This summer, people around the world have suffered extreme heat, droughts, hurricanes and flooding. What we once called Global Warming is now called Global Boiling by the Secretary General of the United Nations and others. Billions of people are vulnerable to the deadly effects. This includes Europeans who have enjoyed temperate weather due to the Gulf Stream, which brings warm water from the Gulf of Mexico. It is part of the Atlantic meridional overturning circulation, or AMOC, and a global conveyer belt.

Water circulates throughout the oceans of the world by moving through currents. Temperature, salinity, dissolved oxygen, carbon dioxide, pH and nutrients are all modified as ocean waters mix and interact with the land and atmosphere. Some of the ocean mixing occurs horizontally and vertically in a type of global conveyer belt, called the Meridional Overturning Current, or MOC. Water with relatively larger amounts of dissolved salts (high salinity) cools and sinks in the North Atlantic. When salt water freezes, only the water molecules solidify. They squeeze out an aqueous solution with higher salt (NaCl) content, which is heavier (denser) than the normal (3.5%) saline solution. Deep water returns to the surface in the Indian and Pacific oceans, due to upwelling.

There is a warm current and a cold, deep, high salinity current. These currents are driven by solar heat and by the properties of water. The equatorial sun warms the water, which evaporates and leaves the water saltier. The Gulf Stream carries warm, salty water from the Florida Straight, up the east coast of the USA and on to Europe. This reduces the difference in temperature between the pole and the equator. Throughout its northward journey, the warm water loses heat and becomes denser. As the water reaches the North Pole, strong winds evaporate off some of the water, leaving it saltier and denser. When the salt water freezes, only the water freezes and a large volume of saltier water sinks. Warmer, saltier water is pulled towards the poles and drives the conveyor. One global circuit takes over one thousand years. It is an important mechanism by which the oceans store and transport heat. The ocean stores more heat in the upper 3 meters than that of the entire atmosphere and it acts as a global heat engine.

Gases and nutrients are also mixed. The concentration of dissolved gases also depends on the temperature and pressure. Sunlight can only penetrate the top 200 meters of the ocean. This is where photosynthesis occurs during the day. Because of photosynthesis, there is more oxygen in the upper 200 meters, and is the environment is oxidizing. Carbon exists as carbon dioxide, iron as Fe(III), nitrogen as nitrate, and sulfur as sulfate. At depths below 200 meters, there is less and less oxygen and there is a reducing environment. There is more methane and less carbon dioxide. Below 200 meters, iron is mostly Fe(II), nitrogen is ammonium and sulfur is a mixture of hydrogen sulfide and the bisulfide ion.

The density of water is determined by its temperature and salinity. When ice freezes, it leaves a surface of more saline water. It becomes denser and starts to sink. This occurs near in the Arctic and Antarctic polar regions, as well as Greenland. At lower latitudes, this seldom occurs. That is because there is a thermal layer or thermocline that separates the upper layer from lower regions in the world’s oceans in all but the polar regions. Most of the heat energy of the sunlight that strikes the Earth is absorbed in the first few centimeters at the ocean's surface, which heats during the day and cools at night as heat energy is lost to space by radiation. The water near the surface layer is mixed by wind and waves.

This distributes heat to deeper water. As a result, the temperature is nearly uniform in the upper 100 - 200 meters, depending on wave strength and the existence of surface turbulence caused by currents. Below this mixed upper layer, the temperature remains relatively stable. The temperature of the deep ocean drops gradually with depth. There is no permanent thermocline in polar regions because both the surface waters and deep waters are very cold. Therefore, there is little temperature difference (or gradient) between the polar surface and deep waters. A small seasonal (summer) thermocline forms but vertical mixing occurs in all seasons. Water above and below the thermocline mix poorly.

At lower latitudes, a global ocean current, or conveyer belt mixes surface waters. Warm, tropical waters evaporate faster and the surface becomes more saline. Phytoplankton and other photosynthetic organisms deplete the upper layer of nutrients. Dead organisms sink to the bottom, decay and make the bottom rich in nutrients. The deep ocean is a huge reservoir of stored nutrients. Organic nutrients are re-mineralized and solubilized. Then they are carried up to the surface. At the same time, winds and waves re-circulate nutrients near the surface.

