Effects of climate change on oceans
There are many effects of climate change on oceans. One of the main ones is an increase in ocean temperatures. More frequent marine heatwaves are linked to this. The rising temperature contributes to a rise in sea levels due to melting ice sheets. Other effects on oceans include sea ice decline, reducing pH values and oxygen levels, as well as increased ocean stratification. All this can lead to changes of ocean currents, for example a weakening of the Atlantic meridional overturning circulation (AMOC).[2] The main root cause of these changes are the emissions of greenhouse gases from human activities, mainly burning of fossil fuels. Carbon dioxide and methane are examples of greenhouse gases. The additional greenhouse effect leads to ocean warming because the ocean takes up most of the additional heat in the climate system.[3] The ocean also absorbs some of the extra carbon dioxide that is in the atmosphere. This causes the pH value of the seawater to drop.[4] Scientists estimate that the ocean absorbs about 25% of all human-caused CO2 emissions.[4]
The various layers of the oceans have different temperatures. For example, the water is colder towards the bottom of the ocean. This temperature stratification will increase as the ocean surface warms due to rising air temperatures.[5]: 471 Connected to this is a decline in mixing of the ocean layers, so that warm water stabilises near the surface. A reduction of cold, deep water circulation follows. The reduced vertical mixing makes it harder for the ocean to absorb heat. So a larger share of future warming goes into the atmosphere and land. One result is an increase in the amount of energy available for tropical cyclones and other storms. Another result is a decrease in nutrients for fish in the upper ocean layers. These changes also reduce the ocean's capacity to store carbon.[6] At the same time, contrasts in salinity are increasing. Salty areas are becoming saltier and fresher areas less salty.[7]
Warmer water cannot contain the same amount of oxygen as cold water. As a result, oxygen from the oceans moves to the atmosphere. Increased thermal stratification may reduce the supply of oxygen from surface waters to deeper waters. This lowers the water's oxygen content even more.[8] The ocean has already lost oxygen throughout its water column. Oxygen minimum zones are increasing in size worldwide.[5]: 471
These changes harm marine ecosystems, and this can lead to biodiversity loss or changes in species distribution.[2] This in turn can affect fishing and coastal tourism. For example, rising water temperatures are harming tropical coral reefs. The direct effect is coral bleaching on these reefs, because they are sensitive to even minor temperature changes. So a small increase in water temperature could have a significant impact in these environments. Another example is loss of sea ice habitats due to warming. This will have severe impacts on polar bears and other animals that rely on it. The effects of climate change on oceans put additional pressures on ocean ecosystems which are already under pressure by other impacts from human activities.[2]
Time scales[edit]
Many ocean-related elements of the climate system respond slowly to warming. For instance, acidification of the deep ocean will continue for millennia, and the same is true for the increase in ocean heat content.[72]: 43 Similarly, sea level rise will continue for centuries or even millennia even if greenhouse gas emissions are brought to zero, due to the slow response of ice sheets to warming and the continued uptake of heat by the oceans, which expand when warmed.[72]: 77
Potential feedback effects[edit]
Methane release from methane clathrate[edit]
Rising ocean temperatures also have the potential to impact methane clathrate reservoirs located under the ocean floor sediments. These trap large amounts of the greenhouse gas methane, which ocean warming has the potential to release. However, it is currently considered unlikely that gas clathrates (mostly methane) in subsea clathrates will lead to a "detectable departure from the emissions trajectory during this century".[72]: 107
In 2004 the global inventory of ocean methane clathrates was estimated to occupy between one and five million cubic kilometres.[134]
