OCEANS – The point of no return
Last week marked World Oceans Day, which is celebrated globally every year on June 8. Although the majority of people are aware of the importance that the oceans have for our planet and all living organisms on it, many of us are not fully aware of how they drastically affect life as we know it. Climate change, driven by increases in greenhouse gases such as carbon dioxide, methane, and nitrous oxide, is deeply affecting the world's oceans. These multiple changes, which are wide-ranging, significantly affect marine ecosystems, sea levels, ocean temperature, and chemistry. For this reason, in the last few decades numerous researches have been devoted to the various dimensions of the impact of climate change on the oceans, but also the consequent effects on marine life and human society globally.
One of the most obvious impacts of climate change on the oceans is the rise in sea level. This increase is primarily the result of two factors, the thermal expansion of seawater caused by warming and the melting of glaciers and polar ice caps. According to the Intergovernmental Panel on Climate Change (IPCC), global sea levels rose by about 15 cm during the 20th century, and the rate of increase has accelerated dramatically in recent decades. Sea level rise poses a significant threat to coastal communities around the world. Low-lying areas are particularly vulnerable to floods, storm surges, and erosion. Cities such as Miami, New York, and Tokyo face increased risks, requiring significant infrastructure investment to mitigate these effects. In addition, small island nations are at risk of becoming uninhabitable, raising concerns about climate refugees and loss of cultural heritage.
Another important role of the ocean is reflected in its role of absorbing excess heat. Because the oceans absorb more than 90% of the excess heat created by the emission of greenhouse gases, there is a rise in sea temperature as a side effect. This warming has several adverse effects on marine ecosystems. For example, coral reefs, which are very sensitive to temperature changes, experience bleaching events when the water gets too warm. Coral bleaching occurs when the symbiotic algae living within the coral tissue are expelled, causing the coral to lose color and vital nutrients, often resulting in widespread coral death. As a result, numerous marine animal species are also forced to migrate to colder waters, disrupting existing ecosystems and the human economies that rely on them. Thus, commercially important fish species such as cod and mackerel are changing their range, affecting the fishing industry and local food supplies.
Another significant consequence of increased carbon dioxide emissions is ocean acidification. When seawater absorbs CO2, it creates carbonic acid in the water, which lowers the pH of the ocean. Available statistics indicate that since the beginning of the Industrial Revolution, the acidity of the ocean surface has increased by about 30%. Ocean acidification has drastic effects on marine life, especially on organisms that build shells and skeletons from calcium carbonates, such as primarily mollusks, crabs, and corals. As the pH of the ocean decreases, it becomes more difficult for these organisms to form their shells, which can lead to reduced survival rates. This in turn affects the entire marine food web, including the species that humans rely on for food.
Climate change also affects ocean circulation patterns, which play a key role in regulating climate and weather systems on planet Earth. Just one of many notable examples can be found in the potential weakening of the Atlantic Meridional Overturning Circulation (AMOC), the main current system in the Atlantic Ocean, which includes the Gulf Stream within its framework. The AMOC helps transport warm water from the tropics into the North Atlantic, thus significantly influencing the weather in Europe and North America. Its slowdown could lead to significant climate change, such as lower temperatures in Europe, changes in tropical monsoon patterns, and disruption of marine ecosystems. This could further have far-reaching effects on global climate stability and human society, which depends on agricultural and fishing systems.
The cumulative effects of ocean warming, acidification, and deoxygenation, which refers to reduced oxygen levels in the ocean, contribute to the reduction of marine biodiversity. Species that cannot adapt or migrate face extinction, leading to a loss of biodiversity that can alter the structure and function of marine ecosystems in the future. Biodiversity loss also affects ecosystem services that people rely on, such as fisheries, tourism, and coastal protection. Healthy marine ecosystems also play a key role in sequestering carbon, helping to mitigate climate change. The decline of these systems could worsen global warming by reducing the ocean's capacity to absorb CO2.
Efforts to address these challenges include reducing greenhouse gas emissions to mitigate further climate change, and protecting and restoring marine ecosystems, but also investing heavily in adaptation measures such as building sea walls and creating marine protected areas. Of course, international cooperation and solid political frameworks are key to effective management and mitigation of the impact of climate change on the oceans. "The ocean has taken the heat for us... If not for the ocean, we would have already felt the effects of climate change much more dramatically. But there are limits to what the ocean can absorb, and we’re pushing those limits." - Hans-Otto Pörtner, Co-Chair of the IPCC Working Group II
