Oceans have a fever — here’s why

Average sea-surface temperature in February reached a new high in 2024. Oceans have been unusually warm for a year now, and the consequences for marine ecosystems can be severe and irreversible

THE AVERAGE global sea surface temperature (SST) — the temperature of the water at the ocean surface — for February 2024 stood at 21.06 degree Celsius, the highest ever in a dataset going back to 1979, the Copernicus Climate Change Service (C3S) said earlier this month. The previous record of 20.98 degree Celsius was set in August 2023.

The average daily SST has been off the charts since March 2023. In the graph alongside, the orange line shows 2023 temperatures, and the grey lines show the temperatures of previous years. The solid back line shows where we are so far in 2024 — and this is clearly above any other year, including 2023.

Why are the oceans getting warmer?

■ Since the middle of the 19th century, human activities such as the increased burning of fossil fuels has released high levels of greenhouse gases (GHGS) in the atmosphere. Carbon dioxide, methane, ozone, and nitrous oxide are some notable GHGS, which trap heat in the atmosphere and contribute to global warming. As a result, the average global temperature has risen at least 1.2 degree Celsius above pre-industrial times.

Almost 90 per cent of the extra heat trapped by GHGS has been absorbed by the oceans, which has made them steadily warmer over the decades.

■ Other factors too have contributed to the rise in ocean temperatures — for instance, El Niño, which refers to an abnormal warming of surface waters in the equatorial Pacific Ocean. The current high global daily average SST, however, began to rise well before the ongoing El Niño event developed fully — and remains unusually high even now, as the weather pattern has peaked and begun to wane.

■ There is also less dust blowing off the Sahara desert of late due to weaker-than-average winds. The dust typically forms a giant umbrella that shades the Atlantic water and reduces ocean temperatures, according to a report in Wired magazine. With this umbrella now partially folded, more of the Sun is beating down on the ocean, the report said.

■ Yet another factor is the drop in the amount of sulphur allowed in marine shipping fuels after international regulations were imposed in 2020 to reduce sulphur dioxide emissions, a health-damaging air pollutant. Sulphur dioxide leads to sulphate aerosols in the atmosphere, which act like a cloud, preventing solar radiation from reaching the ocean surface.

What are the consequences of such warming?

Higher ocean temperatures can have irreversible consequences for marine ecosystems.

■ For instance, warmer oceans lead to an increase in ocean stratification — the natural separation of ocean water into horizontal layers by density, with warmer, lighter, less salty, and nutrient-poor water layering on top of heavier, colder, saltier, nutrient-rich water.

Usually, ocean ecosystems, currents, wind, and tides mix these layers together. But the rise in temperatures has made it harder for water layers to mix. Thus, the surface ocean becomes less dense than the deep ocean, and remains at the surface, where it may get even more warm. Due to this, oceans are able to absorb less carbon dioxide from the atmosphere, and the oxygen absorbed is not able to mix properly with cooler waters below, threatening the survival of marine life.

Nutrients are also not able to travel up to the surface of the oceans from below. This could threaten the population of phytoplankton, the single-celled plants that thrive on the ocean surface and are the base of several marine food webs. Phytoplankton are eaten by zooplankton, which are consumed by other marine animals such as crabs, fish, and sea stars. A fall in the phytoplankton population could lead to a collapse of marine ecosystems.

■ Warmer oceans cause marine heat waves (MHWS), which occur when the surface temperature of a particular region of the sea rises to 3 or 4 degree Celsius above the average temperature for at least five days. Between 1982 and 2016, such heat waves have doubled in frequency, and have become longer and more intense, according to a 2021 study by the UN’S Intergovernmental Panel on Climate Change (IPCC).

MHWS contribute to coral bleaching. Corals are very sensitive to the temperature of the water in which they live. When the water gets too warm, they discharge the algae known as zooxanthellae living in their tissues, causing them to turn entirely white. This coral bleaching reduces reproductivity of corals, and makes them more vulnerable to life-threatening diseases.

Fish kills — the sudden and unexpected death of many fish or other aquatic animals over a short period and mainly within a particular area — can also be attributed to MHWS.

■ Such heat waves impact the migration pattern of aquatic animals. For example, in 2012, an MHW over the northwest Atlantic Ocean caused marine species that like warm water to move northward and migrate earlier than usual, affecting fisheries targeting those species in the United States.

■ According to several studies, higher ocean temperatures may also result in more frequent and more intense storms like hurricanes and cyclones. Warmer temperatures lead to a higher rate of evaporation as well as the transfer of heat from the oceans to the air.

That is why, when storms travel across hot oceans, they gather more water vapour and heat. This results in more powerful winds, heavier rainfall, and more flooding when storms reach the land — meaning heightened devastation for humans.

What now?

These disastrous consequences are set to become worse as the world continues to get warmer. The World Meteorological Organisation (WMO) in its 2023 State of Global Climate report said there was a 66 per cent chance that at least one of the years between 2023 and 2027 would cross the threshold of 1.5 degree Celsius above pre-industrial levels.

The only way to avoid or blunt the impacts is to reduce GHG emissions. But the planet is nowhere close to doing that. In 2023, the concentration of GHGS reached the highest levels ever recorded in the atmosphere, according to C3S.