The biology of Earth is dependent on marine ecosystems4. The oceans are filled with diverse living organisms and covers about 71% of the Earth’s surface10. Unfortunately, the impact of climate change on oceans has been inadequately studied due to the vast size and complexity of this body of water4. Recent studies have indicated that rising atmospheric carbon dioxide (CO2) and climate change are causing shifts in ocean temperature, stratification, circulation, oxygen content, nutrient input, and ocean acidification with a potential risk for irreversible ecological changes3. In addition, there is a large amount of evidence that human activities are a main cause of these rapid environmental changes and have severe and diverse consequences on marine ecosystems3. Specifically, during the last 10-15 years, it has been evident that the oceans have been changing at an extremely fast rate due to changing climates8 and do not seem to be slowing down.
Rising concentrations of atmospheric greenhouse gasses have increased worldwide average temperatures by about 0.2°C per decade over the past 30 years with most of that energy being absorbed by oceans4. Over the past 100 years, the average temperature of the upper layers of the oceans has increased by 0.6°C4. Humans have played a large role in influencing climate, mainly through fossil-fuels, agriculture, and other land-use emissions that change the composition of the atmosphere3.
The uptake of carbon dioxide in the oceans is causing water temperatures to rise that’s resulting in the possible advancement of specific marine plant and animal cycles. Furthermore, this can lead to an increase in productivity in growing seasons and timing of reproduction4 as well as, other physiological functioning, behavior, and demographic traits. Additionally, these changes can cause disturbances in biological interactions3, such as, animal metabolic rates, population growth, and ecosystem processes as they are all temperature-dependent4. The rapid rate at which temperatures are increasing is leading to mortality rate, fitness reduction, population decline, and eventually extinction4 as the organisms are unable to handle the stress that this change causes.
Climbing temperatures create many additional changes, such as rising sea levels (thermal expansion), increased ocean stratification2, melting sea-ice, altering ocean circulation patterns, precipitation, and the input of freshwater3. Sea-ice has drastically declined in the Arctic and along the western Antarctic Peninsula (WAP); it is expected that the Arctic will be sea-ice free starting in the mid- to late twenty-first century3 (around 20404). The warming is causing sea-ice to melt, which in turn, results in the rising of sea levels at a rate of about 3 millimeters (mm) per year1. This is affecting not only the organisms below the sea-ice but also above it4; an example of this would be the polar bear. The polar bear populations over the past few years have drastically declined due to lack of ice availability for hunting5.
Additionally, the warming of upper layers of the ocean causes stratification of the water column. This alters ocean currents, ventilation and reduces mixing in some areas of the ocean and as a result affects oxygen concentrations, nutrient availability, phytoplankton populations, and primary production3&4. Annual primary production has decreased over 6% since the 1980s; varying climates largely influence primary productivity. A species being largely influenced is the phytoplankton, as they decrease more each year due to warming, stratification, and acidification4. Marine organism are not the only ones being influenced by this; changes in these primary productions have great implications for the marine biosphere, carbon sinks, and the biogeochemistry of the planet4. Dissolved oxygen levels play a key role in marine ecosystems; studies state that paleological evidence shows that declining oxygen concentrations have played a major role in many mass extinction events4&6. A decrease in dissolved oxygen leads to an increase in excess amounts of hydrogen sulfide being released into the atmosphere due to oceanic anoxia4&6.
Finally, ocean acidification causes a series of chemical changes such as increased aqueous CO2, total inorganic carbon, reduced pH, carbonate ion and calcium carbonate saturation states3. Absorption of CO2 in the oceans is changing the carbonate chemistry of the seawater, reducing calcification rates and effects physiological processes in certain marine organisms9. Currently, the world’s oceans have absorbed about one-third of anthropogenic CO24; on average, there is a net carbon intake of 2 billion tons by the oceans annually10. The absorbed carbon dioxide acidifies the surface layers of the ocean, with a fixed decrease of 0.02 pH units per decade over the past 30 years and a total decrease of 0.1 pH units since the pre-industrial era4. Polar oceans are especially sensitive because temperatures and acidities are changing at more than twice the global average4. Also, a number of experimental studies have shown that ocean acidification significantly effects the performance of marine organisms7.
In conclusion, climate change is altering ocean temperatures, stratification, oxygen contents, and ocean acidification4. Over the past 100 years, the global average temperature of the upper layers of the oceans has increased by 0.6°C resulting in changes of physiological functioning, behavior, and demographic traits causing disturbances in biological interactions3&4. Dissolved oxygen levels are extremely important to marine ecosystems. Stratification can alter ocean currents, ventilation and reduces mixing in some areas of the ocean and as a result affects oxygen concentrations, nutrient availability, phytoplankton populations, and primary production3&4. Also, ocean acidification significantly effects the performance of marine organisms7. Ocean acidification causes an increase in aqueous CO2, total inorganic carbon, reduced pH, carbonate ion and calcium carbonate saturation states3. The world’s oceans have absorbed about one-third of anthropogenic CO2 which acidifies the surface layers of the ocean, with a total decrease of 0.1 pH units since the pre-industrial period4. Human activities are a major driver for climate change causing severe damage on marine ecosystems3, unless decision makers change how we use energy, from fossil fuels to renewable energy, the health of the ocean will continue to rapidly deplete and mass extinction events of marine organisms can be expected.