Effects of climate change on marine mammals

During the last century, the global average land and sea surface temperature has increased due to an increased greenhouse effect from human activities.[9] From 1960 to through 2019, the average temperature for the upper 2000 meters of the oceans has increased by 0.12 degree Celsius, whereas the ocean surface has warmed up to 1.2 degree Celsius from the pre-industrial era.[10]

The illustration of temperature changes from 1960 to 2019 across each ocean starting at the Southern Ocean around Antarctica (Cheng et. al., 2020)

Marine organisms usually tend to encounter relatively stable temperatures compared with terrestrial species and thus are likely to be more sensitive to temperature change than terrestrial organisms.[11] Therefore, the ocean warming will lead to increased species migration, as endangered species look for a more suitable habitat. If sea temperatures continue to rise, then some fauna may move to cooler water and some range-edge species may disappear from regional waters or experienced a reduced global range.[11] Change in the abundance of some species will alter the food resources available to marine mammals, which then results in marine mammals’ biogeographic shifts. Additionally, if a species cannot successfully migrate to a suitable environment, unless it learns to adapt to rising ocean temperatures, it will face extinction.

Sea level rise is also important when assessing the impacts of global warming on marine mammals, since it affects coastal environments that marine mammals species rely.[12]

Changes in temperatures will impact the location of areas with high primary productivity. Primary producers, such as plankton,[13][14][15][16] are the main food source for marine mammals such as some whales. Species migration will therefore be directly affected by locations of high primary productivity. Water temperature changes also affect ocean turbulence, which has a major impact on the dispersion of plankton and other primary producers.[17] Due to global warming and increased glacier melt, thermohaline circulation patterns may be altered by increasing amounts of freshwater released into oceans and, therefore, changing ocean salinity. Thermohaline circulation is responsible for bringing up cold, nutrient-rich water from the depths of the ocean, a process known as upwelling.[18]

Change in pH since the beginning of the industrial revolution. RCP 2.6 scenario is "low CO2 emissions" . RCP 8.5 scenario is "high CO2 emissions", the path we are currently on. Source: J.-P. GATTUSO et al., 2015

About a quarter of the emitted CO_2, about 26 million tons is absorbed by the ocean every day.[19] Consequently, the dissolution of anthropogenic carbon dioxide (CO₂) in seawater causes a decrease in pH which is corresponding to an increase in acidity of the oceans with consequences for marine biota.  Since the beginning of the industrial revolution, ocean acidity has increased by 30% (the pH decreased from 8.2 to 8.1).[19] It is projected that the ocean will experience severe acidification under RCP 8.5, high CO₂ emission scenario, and less intense acidification under RCP 2.6, low CO₂ emission scenario. Ocean acidification will impact marine organisms (corals, mussels, oysters) in producing their limestone skeleton or shell. When CO₂ dissolves in seawater, it increases protons (H+ ions) but reduces certain molecules, such as carbonate ions in which many oysters needed to produce their limestone skeleton or shell.[19] The shell and the skeleton of these species may become less dense or strong. This also may make coral reefs become more vulnerable to storm damage, and slow down its recovery. In addition, marine organisms may experience changes in growth, development, abundance, and survival in response to ocean acidification

Sea ice, a defining characteristic of polar marine environment, is changing rapidly which has impacts on marine mammals. Climate change models predict changes to the sea ice leading to loss of the sea ice habitat, elevations of water and air temperature, and increased occurrence of severe weather. The loss of sea ice habitat will reduced the abundance of seal prey for marine mammals, particularly polar bears. Initially, polar bears may be favored by an increase in leads in the ice that make more suitable seal habitat available but, as the ice thins further, they will have to travel more, using energy to keep in contact with favored habitat.[20] There also may be some indirect effect of sea ice changes on animal heath due to alterations in pathogen transmission, effect on animals on body condition caused by shift in the prey based/food web, changes in toxicant exposure associated with increased human habitation in the Arctic habitat.[21]

Hypoxia occurs in the variety of coastal environment when the dissolved of oxygen (DO) is depleted to a certain low level, where aquatic organisms, especially benthic fauna, become stressed or die due to the lack of oxygen. [22] Hypoxia occurs when the coastal region enhance Phosphorus release from sediment and increase Nitrate (N) loss. This chemical scenario supports favorable growth for cyanobacteria which contribute to the hypoxia and ultimately sustain eutrophication.[23]  Hypoxia degrades an ecosystem by damaging the bottom fauna habitats, altering the food web, changing the nitrogen and phosphate cycling, decreasing fishery catch, and enhancing the water acidification. [23] There were 500 areas in the world with reported coastal hypoxia in 2011, with Baltic Sea contains the largest hypoxia zone in the world. [24]These numbers are expected to increase due to the worsening condition of coastal areas caused by the excessive anthropogenic nutrient loads that stimulate intensified eutrophication.  The rapidly changing climate in particularly, global warming, also contributes to the increase of Hypoxia occurrence that damaging marine mammals and marine/coastal ecosystem.

Increase frequency of Hypoxia Occurrence in the entire Baltic Sea calculated as the number of profiles with recorded hypoxia relative to the total number of profiles (Conley et. al., 2011)

Polar bears are one of the marine mammals that are most at risk due to climate change. The biggest issue for polar bears related to climate change is the melting of ice as a result of increasing temperatures. When the ice melts, polar bears lose their habitat and food sources. Although polar bears have been known to eat more than 80 species of animals, most of their diet consists of seals, which are also endangered by global warming.[25] There have been an increasing number of polar bear drownings because they become exhausted by having to swim farther to find ice or prey.[26]

Not only are marine mammals impacted by climate change but so is other marine life. An example of this could be coral. When coral is introduced to warming ocean temperatures changes, runoff and pollution, overexposure to sunlight extremely low tides and other stresses, the coral will expel an algae growing on them. They have a symbiotic relationship with the algae. When the coral expels the algae it becomes bleached or "completely white". This is called coral bleaching. The coral then become more vulnerable to disease and death.[27]