Prof. T. Subramoniam*
Emeritus Professor, Department of Zoology, University of Madras, Chennai-600025.
e-mail: thanusub@yahoo.com
*For correspondence
Introduction
Climate change assumes great importance in the context of global warming and its associated changes that affect the living of billions of people through water scarcity, crop shortages, or extremes of weather. It is increasingly understood that warmer temperatures are altering climates all over the planet. We are now only at beginning to understand how the climate change has altered the planet‟s biology and how life on earth is reacting to climate change. A United Nations climate science report released recently voiced the concern of the Intergovernmental Panel on Climate Change (IPCC) that more frequent heat spells and drought, and more erratic precipitation threaten to reduce freshwater supplies and crop yields on all populated continents and particularly in countries in dry regions and the tropics. If the rise in atmospheric concentration of heat-trapping greenhouse gases continues unabated, future food security, human well-being and wealth will be in danger. Taken together, human actions are dismantling the Earth‟s ecosystems eliminating genes, species and biological traits at an alarming rate. The loss of biological diversity will not only alter the functioning of ecosystems but also affect their ability to provide society with the goods and services.
Greenhouse gases and climate change
Climate change has its roots in global warming, a phenomenon that has happened during the last hundred years. Anthropogenic activities have constantly added billions of tones of carbon dioxide to the atmosphere, causing global temperature to rise. While atmospheric air is comprised of 99% nitrogen and oxygen, the remaining one percent is a mixture of other gases such as carbon dioxide, methane, nitrous oxide and chlorofluorocarbons, collectively called as “greenhouse gases”. Among them, carbon dioxide has garnered a great deal of attention from climate change scientists, as it is the most abundant among greenhouse gases. Despite their minor contribution to the atmosphere, greenhouse gases have a considerable effect on climate. Carbon dioxide being the chief contributor to global warming, specific attention has been paid to emission source mainly from anthropogenic activities. It is estimated that the mean global surface temperature has increased by approximately 0.10°C per decade since the late 1950s and this is projected to be 1.4 to 2.1°C above pre-industrial levels by 2050, with temperatures increasing in the arctic at almost twice the global rate in the last century. Although successive IPCC reports have reaffirmed the dire need to control greenhouse gas emissions, only limited success has been achieved in the agreement on limiting the damage between nations.
Impact on marine ecosystems
Anthropogenic carbon emissions have a major impact on the world‟s largest ecosystem-the Ocean. Climate models suggest that frequency of extreme El Ninos would double, as greenhouse-gas emissions continue to rise to warm the globe in this century. Animals and plants living in coastal ecosystems would have to respond to global warming and associated events such as higher frequency of storms, sea level rise, ocean acidification, deoxygenation and the concomitant changes in the physico-chemical properties of the sea.
While climate change over the land masses has been well studied for the obvious reason that any change could have direct spinoff to human condition, awareness on maritime climate is recent. Nevertheless, melting of Arctic ice and shrinking glaciers, along with the observed sea level rise, ocean warming and acidification of sea water are the observed phenomena that provide adequate testimony to global climate change and its impact on marine biosphere. What needs to be understood is the organismic response to situations in which long-term shifts in mean environmental conditions and climate variability move outside the bounds within which adaptations of marine communities have previously been associated.
Ocean acidification
Ocean has been shielding the earth from the worst effects of climate change by absorbing excess CO2 from the atmosphere, driving the ocean along the pH gradient towards more acidification. The absorbed atmospheric CO2 dissolves in water as carbonic acid, making the sea water more acidic with the concomitant change in the physico-chemical properties of the sea water (Fig.1).
Fig. 1 Chemical basis of ocean acidification.
