ocean warming

• The ocean absorbs about 93% of the excess heat caused by climate change and has warmed by an average of more than 1°C over the past century.

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

• 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. A 2018 study showed that the amount of annual marine heatwave days increased globally by over 50% from 1925 to 2016, with heatwaves becoming 34% more frequent and lasting 17% longer.

• 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.

See the Marine Heatwave (MHW) Tracker

Marine heatwaves can devastate ecosystems, effecting species in four main ways:

• distribution: some marine species such as jellyfish like warmer temperatures but most either move on or die. Rogue species range further, following the heatwave, creating a potential threat to those which remain.

• altering food chains: plankton in cooler waters are more nutritious, they are bigger, richer in fats and higher in calories. As temperatures rise the nutrition available and fish stocks react accordingly, generally moving poleward to remain within their preferred temperature and nutritional ranges.

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

• 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.

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.