Role of the Ocean in Moderating Anthropogenic Climate Change

“The ocean moderates anthropogenic climate change at the cost of profound alterations of its physics, chemistry, ecology, and services.” Discuss this statement in relation to ocean acidification and warming, taking environmental impacts and potential future scenarios into consideration

Oceans play a vital role in moderating anthropogenic climate change by aiding in the regulation of both heat and carbon dioxide levels (J.-P Gattuso et al.). This paper will focus on the oceans enormous importance in regulating global climate change in order to understand how it alters the chemical, ecological services of the ocean, taking into account potential future scenarios. Experts have recognised that the ocean absorbs over 90% of the Earth’s additional heat since the 1970s, keeping the atmosphere cooler and regulating the effects of climate change (J.-P Gattuso et al.). However, this comes at profound altering costs for the ocean. As more carbon dioxide (CO₂) is absorbed into the ocean, chemical reactions occur, causing the ocean to become warmer and more acidic (S.C. Doney, et al). Both have very serious consequences including, but not limited to: mass coral bleaching (O. Hoegh-Guldberg, et al), reduced calcification of marine organisms that produce carbonate shells/skeletons (Ulf Rlebesell, et al), sea level rise and ocean deoxygenation. These alterations in the ocean will have tremendous impacts on marine ecology and therefore will affect the fundamental benefits humans derive from the ocean. Without the ocean, present climate change would be far more intense and challenging for human life.

The Earth’s climate has naturally changed significantly throughout history, however strong evidence suggests that recent changes are attributed to human activity, such as burning fossil

fuels[1]. Through human activities CO₂ and other greenhouse gasses are released into the atmosphere, trapping heat and leading to atmospheric warming and climate change. Atmospheric CO₂ levels have increased by almost 40% over the past 250 years, from the preindustrial levels of 280 ppmv (parts per million by volume) to almost 384 ppmv in 2007 (S.C. Doney, et al). Today, the concentration of CO₂ in the Earth’s atmosphere now exceeds 400 ppmv, which is higher than Earth has experienced for over 800,000 years (Luthi et al. 2008). Driven by anthropogenic activity, this rate of increase is an order of magnitude faster than has occurred for millions of years (J.-P Gattuso et al.).

Rising atmospheric CO₂ is alleviated by oceanic uptake, acting as a climate integrator. The ocean has absorbed 93% of Earth’s additional heat since the 1970s, reducing much of atmospheric warming but increasing ocean temperature and sea level (J.-P Gattuso et al.). In addition to this, the ocean has captured 28% of anthropogenic CO₂ emissions since 1750, leading to ocean acidification; and accumulated nearly all the water from melting ice caps, furthering the rise in sea level (J.-P Gattuso et al.). Therefore, by regulating the Earth’s climate, the ocean has been disproportionately impacted by increasing CO₂ at the cost of major changes in its fundamental physics and chemistry. Without the regulation of the ocean, atmospheric CO₂ would be over 450 ppmv today, a level that would have led to even greater changes in climate than seen today (S.C. Doney, et al). Ocean CO₂ uptake has caused pH reductions, altering fundamental chemical balances, ocean warming, resulting in sea level rise and deoxygenation. Increasing atmospheric CO₂ emissions during the 20^th century has caused an increase in the oceans average temperature and sea level, and has depleted seawater carbonate concentrations and acidity (O. Hoegh-Guldberg, et al). These changes in ocean properties greatly affect species’ biogeography, as well as ecosystem dynamics and therefore inevitably affect services on which humans depend.

Ocean acidification is one of the greatest environmental concerns involving a changing climate. Due to the rapid increase of atmospheric CO₂, marine life will struggle to adapt to a reduction in pH, along with many other climate affects, as they have evolved over millions of years in a relatively stable environment[2].The surface ocean absorbs approximately one third of the excess CO₂ in the atmosphere from of all anthropogenic emissions (S.C. Doney, et al). While absorption of CO₂ by the ocean slows the atmospheric greenhouse effect, it puts marine, and as a result, human life at risk. When atmospheric CO₂ dissolves in seawater it combines to produce carbonic acid, which releases H+ ions into the ocean to combine with other molecules (S.C. Doney, et al). The concentration of hydrogen ions have increased by over 30% compared with preindustrial times, increasing the level of ocean acidity². Consequently, the average pH of ocean surface waters has fallen by 0.1 units, from 8.2 to 8.1 pH. This corresponds to a 26% increase in ocean acidity, a rate almost 10 times faster than any time in the last 55 million years[3].

