ocean warming

• The ocean absorbs about 93% of the excess heat caused by climate change.

• In the 20th century it warmed by just over 1°C. By the end of this century temperatures could be up by as much as 4°C.

• Since record keeping began in the 1880s the speed of warming has increased steadily, the rate of warming is now about 4.5 times faster than it was 50 years ago.

• 2023 saw the warmest water temperatures on record to date. 2024 is already set to beat this.

• It is expected that no matter whether we follow a high emissions path or a low one, the ocean will be in a near-permanent heat-wave state by 2100.

• A 1 to 4°C increase may not sound much but a warmer ocean transforms marine biodiversity, changes ocean chemistry, raises sea levels, and fuels extreme weather.

Ocean warming impacts the distribution, food chains and population levels of marine life:

• distribution: as temperatures rise marine life reacts accordingly, generally moving poleward to remain within their preferred temperature and nutritional ranges. Some marine species such as jellyfish like warmer temperatures but most either need to move on or die.

• altering food chains: plankton, which forms the basis of the marine food chain, are more nutritious in cooler water, they are bigger, richer in fats and higher in calories.

• population size: as marine species move away reproduction rates suffer and populations face an inevitable reduction.

Ocean warming affects marine habitats: the marine ecosystems of inter-tidal areas such as mangroves or kelp forests, coral reefs and polar zones, each recognised for their essential role as a nursery for the young of myriad marine species, face damage and destruction with the increase in temperature.

Ocean warming alters weather patterns: warmer water holds more energy, this leads to more intense and more frequent hurricanes, heavier rainfall, causing flooding in some areas and drought in others, and snowstorms.

Ocean warming causes the sea level to rise: water expands as it heats, not only raising sea levels but also eroding polar ice.

Warmer water is also more acidic, holding more carbon dioxide, a shift in the chemical balance which is fatal for any life with a shell or skeleton (such as corals, crabs, clams and snails) as it breaks down the calcium carbonate held within them.

Marine heatwaves, a period of five days or more when the sea surface temperature is significantly warmer than the 30-year historical average for that location and time of year, are on the rise.

The Marine Heatwaves International Working Group conducts research into marine heatwaves, with the goal that by improving scientific understanding of their physical properties and ecological impacts, we can better predict future conditions and protect vulnerable marine habitats and resources.