Oceans are naturally basic, with a pH of about 8.2, but the pH has already slipped nearly to 8.0 and continues to drop. With increasing atmospheric carbon dioxide levels, ocean surface pH would drop from current levels of 7.9-8.3 to between 7.67 and 7.81 by 2100. Recent measurements have indicated that the aragonite saturation state in surface sea water has already dipped to 2.90 omega units. Aragonite is a form of calcium carbonate, utilized by various marine invertebrates ranging from corals to a multitude of phytoplankton that depend on it to build their exoskeletal structures. Increasing acidity could severely weaken these processes. The already altered marine ecosystems by long-term climate changes such as ocean warming and acidification are further undermined by anthropogenically influenced climate change stressors such as pollution, eutrophication, deoxygenation and overfishing. Ocean acidification can lead to (1) alteration, reduction or inhibition of growth of calcified structures by adult organisms or larvae; (2) acid-base balance and metabolic costs; (3) oxygen consumption and thermal tolerance; (4) gamete maturation and activation, fertilization success, egg hatching and embryonic and larval development.
Sea - level rise and enhanced ocean temperature, with their associated climate change could further bring about extremes of weather events, resulting in altered water currents, cyclonic winds and the consequent soil erosion and loss of wetland habitats. The combined effects of these factors have the potential to disintegrate sensitive marine ecosystems such as coral reef, sea grass meadows, and the mangrove. The effect of climate change on threatened or endemic species are of particular concern, because they have highly specific habitat requirements, leading to a poor adaptive capacity to climate change. Furthermore, only very little attempt has been made to assess the interactions between climate change and other anthropogenic stressors on threatened marine species. The changes in abundance and distribution of organisms resulting from the ocean-atmospheric changes are expected to impact the biological and environmental functioning of ecosystem or marine food webs.
Coral reef as a climate change-affected ecosystem
Coral reefs are the most enchanting ecosystem in the world oceans. No wonder, current research on the impacts of climate change on tropical fauna is biased towards corals. As coral reef ecosystem is species-rich and vibrant, global climate change is a real threat to coral reef biodiversity, affecting adversely the distribution of marine species in this dynamic ecosystem. Other human-introduced stressors like chemical pollutants not only interact with, but also exacerbate the effects of climate change. Projected increase in CO2 and temperature over the next 50 years exceed the conditions under which coral reefs have flourished over the past half million years. It has been predicted that coral reefs will change, rather than disappear entirely, with some species already showing greater tolerance to climate change and bleaching than others. It is still a major concern that the accelerating rate of environmental change could exceed the evolutionary capacity of corals and their endosymbiotic zooxanthellae species to adapt.
Coral ecosystem, being an integral part of coastal zone, is also prone to flooding from tropical cyclones, bringing sediments and inducing land subsidence. Coral bleaching has been considered as an illustrative example of marine ecosystem degradation due to ocean warming and climate change. Coral bleaching is known to occur in response to several environmental stress factors such as changes in salinity, temperature, water depth, turbidity, light penetration levels as well as exposure to air and human disturbances, in addition to certain bacterial infection, that occurs at elevated temperatures. Notwithstanding, mass bleaching of corals in several regions of the tropical seas in the last two decades has been related to the powerful warming events such as El Nino in the tropical Pacific Ocean. Coral reef systems may be just approaching a critical survival threshold, if attempts on mitigating global warming are not successful. Despite the fact that some tropical reefs have survived the climate change and regenerated in the past century, the current rate of climate change is at a much faster rate than in the past, making it impossible for the reef system to recover. Additional causes for the coral reef degradation are found in the anthropogenic stressors, including declining water quality, increased terrestrial runoff, and pollution and overfishing. Reportedly, coral cover in the Caribbean inshore reef has declined up to 80%, by virtue of declining water quality, disease and coral bleaching, affecting biodiversity and ecosystem function most adversely.
Coral bleaching is the loss of coloration in live corals that result from the loss of endosymbiotic dinoflagellate zooxanthellae, lodged within the gastro endoderm of the coral polyps and/or a reduction in photosynthetic pigment within the zooxanthellae (Fig.2). Alternatively, if the coral tissues lack pigment of their own, the white skeleton shows through. Corals have an inherent symbiont regulatory mechanism by which algal density could be changed in response to a changing environment. Changes in water depth and the resultant change in light penetration could also exert a stress to expel the zooxanthellae from the gastroendodermal layer, leading to coral bleaching. Corals have the ability to recover and regain their zooxanthellae, on return to normal conditions, but it is associated with reduced growth rate and reproductive capacity. Further, the loss of stress - related bleaching events can lead to mass mortality of coral hosts and associated collapse of reef ecosystem.
