As the world’s largest carbon sink, oceans are responsible for global temperature regulation, oxygen production and play an important role in climate change and global warming. Oceans make up to 70 percent of the Earth’s surface area and host about 3/4 of all known species. Three and a half billion people now live within one hundred kilometers of the coast and greatly depend on the sea for their livelihood. This dependency translates into a life or death matter for the world’s poorest populations, and it’s threatened by current environmental conditions brought on by global climate change. Climate change affects oceans in three ways - through acidification, deoxygenation and sea surface temperature increase. All three of these processes interact with each other and other stressors in the ocean environment.
How climate change affects ocean acidification
Ocean acidification happens when its pH level drops as a result of an uptake of carbon dioxide (CO2) from the atmosphere. Due to carbon emissions being constantly on the rise the oceans’ surface have recorded about 0.1 pH unit drop since the start of the Industrial Revolution in the mid 18th century. Although this change doesn’t seem to be colossal, climate change scientists agree that it is unprecedented and it is highly likely that the current level of ocean acidification was not seen within the last 65 million years.
Current level of ocean acidification mean that many marine organisms and species need to adjust to the changes in the water chemistry with some not having enough time or abilities for such rapid adjustments. Scientists report that ocean acidification already influenced the abundance of phytoplankton which is considered the foundation of aquatic food web and directly contributed to the weakening of some of marine animals (mainly corals and shellfish) skeletons and shells by decreasing their calcification. Fundamental physiological processes such as respiration, photosynthesis, and reproduction have also been shown to respond to CO2 concentrations in seawater.
How climate change affects ocean warming
Global average sea surface temperatures have increased by 0.65°C in the Indian ocean, 0.41°C in the Atlantic and 0.31°C in Pacific ocean if compared with the temperatures in 1950.
Based on the new available data the past five years are the top five warmest years recorded in the ocean. Such enormous amount of heat being added to the oceans is in fact equivalent to every person on Earth running 100 microwave ovens all day and all night.
While climate-driven changes are pervasive in the ocean, the biggest impact are occurring in polar and tropical ecosystems, where higher water temperatures are causing the loss of two important but vulnerable habitats: sea ice and coral reef ecosystems.
Alarmingly, Arctic sea ice is now declining at a rate of nearly 13 percent per decade with 2012 being a year with the lowest amount of summer ice on record. Such drastic change represents a direct loss of important habitat for animals like ringed seals and polar bears that use ice and snow for shelter, hunting, migration, and reproduction, and results in their population decline. Recent research study calculates that by 2100 ringed seals population might experience a decline ranging from 50 to 99 percent. By mid-century disappearing Arctic sea ice resulting from climate change is expected to also cut polar bear population by a third.
Same as polar ecosystems, coral reefs all over the world are also experiencing the significant decline as a result of climate change and related ocean warming. It is estimated that nearly 60 percent of the world’s remaining reefs are at high risk of being lost in the next three decades. Most coral reef scientists agree that the rate of change in climatic conditions is potentially beyond the capacity of coral reefs to recover and adapt. The greatest threat for their survival is bleaching that takes place when prolonged periods of increased sea temperatures cause a breakdown in the relationship between the corals and zooxanthellae (algae) that provides corals with all the necessary nutrients and gives them their bright colours. Bleaching events lead to the loss of corals colour and make them weak with only some corals being able to recover, often with compromised immune system and others being destined to die. Since coral reefs have a greater diversity of animal and plant life than rainforests and also provide food at all levels of the food chain, their loss might have a catastrophic impact not only on the ocean itself (it is estimated that more than 90 percent of marine species are either directly or indirectly dependent on them), but also people who depend on it for food and livelihood.
How climate change affects ocean deoxygenation
Like a chain reaction, ocean warming as one of the consequences of human-induced climate change and constantly increasing levels of carbon emissions leads to ocean deoxygenation when heated ocean water holds less soluble oxygen and the circulation and oxygen exchange between atmosphere and water is reduced. All that further intensifies stress on marine life and contributes to growing of coastal dead zones where no life is possible beneath the surface waters.
It is estimated that globally, oceanic oxygen reserves have already been reduced by 2% over a period of just 50 years (1960-2010).
Loss of oceanic oxygen is also intensified by other human-related factors including excess nutrient runoff from agriculture, sewage and fossil fuels.
How climate change affects The Great Barrier Reef
The largest living structure on Earth and one of the seven world's natural wonders, The Great Barrier Reef can be seen even from the Moon. It is comprised of roughly 2,900 individual reefs and 940 islands and cays stretching for 2,300 kilometers in northeast Australia. Apart from being a massive storehouse of unique biodiversity, the Reef is a strong economic asset that contributes to the Australian economy with around $6.4 billion each year. In its recent report Deloitte estimated that the economic, social and iconic value of the Reef equals to around $56 billion.
In the recent years though the Great Barrier Reef experienced the consequences of climate change that led to the disappearance of a large number of its corals. In two consecutive years - 2016 and 2017 the Reef experienced unprecedented back-to-back bleaching events caused by marine heatwaves.
The first bleaching event destroyed primarily corals in the northernmost section of the Reef between Lizard Island and the Torres Strait, while 2017 bleaching event killed corals mainly in the Reef’s central section. As a result, almost a third of the coral across the entire Great Barrier Reef died due to thermal stress. Currently, average coral cover in the northern section of the reef is at its lowest point on record, while coral cover in the central part is known to decline from 22 percent in 2016 to 14 percent in 2018, mainly due to the 2017 bleaching event. Coral mortality is tragic by its own, but in addition it leads to decline in the diversity of fish species, as well as the number of juvenile fish settling on the reef. Coral reef scientists report that following two massive bleaching events juvenile fish at Lizard Island have suffered a 40 percent decline in settlement.
The outlook for the Great Barrier Reef is not that shiny since the return period for global bleaching events has decreased from 27 years in the 80s to only 6 years now. The situation is supposed to worsen further with extreme bleaching events start to occur every two years by 2034. According to scientists, this would certainly mean the destruction of world’s coral reefs including the Great Barrier Reef - the corals that won’t die right away simply won’t have enough time to recover. Scientists warn that unless deep cuts in greenhouse gas emissions are made in order to keep global average temperature rise to no more than 1.5°C above pre-industrial levels, the Reef and its corals have no chance to survive. Rise of 2°C above pre-industrial levels would mean that 99 percent of warm water tropical coral reefs will cease to exist.
Although the situation around oceans continues to worsen there is still a glimpse of hope! In order to save them greenhouse gas emissions must peak this year at the latest and go steeply downwards thereafter, reaching net zero emissions by 2050 at the latest. For Australia, this means replacing coal and gas fired power stations with renewable energy, switching to electric modes of transportation and leaving fossil fuels in the ground. For now, the Federal Government's proposed policy - the National Energy Guarantee - that has an emissions reduction target of 27 percent below 2005 levels by 2030, makes achieving this goal impossible.