the seas provide to humankind is under threat: climate change is making the oceans less and less efficient at absorbing CO2. Corinne Le Quéré, professor of climate change science at the University of East Anglia in the UK, accurately maps out the complex interaction between the oceans and the atmosphere. In 2020, she received the Dr A.H. Heineken Prize for Environmental Sciences for her work.

Lees dit interview in het Nederlands (NewScientist)

Every day, climate scientist Corinne Le Quéré uses advanced models to study how the world’s oceans respond to the changing climate. A little ironic: she only saw the ocean for the first time when she was eighteen. But her interest in the environment had already been aroused. ‘I grew up in the middle of the forests in Canada,’ says Le Quéré. ‘I was always very close to nature and often went camping in the wild during the holidays. My entire childhood was the true Canadian holiday that everyone dreams of. When I first saw the ocean at 18, it was fascinating, but my interest had already been piqued from growing up so close to nature, enjoying the weather and the elements.’ When Le Quéré went to university in the 1980s and 1990s, environmental science was still an unknown field of research. Le Quéré studied physics and went on from there to study oceanography and climate.

‘The oceans are doing us a great service,’ says Le Quéré. ‘They absorb about a quarter of the CO2 that we emit and therefore have a dampening effect on climate change. But this carbon reservoir is in turn susceptible to climate change, through changes in sea surface temperature, wind strengths, ocean circulation, and marine ecosystems.’ Le Quéré maps out this complex interplay and translates it into computer models, with which she studies how these processes have changed in the past, and what we can expect for the future.

In the depths
The oceans currently act as a so-called carbon sink: a carbon reservoir that absorbs more carbon than it emits. As a result, they slow down climate change. But at the same time, this carbon sink function is under threat from climate change. To understand why, it is first useful to know why the oceans currently act as a carbon sink. Human CO2 emissions increase the concentration of CO2 in the atmosphere. This leads to a difference in the ‘pressure’ of CO2 between the atmosphere and the ocean. To cancel out this difference, CO2 dissolves in the ocean. ‘What makes the ocean so absorbent are the chemical reactions that convert the CO2into bicarbonate,’ says Le Quéré. The benefit of this is that it isolates the carbon from the atmosphere. The bicarbonate is then transported to the depths of the ocean by ocean currents. This allows the ocean to continue to absorb new CO2. This process slows down climate change through CO2 emissions. The drawback is that this process also acidifies the ocean.’ This acidification is bad news for some organisms. For example, certain shells and fish bones may dissolve due to the higher acidity level. ‘We do not yet know what the consequences of this will be for the carbon cycle, but in any case, it will have a great impact on the ocean ecosystem.’

The factor that most determines the amount of CO2 the ocean can absorb is the transport of carbon into the depths. Because when it is removed from the ocean surface, the concentration there becomes lower, and the ocean can absorb new CO2. But that is not the only factor. ‘Climate change is warming the atmosphere and therefore surface water,’ says Le Quéré. ‘And CO2 is less soluble in hot water than in cold water.’ So, it is bad news: by warming up the climate, we are reducing the ocean’s absorption capacity, which we so desperately need to limit climate change.

Against the wind
Early in her career, Le Quéré made an important discovery. She was the first to use observations to show that the efficiency of CO2 absorption by the ocean had been reduced. ‘I analysed the Antarctic Ocean because the wind had increased there. This extra wind causes deep, carbon-rich waters to rise to the surface faster than normal. As a result, more natural carbon is emitted back into the atmosphere, and less man-made CO2 is absorbed.’

The innovative aspect of this research was that Le Quéré did not look at measurements in the ocean itself, but rather at the CO2concentrations in the air above the ocean. ‘We collected atmospheric measurements taken over a period of 25 years. Then we used a method called ‘inversion’. Suppose you measure the CO2 concentration at point A and point B, and the wind blows from A to B. If the CO2 concentration in point A is higher than in point B, you know that CO2 has ‘disappeared’. That CO2 has been absorbed by the ocean. So, by comparing two concentration measurements at a time, you can determine how much CO2 has been absorbed by the ocean between those points. We collected as many measurements as possible of CO2 concentrations around the Antarctic Ocean and combined them with weather data. This enabled us to demonstrate that the ocean’s carbon absorption had not increased, while CO2 emissions had risen by 40%. If efficiency had remained the same, carbon absorption should have followed the emissions, but it did not.’

The hole in the ozone layer was one of the main causes of the stronger winds in the area. It changed the circulation of air and accelerated the winds. ‘This raises a new major research question,’ says Le Quéré. ‘Because climate change also increases wind strength. In fact, climate change increases the winds in all seasons, while the hole in the ozone layer does so only in summer. I am currently trying to study how much the wind will increase in the coming period, and whether the effect on CO2 absorption will disappear when the ozone layer recovers or get worse due to climate change.’ Le Quéré studies the impact of climate change not only in the Antarctic Ocean, but all over the world. She was also the first to quantify the impact of climate change on the carbon sink function of the world’s oceans. ‘We demonstrated that it has become a little weaker due to climate change. A small part of this was due to warming of the ocean surface, but most was due to changes in the wind,’ says Le Quéré.

And then there is another important factor for the carbon cycle in the oceans: ecosystems. ‘These ecosystems live in the upper part of the ocean,’ says Le Quéré. ‘But dead organisms or excrement can sink to the depths of the ocean. This is all material that contains carbon, and so a lot of carbon is transferred from the ocean surface to the depths of the ocean. This is part of a natural cycle: ocean currents eventually bring this carbon to the surface elsewhere. Initially, this cycle was evenly balanced. But acidification, warming, and oxygen depletion have affected such ecosystems. This also affects the carbon cycle.’