The North Atlantic Ocean is saltier and colder than the Pacific because it is much smaller and is somewhat isolated by North and South America on the west and Europe and Africa on the east. The warm water of the Great Conveyor Belt evaporates out of the North Atlantic, leaving behind saltier waters, and the cold continental winds off the northern parts of North America cool the waters. Salty, cool waters settle to the bottom of the sea, most at a point a few hundred kilometers south of the southern tip of Greenland, producing a whirlpool of falling water that's 3 to 6 km in diameter.

While the whirlpool rarely breaks the surface, during certain times of year it does produce an indentation and current in the ocean that can tilt ships and be seen from space. This falling column of cold, salt-laden water pours itself to the bottom of the Atlantic, where it forms an undersea river forty times larger than all the rivers on land combined, flowing south down to and around the southern tip of Africa, where it finally reaches the Pacific. The water is so deep and dense (because of its cold and salinity) that it often doesn't surface in the Pacific for as much as a thousand years after it first sinks in the North Atlantic off the coast of Greenland. Two things could prevent high-latitude water from sinking: surface water warming and decreased salinity caused by freshwater runoff from melting ice (especially from glaciers) and rain. Both of these things are happening.

Recently, scientists have published peer-reviewed articles describing the potential collapse of global currents, including the Gulf Stream and AMOC that warm England and Europe1-4. If this happens, heat that currently moves north from the Equator to Europe will instead heat up Mexico, the Southwest USA, and much of South America, along with the rest of the Southern Hemisphere5-6. One analysis suggested that there is a 95% chance that the AMOC will stop sometime between 2025 and 2095. The most likely time for this collapse would be around 20572.

If the AMOC collapses, the North Atlantic will cool off, while North America, Asia, Africa, Australia, Pacific islands and especially Antarctica will heat up even faster than they are now. This will increase the melting of ice in Antarctica, thus raising sea levels and putting coastal regions throughout much of the world underwater permanently. Food production in Europe will decrease dramatically because of the cold. It could be even worse in the rest of the world, as it heats up.

Another study grabbed headlines in popular media. Scientists found that the ice sheet on Greenland was not as stable as they had thought. Previously, data suggested that Greenland had been almost completely covered with an ice sheet for a million years. Scientists had drilled into the ice sheet back in the 1960s to learn about Greenland’s ancient climate. They also collected about one meter of sediment that was beneath the ice but never looked at it very closely. Just recently, scientists did look at this sediment. Much to their surprise, Danish scientists found signs of life, such as freshwater diatoms. They concluded that there was an ice-free environment in northwest Greenland 416 ± 38 thousand years ago7. So, the Greenland ice sheet could melt much faster than previously expected. In a bit of irony, the melting ice will lead to the collapse of the AMOC, with subsequent cooling in Greenland. Even though some ice could form there again, it will melt even faster in Antarctica and the mountains in Asia, South America and Africa.

However, we must not give up hope. Even though the USA and other countries support the production of fossil fuels, wind, solar and other ways to produce energy are becoming cheaper. Still, there are things that each of us can do to help – especially those who live in the USA. We can vote. As more young people reach voting age, they will help us vote out the only political party in the world that denies climate change and gives tax breaks to companies that produce fossil fuels and mass-produced meat. If such tax breaks were given to people who produce clean energy and grow fruits and vegetables, fossil fuels and meat will not be able to compete on the free market.

Notes

1 National Ocean Service. What is the Atlantic Meridional Overturning Circulation? National Oceanographic and Atmospheric Administration (NOAA), 2023.
2 Ditlevsen, P. & Ditlevsen, S. Warning of a forthcoming collapse of the Atlantic meridional overturning circulation. Nature Communications, Volume 14, article 4254, 2023.
3 Wang, Q. et al. A Review of Arctic–Subarctic ocean linkages: past changes, mechanisms, and future projections. Ocean-Land-Atmosphere Research, Volume 2, article 0013, 2023.
4 Orihuela-Pinto, B. et al. Interbasin and interhemispheric impacts of a collapsed Atlantic Overturning Circulation. Nature Climate Change Volume 12, p. 558-565, 2022.
5 Caesar, L. et al. Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, Volume 556, p. 191-196, 2018.
6 Liu, W. et al. Overlooked possibility of a collapsed Atlantic Meridional Overturning Circulation in warming climate. Science Advances, volume 3, article e1601666, 2017.
7 Christ, A.J. et al. Deglaciation of northwestern Greenland during Marine Isotope Stage 11. Science, Volume 381, p. 330-335, 2023.