Introduction: Marine Ecosystem
The oceans encompass over 70% of the Earth’s surface, emphasizing their importance to the global environment. In addition to having a huge effect on global heat transfer and precipitation, the seas are home to a diverse range of marine creatures.
Humans largely impact climate through emissions from fossil fuels, industrial, agricultural, and other land-use activities that affect the atmospheric composition. Long-lived, heat-trapping greenhouse gas emissions (CO2, CH4, N2O, tropospheric ozone, and chlorofluorocarbons) warm the Earth’s surface worldwide, but shorter-lived aerosols can either warm or cool the planet’s surface regionally.
The IPCC assessment 2007 report highlights significant physical changes in the ocean. Upper-ocean heat content increased significantly during the 1950s, with mean worldwide SST increasing by around 0.4°C over this period. According to several studies and research publications, sea-ice extent has decreased drastically in the Arctic and western Antarctic Peninsula, mainly during the summer.
Climate change has a diverse impact on regional wind patterns and hence ocean circulation. A strengthening of the Southern Hemisphere’s mid-latitude westerlies supports the spin-up of subtropical circulations and a poleward shift in the Antarctic Circumpolar Current. Changes in temperature in coastal and marine habitats will affect organism metabolism and impact ecological processes such as productivity and species interactions. Species are suited to certain temperature ranges in their habitat. As temperatures fluctuate, species’ geographic ranges expand or decrease, resulting in novel species combinations that interact in unpredictable ways. Species that are unable to move or compete for resources with other species may suffer local or global extinction.
Because of the increased melting of glaciers in the world’s polar areas, sea-level rise has accelerated in recent decades. According to the most recent World Meteorological Organization statistics, the global mean sea level hit a new high in 2021, growing an average of 4.5 millimeters per year from 2013 to 2021.
Sea-level rise has intensified severe occurrences such as deadly storm surges and coastal hazards such as flooding, erosion, and landslides, which are now expected to occur at least once a year in many regions.
Main causes behind sea level rise:-
- The melting of the Antarctic and Greenland Ice Sheets.
- The thermal expansion of ocean surface waters.
- The melting of land glaciers and small ice caps.
- The thermal expansion of deep-ocean waters.
Marine heatwaves have increased in frequency, becoming longer-lasting, more severe, and more widespread. According to the IPCC, human activity is the primary cause of the observed rise in ocean heat since the 1970s.
Between 2006 and 2015, the majority of heatwaves occurred, resulting in significant coral bleaching and reef destruction. Nearly 60% of the world’s ocean surface suffered at least one period of marine heatwaves in 2021. According to the UN Environment Programme, if the ocean continues to warm, all of the world’s coral reefs would bleach by the end of the century.
Loss of Marine Biodiversity
Rising temperatures raise the possibility of permanent loss of marine and coastal ecosystems. Today, significant effects have been seen, including coral reef and mangrove damage, as well as species migration to higher latitudes where the water may be cooler. According to the latest UN Educational, Scientific, and Cultural Organization projections, more than half of the world’s marine species may be at threat of extinction by 2100.
With today’s 1.1°C temperature increase, an estimated 60% of the world’s marine ecosystems have already been destroyed or are being utilized in an unsustainable manner. A 1.5°C increase threatens to kill 70 to 90% of coral reefs, while a 2°C increase means a nearly 100% loss – a point of no return in the marine ecosystem.
Fig. Heat uptake by the top 700 m of the ocean, as determined by differences between the averages over two 5- or 20-year intervals converted to heat flux into the ocean (W m–2), either from observationally-based analyses or a 38-member ensemble of Coupled Model Intercomparison Project Phase 5 (CMIP5) Earth System Models (IPCC).
Impact on Ocean Oxygen
The balance between oxygen generation during photosynthesis, temperature-controlled solubility, and air-sea exchange control ocean oxygen (O2). Deeper in the water column, oxygen consumption during respiration, as well as redistribution via ocean circulation and mixing, are prominent activities. Theoretically, a warmer, more stratified ocean would contain less oxygen due to the combined effect of decreased gas solubility and increased interior respiration of organic matter due to improved physical separation of subsurface waters. Several studies reflect that the north Pacific and Southern Oceans have shown the largest overall oxygen declines.
Fig. Absolute change in dissolved oxygen (umol kg–1 per decade) between water depths of (a) 0 and 1200 m, and (b) 1200 m and the sea floor over the period 1960–2010 (IPCC).
Increase in Acidification
CO2 and ocean pH (as seen in the above Figure) have a direct relationship: when CO2 levels rise, pH falls. Many aquatic species and habitats are at risk as a result of this. Over the last 200 years, the oceans have absorbed around half of the anthropogenically created CO2, with an additional 1 million tonnes of CO2 diffusing into the global ocean every hour.
The rate of oceanic CO2 absorption varies geographically depending on wind intensity and temperature. Colder waters may accommodate more dissolved CO2 than warm seas, making them more acidic.
One of the most significant effects of ocean acidification on marine life results from interactions between acidity and carbonate availability. A wide range of marine animals requires calcium carbonate for their skeletons, including small coccolithophores (a form of phytoplankton), pelagic and benthic mollusks, fist-sized starfish and urchins, and giant corals (e.g. fish otoliths). All of them are predicted to suffer when rising acidity limits carbonate availability and species-level effects may cascade to wider community change in the marine ecosystem.
Combined Effects of Climate Change and Other Impacts
Most marine species and habitats are currently under multiple pressures. These include, in addition to climate change, fishing, increased UV exposure, pollution, alien introductions, and illness.
Increased in the frequency of Oceanic generated Disasters
Many experts believe that warmer waters will almost certainly enhance storminess because more heat promotes evaporation, which generates storms. Furthermore, the eventual intensity of a hurricane is heavily influenced by the temperature of deep seawater (which has also risen) churned upward as the storm passes overhead. However, the linkages between global warming and hurricane activity are complicated, with rising ocean temperatures, shifting energy distributions, and changed atmospheric dynamics all having an impact. For example, as air temperature rises, so does atmospheric stability, limiting convective movement and hence reducing tropical storm generation.
Change in Deepwater Circulation System
Deep-sea sediments and computer models show that changes in the global deep-water circulation pattern can have a substantial and rapid effect on climate. The circulation in the North Atlantic Ocean, which is a major supply of deep water (below Figure), is especially vulnerable to these changes. The sinking of cold, high-salinity, dense surface waters at high latitudes, notably in the North Atlantic, is what drives deep-water circulation.
Surface waters would absorb and disperse heat from solar radiation far less efficiently if they stopped sinking because they were too warm and/or too diluted by melting ice (and so low in density). This would almost certainly result in significantly greater surface water temperatures and far higher land temperatures than are currently observed.
Fig. Circulation in the North Atlantic Ocean A perspective image of North Atlantic Ocean circulation shows that the Gulf Stream transports a large quantity of heat northward, warming the region. As this water cools, it produces a massive amount of cold, salty, dense water known as North Atlantic Deep Water, which sinks into the deep-ocean basin and travels south. A disruption in this circulation pattern might have significant effects on the world climate.
Impact on Mangrove Forest
Mangrove forests in Florida are on the move as well. A study that compared satellite images from 1984 to 2011 concluded that Florida’s mangroves have migrated north along the U.S. East Coast by about 12 square kilometers (4.6 square miles). The mangroves’ gains come mainly at the expense of salt marshes, which thrive in areas normally too cold for mangroves. The researchers found that mangroves have expanded into places where it had been historically too cold in the winter for mangroves to survive, but winter lows are now warm enough to support the growth of mangroves.