How Does the Ocean Regulate
the Global Climate?
To mark World Ocean Day and in connection with the United Nations Ocean Conference (UNOC), the Office for Climate Education (OCE) presents a series of four articles to explore the theme “Ocean and Climate” in the classroom.
This first article explores the ocean’s regulatory role through three key mechanisms: ocean circulation, stratification, and the El Niño phenomenon. These are essential concepts to introduce to students in order to understand the major climate balances.
-
Ocean Circulation
The ocean absorbs most of the solar energy received by the Earth, especially near the equator. This heat is then redistributed around the globe via ocean circulation, which is driven both by winds—creating surface currents—and by differences in water density, which generate the thermohaline circulation.
In fact, the colder or saltier seawater becomes, the denser it is, causing it to "sink" into deeper layers. This process occurs mainly in high latitudes, where heat loss to the atmosphere and sea ice formation cause significant changes in temperature and salinity.
This circulation forms a loop: in the Atlantic, where it is strongest, warm surface waters move northward across the equator, before cooling and mixing into the deep ocean at higher latitudes. Returning southward in the deep ocean, these waters cross the equator again to reach the Southern Ocean around Antarctica, before eventually resurfacing centuries later and rejoining the surface currents in the Atlantic.
But this "ocean machine" is under threat. Warming surface waters and freshwater input from melting ice at high latitudes reduce water density, slowing down this circulation. This could lead to major climate disruptions.
Ressources vidéos
- The régulating role of the ocean on the climate - Sébastien Carassou | OCE
- The oceanic machine – Sébastien Carassou | OCE
- Thermohaline circulation explained – Jean-Baptiste Sallée | OCE
-
Stratification and Deoxygenation
In the ocean, water masses are arranged in different layers according to their density, which varies with salinity and temperature. This is called ocean stratification. With climate change, surface waters are becoming warmer and less dense compared to deeper waters. This increased contrast in density and temperature between surface and deep layers reduces mixing, disrupting the exchange of oxygen and nutrients between layers.
By 2090, in all scenarios studied by the IPCC, projections show a decline in oxygen and nutrient levels in the ocean, which will have significant effects on marine life.
Classroom Activity Ideas:
- The impact of temperature on ocean currents
- Understanding thermal inertia
- Marine currents regulate the climate (advanced students)
-
The El Niño Phenomenon: An Example of Climate-Ocean Interaction
In some years, abnormally warm conditions develop in the central and eastern Pacific. This event, known as El Niño, results from an unusual energy exchange between the ocean and the atmosphere. It occurs fairly regularly, every few years, and causes major global impacts for both people and the environment.
On the coasts of Peru and Chile, normally abundant fishing halts during El Niño, as nutrients transported by cold deep waters can no longer reach the surface to feed fish. In the western Pacific, the usual intense rains shift eastward, causing devastating droughts in Indonesia, the Philippines, and Australia. Summer monsoons, which half of the world’s population depend on, are disrupted—especially in China, India, Australia, the Sahel, and Brazil.
Scientists believe that extreme El Niño events may become more frequent in the 21st century, due to ocean warming.
Additional Resources