Fig. 2 Coral reef images taken from Lakshadweep. Photo courtesy: Dr. T. T. Ajith Kumar (Annamalai University).
Ocean acidification and lowering of pH of the sea water in the coral reef region could also affect the calcifying process of coral reef. Corals use aragonite, a form of calcium carbonate, in their exoskeleton formation. Any reduction in the pH and the lowering of carbonate ion concentration would adversely affect calcification process in the coral reef making. For example, coralline algae, which grow as a crust over and between the fragments and gaps in the coral reef, giving a structural continuity to the reef, may also get affected in their calcifying process by ocean acidification. Similarly, coral reef associated invertebrates such as echinoderm, tube-dwelling polychaetes and crustaceans that make home in the coral reef would also find their calcifying process more energy demanding.
Effects on mangrove and sea grass beds
These are the two other offshore ecosystems in the tropical seas, which are under the impact of global warming. Rising of sea levels has serious consequences on the survival of mangrove ecosystem. In India, Sundarban biosphere is already affected by climatic changes. Sea level rising, coupled with extreme weather events like tropical cyclone, caused submersion of several islands. With the predicted sea level rise of 28 cm, the Bengal tiger population could be declined by 96%. Mangroves, salt marshes and sea grasses are among the most cost- effective carbon capture and storage systems on the planet. Hence, conservation and prevention of ecosystem loss are important strategies for reducing CO2 emission. These ecosystems are important in protecting the shoreline by restricting the water movements. They also provide shelter and food for the commercially significant finfish and shellfish. The sea grass beds in particular provide shelter and food for mega herbivores including green turtles and dugongs.
Effects on marine microbes
Climate change also affects marine microorganisms by changing their composition, diversity, productivity, rate of
metabolism and food web dynamics. Changes in climate with increased sea water temperature create a marine environment conducive for harmful algal bloom, causing considerable mortality of fish, eutrophication and widespread disease outbreak. For some species of corals, bleaching, caused by microbial disease outbreak, has been linked with increase in water temperature. Microorganisms dominate the marine food web, and are intimately connected with food production for all life in the ocean. Intriguingly, microorganisms have a crucial role of altering the climate by accelerating the pace of climate change by adding more greenhouse gases into the seawater. For example, methane is produced and consumed in anoxic coastal sediments through microbial processes. However, marine microorganisms are also involved in mitigating measures against climate change related activities. For instance, marine microalgae, constituting 90% of the total marine biomass, play pivotal role in carbon sequestration by virtue of their increased photosynthetic capacity.
In general, organisms respond and adapt to the environmental factors favorably to live and reproduce, in a given ecosystem. Many species such as the corals also show resilience to the impact of environmental changes that occur seasonally. However, when there are long-term shifts in the mean environmental conditions and climate variability, caused by rising sea water temperature and the sea level rise, the recovery is poor and unpredicted.
Conclusion
In conclusion, it may be said that global climate change has profound and diverse effects on organisms inhabiting both terrestrial and marine ecosystems. It is therefore imperative that any study on climate change should focus attention on the manner in which organisms, in general, respond to changes in the environmental conditions due to long-term climate change that is currently affecting the planet earth and ocean. Ecological impact of global warming on marine ecosystem as well as the biotic impact on marine organisms in particular is of great concern, as evidence of ecosystem degradation and species extinction are constantly coming to the forefront. A proper understanding of the physiological as well as behavioral responses of marine organisms that are under threat of climate change is required to protect them and the ecosystem they occupy. Climate change is not just an environmental issue, but a discipline for integrative research.
References
Haque, M. A. (2019). Climate change: causes and consequences. Dream 2047. 21 (10): 23 - 27.
Shephali Sachan (2019). Climate Change and India. Dream 2047. 21 (11): 30 - 31.
Stephen Barker and Andy Ridgwell (2012). Ocean Acidification. Nat. Edu. Knowledge. 3 (10): 21.
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