But how exactly is not yet clear. That is why Le Quéré creates models to map the complex interactions within ecosystems. ‘We have developed a way of representing ecosystems that does justice to the diversity in ecosystems but is not too complex to model. To do this, we divide organisms into groups that function in a similar way, for example because they all have shells, eat the same kind of food, show similar behaviour, or have a similar size.’ Le Quéré focuses on micro-organisms. ‘When we think of marine ecosystems, we often think of animals like fish or whales. But they do not affect the carbon cycle that much. The small organisms play a much bigger role.’

For each of these groups of organisms, Le Quéré and colleagues collect observations – for example, of their growth as a function of temperature or the amount of nutrients available. They do this in the laboratory or with measurements from ships. All this information is included in the model and used to calculate various scenarios. At present, they do not foresee any major changes in the carbon sink function as a result of changing ecosystems. But whether this will remain so in the long run is questionable. ‘We know that there have been major shifts in ecosystems in previous geological eras,’ says Le Quéré. ‘There have been situations in which organisms reuse all the carbon-rich material on the surface. Hardly any carbon sank to the bottom.’ Such a situation would be anything but favourable, as no carbon would be transported to the depths. ‘We are closely monitoring the ecosystems, to understand if or when this will happen again.’

Modelling ecosystems is not easy. ‘You do not have natural laws like in physics,’ says Le Quéré. ‘It is much more unpredictable. You really have to observe ecosystems and try to mimic that behaviour in a model. When I first published my model based on the groups of similar organisms, two critical publications immediately responded. One claimed the model was far too complex; the author talked about “running before we can walk”. The other said that our model was far too simple. So, I thought: we have probably got it about right’, Le Quéré laughs. ‘They were both a bit right, of course. But now, some 17 years later, there are many new options. In addition to more computing power, we have, for example, underwater cameras that continuously take pictures in the ocean. With artificially intelligent computer programmes, we can detect which organisms are in a photo. This allows us to automatically process millions of observations.’

In the coming years, Le Quéré hopes to learn more about the stability of ecosystems. ‘I push my model to the limit, with, for example, extreme temperatures, acidification, oxygen depletion, or pollution, and see when the ecosystems collapse. This way, I can analyse how things go wrong in these cases. Then we can look in the ocean to see if there are any indicators that things are going in the wrong direction.’

In addition to her research, Le Quéré also advises governments on their CO2 emissions and how to deal with climate change. ‘I think it is very important for scientists to speak out, to show the evidence, so that governments and people can make informed decisions. That is why, throughout my career, I have spent about one day a week making science available to the public, and especially to policy makers.’ Currently, Le Quéré has a seat on the Climate Change Committee in the UK and chairs the French High Council on Climate. These are both independent advisory bodies to the governments. ‘There is much evidence that the recommendations of these commissions have accelerated action on climate change’, says Le Quéré.

In addition, Le Quéré initiated the Global Carbon Budget with colleagues in 2004. ‘At an annual meeting of the Global Carbon Project, the alliance of all global research on the carbon cycle, we discussed what the research community could do to support policy makers. Policy makers meet annually in climate summits, but we in the scientific community only issue an IPCC report once every six or seven years. To fill this gap, we decided to publish an update every year on how much CO2 was emitted the previous year and where that carbon ended up: in the atmosphere, the ocean, or on land.’ Le Quéré directed this publication, which was christened the Global Carbon Budget, for thirteen years. ‘It started out as a way of advising policy makers, but eventually it also proved to be a great boost for research regarding the carbon cycle.’

Corinne Le Quéré (Magog, Canada, 1966) studied physics at the University of Montreal in Canada. She then obtained her master’s degree in atmospheric and oceanic sciences from McGill University, also in Montreal. In 1999, she obtained a PhD in oceanography from Pierre and Marie Curie University in Paris (now Sorbonne University). Following several positions, she was appointed Royal Society research professor of climate change science at the School of Environmental Sciences at the University of East Anglia in Norwich, UK, in 2019. Le Quéré initiated the annual publication Global Carbon Budget in 2004 and was author of several IPCC reports. She currently has a seat on the Climate Change Committee in the UK and chairs the French High Council on Climate.

Corinne Le Quéré studies how the carbon cycle in our oceans is changing under the influence of climate change. Every year, the oceans absorb an average of one quarter of the CO2 emitted by humans from the atmosphere. This carbon reservoir is in turn susceptible to climate change, through changes in sea surface temperature, wind strength, ocean circulation, and marine ecosystems. Le Quéré studies the interactions between these components in the recent past, and makes projections for the future. She was the first to demonstrate that increases in winds in the Antarctic Ocean are leading to less efficient absorption of CO2, and linked this to the depletion of the ozone layer. Le Quéré currently focuses on studying the different processes and ecosystems in the sea with great details. Thanks to new measurement techniques, she can create a complex model of how marine ecosystems respond to climate change and how this affects the carbon cycle.

Heineken Prizes
Every two years, the Royal Netherlands Academy of Arts and Sciences awards the Heineken Prizes to five renowned international researchers and one artist. The first of the prizes, the Dr H.P. Heineken Prize for Biochemistry and Biophysics, was established in 1964 by Alfred H. Heineken, in honour of his father, Dr Henry P. Heineken. To this award were subsequently added Heineken Prizes for Art (1988), Medicine (1989), Environmental Sciences (1990), and History (1990). The daughter of Alfred Heineken, Charlene L. de Carvalho-Heineken, is continuing this tradition. The C.L. de Carvalho-Heineken Prize for Cognitive Sciences (2006) is named after her.


Corinne le Quéré — Oceanographer