Suresh Kumar Govindarajan

Climate change has a significant impact on oceanography, particularly in the context of ocean acidity, which is often referred to as ocean acidification. Here are the answers to your questions:

Marching towards increased ocean acidity: Yes, oceans are heavily affected by climate change, and ocean acidity is increasing as a result of atmospheric CO₂ absorption. The excess carbon dioxide emitted into the atmosphere due to human activities (such as burning fossil fuels) is absorbed by the oceans. When CO₂ reacts with water, carbonic acid (H₂CO₃) is formed, which then dissociates in water, lowering the pH. This leads to increased ocean acidity.

Has the increase in dissolved CO₂ significantly lowered ocean pH? Yes, the increase in atmospheric CO₂ concentration leads to a significant rise in dissolved CO₂ in the ocean, which causes the water's pH to drop. Oceans have become more acidic by about 0.1 pH units since the beginning of industrialization, and projections indicate that this trend will continue. The decrease in ocean pH presents a major challenge for marine life.

Has the concentration of carbonate ions decreased significantly below critical saturation thresholds for calcium carbonate? Yes, the increase in ocean acidity causes a reduction in the concentration of carbonate ions (CO₃²⁻), which are important for many marine organisms, such as corals, mollusks, and certain plankton species. Carbonate ions are necessary for the formation of calcium carbonate (CaCO₃), which organisms use to build their shells or skeletons. As ocean acidity increases, the solubility of calcium carbonate rises, making it harder for organisms to accumulate it. This can lead to negative effects on the growth and survival of these organisms, and in some cases, it may even result in a reduction in coral reef areas or the degradation of marine ecosystems.

Marching towards significant variations in ocean salinity as a result of climate change: Yes, climate change leads to significant variations in ocean salinity. Changes in temperature and precipitation, as well as the melting of polar ice caps and glaciers, affect the distribution and concentration of salt in the ocean. For example, global warming leads to increased evaporation in some regions and increased precipitation in others, which can cause changes in salinity. Additionally, the melting of sea ice, caused by rising temperatures, can introduce larger amounts of fresh water into the ocean, which decreases salinity in those areas.

Have changes in ocean currents, the release of saltwater from sea ice, and the net flow of freshwater into the ocean significantly altered ocean salinity with effects on the structure of mixed layers, density stratification, and vertical movement of nutrients and marine organisms? Yes, changes in ocean currents, the melting of sea ice, and the introduction of fresh water into the oceans can significantly alter salinity, which has profound consequences for ocean structure and dynamics. The impact on mixed ocean layers and density stratification can lead to changes in vertical water movement, disrupting the natural processes of water mixing from different layers. Ocean stratification is formed by differences in water density, which depend on temperature and salinity. When fresh water is introduced into the ocean (for example, due to sea ice melting), it decreases the density of water in surface layers, which can hinder natural mixing of deeper water layers, as the density difference becomes larger.

These changes can have a significant impact on the vertical movement of nutrients and marine organisms. Nutrients in deeper waters may remain trapped in lower layers if there is insufficient mixing with surface waters, which can limit the availability of these nutrients to surface-dwelling marine organisms. This may affect plankton production and, as a consequence, the entire food chain, threatening marine ecosystems.

Have climate changes led to significant ocean warming, and in turn, increased stratification which has essentially reduced oxygen levels in the ocean? Yes, climate change has led to significant ocean warming. The temperature of ocean surfaces has increased as a result of global warming, which has resulted in increased ocean stratification. Stratification occurs when warmer, lighter water is held at the surface, while cooler, denser water remains deeper. This surface warm water makes it more difficult for natural mixing of ocean layers, reducing circulation and the transfer of oxygen into deeper layers. As warmer surface waters hold oxygen, deeper layers of the ocean become more oxygen-poor. This can lead to a significant decrease in oxygen concentrations in deeper ocean waters.

Has this led to the expansion of oxygen minimum zones in the open ocean? Yes, ocean warming and increased stratification have led to the expansion of oxygen minimum zones in the open ocean. These zones, known as anoxic zones, where oxygen levels are low, are expanding as deeper waters become increasingly oxygen-depleted. Ocean currents and mixing that would normally transfer oxygen to deeper layers become less efficient, causing these oxygen minimum zones to increase in both extent and depth. These changes can have serious consequences for marine ecosystems, as many marine species depend on oxygen and may be threatened in these zones.

