This article comes from the journal Sciences et Avenir – La Recherche no. 893 of July and August 2021.
Sciences et Avenir: Why do climatologists talk about the “ocean” in the singular?
Eric Guilyardi: For the climate, all the water bodies of the ocean are connected and the role of the latter in the “climate machine” is global: therefore there is only one. On the other hand, geography distinguishes the Atlantic, Pacific and Indian Oceans and many seas (Nordic, Mediterranean …) which are separated by continents, connected by straits and canals and which are historical and current theater of human activities. The climatic role of the ocean and its regional variations follow another division: latitude, Southern Ocean, distribution of water masses at depth, eastern and western edges of basins, etc. The effects of climate change include a combination of geography and climate.
What is its interaction with the atmosphere?
The tropics get more heat from higher latitudes. To have a stable system, this excess heat is transported to the pillars. This role is performed by the two fluid envelopes of the planets, the atmosphere and the ocean. This is a major factor in tropical and subtropical areas, and the atmosphere takes on medium and high latitudes.
However, if the ocean and atmosphere work together, they do not have the same climatic properties. In a pair formed with the atmosphere, the ocean is a slow partner because it has strong thermal inertia: its first two to three meters contain as much heat as the entire atmospheric column that overhangs it (i.e., about 80 kilometers). On average 4000 meters deep, its thermal inertia is therefore huge and makes it a real heat accumulator of the air conditioning machine.
In addition, most of this heat storage is insulated from the atmosphere. In fact, the ocean is opaque because the solar current penetrates only a few tens of meters. It also heats up from above, which tends to stabilize it, and the hot water stays above the cold water.
The ocean is therefore strongly “stratified,” which accordingly reduces exchanges between the deep ocean and the surface. The difference in inertia between the ocean and the atmosphere means that an atmospheric depression has a lifespan of several weeks, while ocean vortices can be tracked on the surface for several years, and multiple deep circulation paths span several hundred years.
What exactly is the role of the ocean in climate?
It’s double. First, it regulates the rapid movements of the atmosphere, “smoothing” them in time and space. The ocean is therefore a giant thermostat for the planet. For example, the heat stored in the summer in the surface ocean returns to the atmosphere in the winter, which explains, for example, the winter mildness of the oceanic climate of Brittany compared to that of the more continental, Alsace. Then the ocean plays a major role in slow climate variations. The El Niño phenomenon, this surface water warming that occurs every three to seven years in the tropical Pacific, illustrates this role of the ocean particularly well. Decadally, ocean circulation is also experiencing upheavals that are leaving a mark on the global climate. We know that the droughts in the Sahel in the 1970s and 1980s were associated with higher sea surface temperatures in the North Atlantic.
What changes with increasing greenhouse gas emissions in humans?
Thanks to its sluggishness and deep water masses, the ocean absorbs over 90% of the additional heating and about 30% of the carbon dioxide we emit. Over thirty years, the heat absorbed has more than doubled. Warming during the first two kilometers of the water column occurred mainly around Antarctica, a region that has absorbed between a third and a half of the heat accumulated over the past fifty years.
The first consequence is that water is expanding by warming and raising global ocean levels. With this thermal phenomenon, there is an increasing influx of fresh water from the melting of continental glaciers and caps of Greenland and Antarctica.
Currently, the contribution of glaciers and thermal expansion cause a total sea level rise of 3.6 mm per year. And that speed should be accelerated due to the thermal inertia of these water masses, even in the case of a sharp reduction in greenhouse gas emissions.
The first 2 to 3 meters of the ocean they contain as much heat as an atmospheric column that towers over them at 80 km above sea level. (JAMPITER / GETTY PICTURES)
Today, what are we observing?
As the heat is mainly absorbed by the surface layers, the temperature difference with the deeper zones is emphasized, so that the vertical exchange of water is more difficult. The ocean becomes even more stratified, especially in the first 200 meters. As a result, nutrients are no longer lifted from the depths, and oxygen penetrates less from the surface, a connection that affects biological life. There is also an increase in “sea heat waves” where water temperatures rise more than 2 ° C above normal for more than five days, affecting marine ecosystems such as coral. Finally, the absorption of carbon dioxide, the result of the burning of fossil fuels, acidifies the ocean, again with an impact on marine life.
Will storms be more frequent?
For tropical cyclones, the synthesis of IPCC indicates a decrease in their number, but an increase in the strongest. Indeed, warm ocean water is a source of cyclone energy and atmospheric stability is essential for their creation. Ocean warming is therefore causing a more powerful “engine” of cyclones, but scenarios also show that the atmosphere is becoming more unstable, limiting their numbers.
What are the impacts on human activities that we should anticipate the most?
Rising ocean levels will affect a quarter of humanity living within 100 km of the coast and several tens of meters above sea level. Stronger storms and cyclones, which will create their effects further inland, will increase risks for populations, especially the most vulnerable. Another area of concern is the fishery resource, although other factors are involved. As a result of the change in the ocean, fish are moving north at speeds of several kilometers a year. Acidification of water affects the shells of animals at the bottom of the food chain, such as zooplankton. According to the scenarios, this decline in primary production could lead to a 15% reduction in the total biomass of all marine animals, including those fed by humans, by 2100. Coastal ecosystems such as wetlands, lagoons, mangroves, coral reefs are threatened by rising temperatures, sea heat waves and increased cyclone force. These natural environments of great biodiversity are also effective barriers to sea level rise and extreme events.
Can scientists really distinguish what a natural weather phenomenon is from events exacerbated by climate change?
It is a research sector that opened fifteen years ago. Scientists call this “attributing extreme events”. To what extent is the strong heat, the great flood, the extraordinary hurricane partly caused by global warming? To answer this question, we conduct simulations with and without anthropogenic effects and analyze differences in the probabilities of such events. For example, we find that a “centenary” storm in a particular region can now occur every ten years or that the duration of heat waves ranges from 3 days a year in France before the 1990s to 6 or 7 today … and 30 in the future according to the scenarios without reducing emissions.
Biographical expression of Éric Guilyard:
1966 Born in Paris.
1997 Thesis at Cerfacs (University of Toulouse-III) and at Locean (CNRS, Sorbonne University, IRD, MNHN) on the climatic role of the ocean.
2012-2014 Principal author of the 5th IPCC report.
2015 Liberation What would you do to know? climatologists before misinformation, an essay in collaboration with Catherine Guilyardi (published by Le Pommier).
2018 Deputy Director of Locean-IPSL.
2018 Selected highly cited researcher.
2020 Chairman of the Office of Climate Education which develops educational resources for teachers.
To learn more:
Find Eric Guilyardi in the summer issue of the magazine Fundamentals of science and the future – research (No. 106) devoted to great scientific discoveries which turned the world upside down, from X-rays to atoms, from black holes to Pompeii, to global warming. It conjures up discoveries made possible by studying the impact of humans on the climate. This number also allows us to discover that Einstein was not a lone genius, just Galileo, Newton or Marie Curie. Historians, on the other hand, show that discovery is a collective adventure, where chance sometimes gives impetus.