Climate change impacts on the marine environment

Corals become "bleached" when water temperatures rise too high and are sustained for too ... / ©: Cat Holloway / WWF-Canon
Corals become "bleached" when water temperatures rise too high and are sustained for too long, Fiji.
© Cat Holloway / WWF-Canon
Climate change is affecting ocean temperatures, the supply of nutrients, ocean chemistry, food chains, wind systems, ocean currents and extreme events such as cyclones. All of these, in turn, affect the distribution, abundance, breeding cycles and migrations of marine plants and animals that millions of people rely on for food and income.

Evidence is emerging that marine organisms may be responding faster to climate change than land-based plants and animals. As the climate warms, marine plants and animals are shifting towards the poles changing marine food webs and impacting the plants, and animals (including people) that depend on them. The slower ocean dynamics also means that some changes, such as ocean acidification, will be irreversible this century.

The key impacts of climate change on our marine environment include:

Coral bleaching: Coral bleaching is the whitening of corals, due to stress-induced expulsion or death of their symbiotic protozoa, zooxanthellae, or the loss of pigmentation within the protozoa.

Once bleaching begins, it tends to continue even without continuing stress. If the coral colony survives the stressful period, zooxanthellae often require weeks or months to return to normal density. New residents may be of a different species and change the make up of marine ecosystems dramatically.  Often biological diversity is reduced making the reef even less resilient to future environmental change. Some species of zooxanthellae and corals are more resistant to stress than others.

Temperature change is the most common cause of coral bleaching. Large coral colonies, such as porites, are able to withstand extreme temperature shocks. Other, more fragile branching corals, such as table coral, are more susceptible to stress following temperature change. 

Increasing ocean acidification likely exacerbates the bleaching effects of thermal stress. The Great Barrier Reef experienced bleaching in 1980, 1982, 1992, 1994, 1998, 2002 and 2006. While most areas recovered with relatively low levels of coral death, some locations suffered severe damage, with up to 90% of corals killed. The most widespread and intense events occurred in the summers of 1998 and 2002, affecting about 42% and 54% of reefs, respectively. Under the Intergovernmental Panel on Climate Change’s (IPCC’s) moderate warming scenarios, corals on the Great Barrier Reef are very likely to regularly experience summer temperatures high enough to induce bleaching.

Rising sea levels: Global sea levels may rise by more than 60 centimetres during the next 100 years due to the melting of glaciers and polar ice, and thermal expansion of warmer water. Rising water levels will have serious impacts on marine ecosystems. The amount of light reaching offshore plants and algae dependent on photosynthesis could be reduced, while coastal habitats are already being flooded.

Acidic oceans: After absorbing a large proportion of the carbon dioxide released by human activities, our oceans are becoming acidic. In fact, the IPCC has reported that the uptake of anthropogenic carbon since 1750 has led to the oceans becoming more acidic.  Increasing atmospheric carbon dioxide concentrations lead to further acidification. Projections estimate a reduction in average global surface ocean pH of between 0.14 and 0.35 units over the course of the 21st century.

While the effects of ocean acidification on the marine biosphere are not yet fully documented, the progressive acidification of our oceans is expected to have negative impacts on marine shell-forming organisms (e.g. corals) and their dependent species. Fish, squid, and other gilled marine animals may also find it harder to “breathe” as extracting the dissolved oxygen from acidic waters becomes more difficult.

Altered lifestyles and locations: Rising temperatures can directly affect the metabolism, life cycle, and behaviour of marine species. For many species, temperature is a cue for reproduction, so temperature changes could affect successful breeding. Temperatures also determine the number of male and female offspring born to marine turtles, as well as some fish and copepods (tiny, shrimp-like animals on which many other marine animals feed).

Changing climate could, therefore, skew sex ratios and threaten population survival. As the oceans warm, the location of the ideal water temperature may shift for many species. A study has shown that fish in the North Sea have moved further north or into deeper water in response to rising sea temperatures. Other species may lose their homes for other reasons. The distribution of penguin species on the Antarctic Peninsula, for example, is changing due to the reductions in sea ice caused by global warming.

Stormy weather: Most scientists believe that global warming will herald a new era of extreme and unpredictable weather. Tropical storms and heavier rainfall may increase, causing physical damage to coral reefs, other coastal ecosystems, and coastal communities. Hurricanes Hugo and Marilyn, which hit the US Virgin Islands National Park in 1989 and 1995, respectively, did massive damage to coral ecosystems.