Have climate changes significantly affected the fundamental structure of ocean heating; in turn, affecting the intensity of upper ocean stratification and the timing and strength of coastal upwelling? Yes, climate change has significantly affected the structure of ocean heating. The warming of the ocean surface due to global warming leads to increased stratification of the upper ocean, where warmer water remains at the surface, and cooler water stays deeper. This increased stratification can lead to reduced water circulation between the surface and deeper layers, affecting coastal currents and coastal upwelling (the rise in sea level in coastal areas). These changes are linked to changes in ocean current dynamics and can result in alterations to coastal ecosystems, as the intensity and duration of coastal upwelling depend on deep ocean circulation, which becomes more restricted due to increased stratification. Has there been a change in the vertical transport of oxygen-rich and nutrient-rich waters that affect the productivity of fisheries and marine ecosystems? Yes, climate change has led to changes in vertical water transport. Under normal conditions, warm surface waters mix with cooler, nutrient-rich waters from deeper layers, allowing oxygen and nutrients to be available for organic processes in the upper ocean layers. However, increased stratification that accompanies ocean warming hinders this natural mixing, as warmer surface water prevents deeper mixing. As a result, lower levels of oxygen and nutrients in surface waters can significantly impact phytoplankton production, which is the base of the marine food chain. This can reduce fishery productivity and affect the health and stability of marine ecosystems, as many species depend on the availability of these resources.

• Have we already experienced the end of marine heatwaves, which are a result of climate change, pushing water temperatures above critical threshold values? No, we have not seen the end of marine heatwaves. On the contrary, climate change continues to lead to more frequent and intense marine heatwaves. These heatwaves are periods when sea temperatures exceed critical thresholds, causing severe consequences for marine ecosystems. It is expected that this phenomenon will become increasingly frequent as global temperatures rise, impacting biological processes in the oceans.
• Have marine heatwaves led to (a) coral bleaching episodes? Yes, marine heatwaves have led to coral bleaching episodes. When sea temperatures exceed a certain threshold, corals experience stress and expel their symbiotic algae (zooxanthellae), which provide them with nutrients. This leads to coral bleaching, as corals lose their color and turn white. If high temperatures persist for long periods, corals may die, which has severe consequences for coral reef ecosystems.
• Have marine heatwaves led to (b) harmful algal blooms that significantly disrupt ecosystems, tourism, and human health? Yes, marine heatwaves have led to harmful algal blooms, which can have catastrophic effects on marine ecosystems. These algal blooms, often referred to as toxic algal blooms, can cause poisoning in marine organisms, as well as degrade water quality, threatening both tourism and human health. The accumulation of large amounts of algae on the surface can also deplete oxygen levels in the water, impacting marine life.
• Have marine heatwaves led to (c) changes in species composition? Yes, marine heatwaves have caused changes in species composition in marine ecosystems. Most species that are sensitive to temperature changes, such as corals, certain fish, and plankton, may be forced to migrate to cooler areas or may go extinct if sea temperatures are too high. On the other hand, some species that are more resistant to higher temperatures may thrive, leading to ecosystem imbalances and changes in the structure of marine communities.

To date, have there been significant shifts in thermal zones that affect the suitability of habitats for fisheries and marine/coastal species, as well as migration pathways in any ocean as a result of the increase in ocean average temperature (linked to climate change)? Yes, to date, there have been significant shifts in the thermal zones of the oceans as a result of climate change, affecting the suitability of habitats for fisheries and marine/coastal species, as well as migration pathways. The increase in average ocean temperatures leads to a shift in thermal zones toward higher latitudes, changing the habitat range of many species. These changes cause some species to migrate to cooler waters, while others that prefer warmer temperatures expand their territories. This impacts fisheries production, as fish and other marine animals change their migration routes, often moving outside traditional fishing areas. Additionally, coastal species can be threatened due to changes in water temperature, as they depend on specific temperature ranges. These shifts can disrupt ecosystems and affect economies reliant on fisheries and other marine resources.