Ocean Warming
Authors: IUCN (International Union for Conservation of Nature)
DOI / Source: IUCN
Date: November 2017
Reading level: Beginner
Why This Matters for Freedivers
Warmer seas change what freedivers see and rely on: fish distributions, reef health, visibility, storms, and even disease risks in marine life. This brief is a quick, credible overview you can share with divers to connect “what we feel in the water” to the bigger climate drivers, and it points to practical responses like emissions reduction and marine protection.
Synopsis
This two-page IUCN issues brief explains a simple but huge idea: the ocean is acting like Earth’s heat sponge. As greenhouse gases rise, the ocean absorbs most of the extra heat, which pushes ocean temperatures upward. According to the brief, the IPCC reported the ocean absorbed more than 93% of the excess heat since the 1970s, which has slowed (buffered) how fast air temperatures would otherwise have risen.
It summarizes the evidence for warming at the surface and at depth. The brief cites NOAA data showing global average sea surface temperature has increased by about 0.13°C per decade over the past 100 years, and notes research suggesting a large fraction of the excess heat is also accumulating deeper than 700 m. It also highlights that warming isn’t evenly distributed, with strong warming in the Southern Hemisphere contributing to subsurface melting of Antarctic ice shelves.
Then it focuses on impacts. For ecosystems, warming can drive coral bleaching and mortality, and disrupt breeding grounds and distributions for fish, seabirds, and marine mammals as species move to chase their preferred conditions. For people, the brief connects ocean warming to food security (fish stocks shifting and disease vulnerability increasing), economic losses, and reduced coastal protection where reefs and mangroves are damaged. It also links rising sea surface temperatures to more severe hurricanes and intensified El Niño impacts (droughts and floods), and notes warming can increase and spread diseases in marine species, with potential risks to humans through seafood consumption or wound exposure.
Finally, the brief outlines what can be done in four buckets: cut greenhouse gas emissions in line with the Paris Agreement goals (well below 2°C), protect marine ecosystems through well-managed marine protected areas, restore damaged coastal habitats to improve resilience, and improve human adaptation through fisheries policies (precautionary catch limits, removal of harmful subsidies), coastal setback zones, and better monitoring/forecasting of marine disease outbreaks.
Abstract
The ocean absorbs the majority of excess heat from greenhouse gas emissions, leading to rising ocean temperatures. Ocean warming affects marine ecosystems by driving changes such as coral bleaching, increased mortality risks, and shifts in species distributions and breeding grounds. These impacts also affect humans through risks to food security and livelihoods, reduced coastal protection, more extreme weather events, and increased marine disease prevalence. Limiting greenhouse gas emissions in line with the Paris Agreement, expanding and managing marine protected areas, restoring ecosystems, and strengthening adaptation and scientific monitoring are key actions to reduce harms from ocean warming.