Rising concentrations of greenhouse gases in the atmosphere raise three major threats to ocean life: increasing temperatures, reduced oxygen content and acidification.
Already, rising water temperatures are pushing many fish stocks towards higher latitudes1. The biological consequences are hard to predict with confidence; but some species are likely to find themselves in areas where their normal diet does not exist, or where there is no nursery habitat. For static organisms such as coral, the impacts can be more immediate2.
The last 50 years have seen a 10-fold increase in the number of ‘dead zones’ – areas of ocean severely depleted in dissolved oxygen3. Although these are predominantly around coasts and have as their primary cause nutrient runoff from farmland, climate change is having a similar effect, by a different mechanism, on the high seas. This is the second major threat from greenhouse gas emissions, and one that is only now being explored. Elevated sea temperatures mean that layers of water mix less thoroughly, enlarging the ‘oxygen minimum zone’ – the layers where oxygen levels are too low to support life. This is already reducing the proportion of the Atlantic Ocean in which species such as blue marlin can live4, and may have similar impacts in other regions.
The third threat is ocean acidification. Seawater absorbs about a quarter of the carbon dioxide being added to the atmosphere by human activity5. This changes the pH, the acidity, of the ocean. It is a minuscule change by human standards, but profound for creatures that need to form shells or other ‘hard parts’, such as molluscs and corals6. And scientists are now warning that acidification is occurring at a rate faster than at any time in geological history7. In the Antarctic, the effects of acidification are already being seen in sea butterflies – ecologically important 8. Experiments in the laboratory, and in places where CO2 naturally seeps from the seabed, indicate that acidification is set to have a profound impact on ocean ecosystems9. The impacts for humanity are likely to include a reduction in food supply.
There is some evidence that ecosystems are more resilient to the warming and acidifying trends10 when they are healthy. Conversely, overfishing has already been seen to combine with the effects of climate change to produce large-scale changes in ecosystems11. Rebuilding depleted fish stocks, ending trawling that destroys seabed habitat, and restoring degraded ecosystems could all make ocean life more resilient to impacts of greenhouse gas emissions.
Among the other issues threatening the health of the global ocean are pollution, invasive species and noise. Plastic particles bearing hydrocarbons are now found in seafloor sediments12, invasive crabs have taken over long stretches of Russian and European coast13 and have even reached Antarctica14, while noise is stressing cetaceans15 and perhaps impairing their capacity to communicate over long distances and feed.
These threats are not fully understood, but their impacts are likely to place further stress on the global ocean.