This equilibrium reaction demonstrates the chemistry change that occurs when CO₂ dissolves in seawater. Once dissolved, CO₂ gas reacts with water to form carbonic acid, which can then dissociate by losing hydrogen ions to form bicarbonate and carbonate ions.

A lowered ocean pH has tremendous impacts on marine biodiversity and ecosystems. Ocean acidification will directly impact a wide range of marine organisms that build shells from calcium carbonate, including molluscs, echinoderms, corals and algae (Ulf Rlebesell, et al). This is a result of carbonic acid dissociating to form bicarbonate ions in water, reducing the availability of carbonate to biological systems. As a result of decreasing carbonate ions, rates of calcification among many calcium carbonate secreting organisms are reduced (Ulf Rlebesell, et al). One major organism that will be affected are coral reefs, which will erode at a faster rate than they are created, causing detrimental impacts to marine life that rely on them, altering the food web and ecosystems (O. Hoegh-Guldberg, et al).Coral reefs also provide many services for humans, they: provide millions of jobs to local people through tourism, recreational activities and many other services, with the Great barrier reef generating more than $5.2[4] billion to the Australian economy; act as a physical barrier for coastlines, protecting them from the destructive action of waves, floods and tropical storms and thereby help to prevent loss of life, property damage and erosion that has been valued at $9 billion per year; and a loss of marine life, including fish, oysters and mussels which has the potential to affect food security. Responses to future levels of ocean acidification expected by the end of the century include reduced calcification, reduced rates of repair and weakened calcified structures (J.-P Gattuso et al.). Reproductive success, early life-stage survival, feeding rate, and stress-response mechanisms may also be affected.

Furthermore, experiments have shown that acidification affects ocean physics by reducing sound absorption and thus allowing sound to travel much further (Hester et al. 2008). Therefore, as the ocean becomes more acidic, noise from ships and other ocean vessels will be able to travel further and as a result interfere with whales and other organisms that rely on sound to navigate, hunt and communicate[5].

Finally, the capacity of the ocean to absorb CO₂ decreases as ocean acidification increases, impacting carbon storage and climate regulation and thus creating more environmental problems in the future. If greenhouse gas emissions continue as they are doing at present, seawater could increase its acidity by 0.4 units by 2100[6]. Acidification impacts on processes so fundamental to the overall structure and function of marine ecosystems that any significant changes could have far-reaching consequences for the oceans of the future and the millions of people that depend on its food and other resources for their livelihoods

Equally important, the ocean will also become warmer due to the absorption of excess heat in the atmosphere. Increasing concentrations of greenhouse gases are preventing heat from escaping into space freely, and therefore trapping it in the Earth’s atmosphere. As there is more heat in the atmosphere, more of it is absorbed into the ocean causing it to become warmer. More than 90% of the warming that has happening on Earth over the past 50 years has occurred in the ocean[7], with most of it occurring in the upper oceans⁶. This means that although the atmosphere has been spared from the full extent of global warming, heat already stored in the ocean will eventually be released, committing Earth to additional warming in the future[8].

Sea level rise is one of the many consequences of ocean warming. During the 20^th century, increasing CO2 levels drove the ocean’s average temperature to increase by 0.74˚C and sea water level by 17cm (O. Hoegh-Guldberg, et al). As water warms, it expands and the ocean surface rises. Currently, most of the excess heat in the ocean is in the surface layers, however over time this heat will diffuse downward, increasing expansion and triggering further changes in sea level (Domingues et al. 2008). Additional sea-level rise is caused by the melting of ice caps, as a result of a warmer atmosphere and ocean. These combining factors threaten natural ecosystems and human structures near coastlines around the world. By 2100, studies conclude that sea levels could increase by 1-3 metres, depending on future global emissions[9]. A change this significant causes storm surges and flooding to be more dangerous and to occur more regularly (McMullen and Jabbour 2009).

