“The ocean is a giant thermostat for the planet”

This article comes from the journal Science et Avenir – La Recherche en ° 893 dated July and August 2021.

Science and Avenir: Why do climatologists speak of “ocean” in the singular?

Eric Guiliardi: As for climate, all water bodies in the ocean are connected and the role of the latter within the “climate machine” is global: therefore there is only one. On the other hand, geography distinguishes the Atlantic, Pacific and Indian oceans and several seas (Nordic, Mediterranean…) which are separated by continents, connected by straits and canals and which are the historical and present theaters of human activities. The climatic role of the ocean and its regional variations follows another division: latitude, the Southern Ocean, the distribution of water masses at depth, the eastern and western sides of the basins, etc. The effects of climate change involve a combination of geography and climate.

What is its interaction with the atmosphere?

The tropics receive more heat than the higher latitudes. For a stationary system, this excess heat is carried to the poles. This role is played by the two liquid envelopes of the planet, the atmosphere and the ocean. It is the main contributor to tropical and sub-tropical regions, and the atmosphere tends to dominate at mid and high latitudes.

However, if the oceans and the atmosphere work together, they do not have the same climatic properties. In pairs formed with the atmosphere, the ocean is the slow participant because it has a 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 km). . An average of 4000 meters deep, its thermal inertia is enormous, and makes it the real heat reservoir of the climatic machine.

In addition, much of this heat store is isolated from the atmosphere. Indeed, the ocean is opaque because the solar flux penetrates it only a few tens of meters. It is also heated from above, which tends to freeze it, with hot water remaining on top of cold water.

The ocean is therefore strongly “stratified”, reducing the exchange between the deep ocean and the surface. The difference in inertia between the ocean and the atmosphere means that an atmospheric depression has a lifetime of a few weeks, while oceanic depressions can be followed for many years at the surface, and many paths of deep circulation can be traced over several hundred years. is applied.

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What is the exact role of the ocean in climate?

It’s double. First, it controls the rapid movement of the atmosphere, “smoothing” them in time and space. Thus the ocean is a giant thermostat for the planet. For example, heat stored in the surface ocean in summer is returned to the atmosphere in winter, which explains, for example, the winter mildness of the oceanic climate of Brittany, more continental than that of Alsace. The ocean then plays a major role in the slow variations in climate. The El Nio event, this warming of surface waters every three to seven years in the tropical Pacific, illustrates this role of the ocean particularly well. On a decadal scale, ocean circulation also experiences upheavals that leave their mark on the global climate. We know that drought in the Sahel in the 1970s and 1980s was associated with higher sea surface temperatures in the North Atlantic.

What changes is caused by an increase in human greenhouse gas emissions?

The ocean, due to its inertia and deep water mass, absorbs more than 90% of the additional warming and about 30% of the carbon dioxide that we emit. For thirty years, the absorbed heat has more than doubled. The heat in the first two kilometers of the water column has mainly occurred around Antarctica, a region that has absorbed a third of the heat accumulated over the past fifty years.

The first consequence is that water expands as it warms and raises global sea levels. In addition to this thermal phenomenon, the melting of continental glaciers and the influx of fresh water from the capes of Greenland and Antarctica is increasing.

Currently, the contribution of glaciers and thermal expansion results in an overall rise in sea level of 3.6 mm per year. And even in the event of a sharp reduction in greenhouse gas emissions, this rate should accelerate due to the thermal inertia of these water masses.

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first 2 to 3 meters of sea It contains as much heat as the atmospheric column that hangs them at a height of more than 80 km. (Jumpitor/Getty Images)

What do we see today?

Since heat is mainly absorbed by surface layers, the temperature difference increases with deeper regions, making vertical water exchange more difficult. The ocean becomes even more stratified, especially in its first 200 m. As a result, nutrients no longer rise from the depths and oxygen penetrates less from the surface, a combination that affects biological life. There has also been an increase in “marine heat waves,” which see water temperatures rising by more than 2 degrees Celsius above normal for more than five days, affecting marine ecosystems such as corals. Finally, the absorption of carbon dioxide, the result of the combustion of fossil fuels, re-acidifies the ocean, with effects on marine life.

Will hurricanes come more often?

For tropical cyclones, the IPCC synthesis indicates a decrease in their number but the intensity of the most powerful. Indeed, warm ocean water is the energy source of cyclones, and the stability of the atmosphere is essential for their formation. So ocean warming creates a more powerful “engine” of cyclones, but scenarios also show that the atmosphere becomes more unstable, thus limiting their numbers.

What are the impacts on human activities that we should most expect?

Sea level rise will affect a quarter of mankind who live within 100 km of coasts and a few tens of meters above sea level. Strong storms and cyclones generate their effects further inland, increasing the risk to populations, especially the most vulnerable. Another area of ​​concern is fishery resources, although other factors are involved. As a result of changes in the ocean, fish move north at the rate of several kilometers per year. Water acidification affects the shells of animals at the base of the food chain, such as zooplankton. According to scenarios, this drop in primary production could lead to a 15% reduction in the overall biomass of all marine animals, including those on which humans feed, by 2100. Coastal ecosystems such as swamps, lagoons, mangroves, coral reefs are threatened by both rising temperatures, ocean heat waves and the increasing strength of cyclones. These natural environments of great biodiversity are also effective barriers against sea level rise and extreme events.

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Can scientists really differentiate between a natural weather phenomenon and a phenomenon exacerbated by climate change?

This is a research area which was opened about fifteen years ago. Scientists call this the “attribution of extreme events.” To what extent was a strong heatwave, a major flood, an extraordinary storm partly caused by global warming? To answer this question, we run simulations with and without anthropogenic effects and analyze the difference in the likelihood of such events occurring. We find for example that “century” storms in a given region may now occur every ten years or that the duration of heat waves has increased from 3 days per year in France before the 1990s to 6 or 7 today. … and as of 30 in the future for scenarios in which there is no reduction in emissions.

Bio Express D’Rique Guillardi:

1966 Born in Paris.

1997 Thesis in Cerfacs (University of Toulouse-III) and Locean (CNRS, Sorbonne University, IRD, MNHN) on the climate role of the ocean.

2012-2014 Lead author of the 5th IPCC report.

2015 to issue What would you do if you knew? Climatologists are facing disinformation, Essays in collaboration with Catherine Guillardi (Le Pomier ed.).

2018 Lokian- Deputy Director of IPSL.

2018 lu Highly cited researcher.

2020 President of the Office for Climate Education, which develops educational resources for teachers.

to know more :

Find ric Guillardi in the summer issue Science and the Essentials of the Future – Research (N ° 106) is dedicated to the great scientific discoveries that have changed the world, from X-rays to atoms, from black holes to Pompeii, to global warming. He gives examples of discoveries made possible by studying the effects of humans on climate. This number also allows us to discover that Einstein was not a lone genius, no more so than Galileo, Newton or Marie Curie. Historians show the opposite that discovery is a collective adventure, where chance sometimes gives a boost.

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