As temperatures rise, mass coral bleaching events are increasing in frequency and intensity. Coral bleaching is a global crisis caused by increased ocean temperatures driven by anthropogenic carbon pollution. Higher ocean temperatures causes thermal stress in corals contributing to coral bleaching and infectious disease (O. Hoegh-Guldberg, et al). Higher temperatures saw the bleaching of the Great Barrier Reef in 2016 and 2017, killing approximately 50% of its coral[10]. Coral bleaching causes reduced growth rates, reproductive capacity, increased susceptibility to diseases and elevated mortality rates[11]. Changes in coral communities affect the species that depend on them, such as fish and invertebrates that rely on live coral for food or shelter, making them more vulnerable to population declines. Degraded coral reefs impact human services, such as commercial fishing, tourism and coastal protection. A UNESCO report predicts that the coral reef in the 29 reef-containing World Heritage sites would cease to exist as functioning coral reef ecosystems by the end of this century if carbon emissions continue at this rate[12]. This dramatically alters the function of the ecosystem, as well as the goods and services coral reefs provide to humans.

Furthermore, a warming ocean additionally causes deoxygenation[13]. Ocean deoxygenation is a result of warmer oceans, which cause oxygen to become less soluble and increases stratification of ocean layers therefore inhibiting the production of oxygen from photosynthesis¹². Healthy marine ecosystems require oxygen in order to grow, reproduce and survive. Thereby, the implications for this are extreme, including limitations for ocean productivity, nutrient cycling, carbon cycling and marine habitat. Long term ocean monitoring shows that oxygen concentrations in the ocean have declined by more than 2% between 1960 and 2010[14]. Reports predict that they will decrease by a further 3-6% during the 21st century in response to surface warming[15]. Oxygen plays a direct role in the biogeochemical recycling of carbon, nitrogen and many other important elements, and is also fundamental for all aerobic life[16]. Deoxygenation will be catastrophic to the marine environment at the local level, and in turn economic and socio-economic impacts will impair the human society at the regional and global level.

The evidence presented in this essay examines the ability of the ocean to strongly influence the climate system, at the cost of its own health. The ocean absorbs much of the Earth’s additional heat and CO₂ emissions, which has aided in the cooling of the Earth but has severely impacted the ocean, causing an increase in ocean temperature and acidification. Without the ocean, present climate change would be far more intense and challenging for human life. The rising temperatures, coupled with ocean acidification, affect marine species and ecosystems, and consequently, the fundamental benefits humans derive from the ocean. If fossil fuels and greenhouse emissions are not decreased by 2030, there will be an onset of alterations in our oceans that we will no longer be able to fix. The ocean has a very important role in moderating our climate system, however this comes at the cost of altering fundamental chemical, physical and ecological processors which in turn affect the fundamental benefits humans derive from the ocean.

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[1] Climate Change: Vital Signs of the Planet. (2019). Climate Change Evidence: How Do We Know?. [online] Available at: https://climate.nasa.gov/evidence/.

[2] https://ocean.si.edu/ocean-life/invertebrates/ocean-acidification

[3] Ocean acidification and its effects | CoastAdapt. (2019). Retrieved from https://coastadapt.com.au/ocean-acidification-and-its-effects

[4] Tourism in the Great Barrier Reef | SELTMP. (2019). Retrieved from https://seltmp.eatlas.org.au/seltmp/tourists-tourism-operators

[5] https://www.whoi.edu/oceanus/feature/will-more-acidic-oceans-be-noisier/

[6] Ocean acidification and its effects | CoastAdapt. (2019). Retrieved from https://coastadapt.com.au/ocean-acidification-and-its-effects

[7] Climate Change: Ocean Heat Content | NOAA Climate.gov. (2019). Retrieved from https://www.climate.gov/news-features/understanding-climate/climate-change-ocean-heat-content

[8] https://www.climate.gov/news-features/understanding-climate/climate-change-ocean-heat-content

[9] https://www.environment.gov.au/climate-change/climate-science-data/climate-science/climate-change-future

[10] Scientist reveals bleaching has killed almost 50% of the Great Barrier Reef’s coral. (2019). Retrieved from https://www.wwf.org.au/news/news/2017/scientist-reveals-bleaching-has-killed-almost-50-of-the-great-barrier-reef-s-coral

[11] Bleaching Impacts | Reef Resilience. (2019). Retrieved from https://reefresilience.org/coral-reefs/stressors/bleaching/bleaching-impacts/

[12] Coral reefs and climate change. (2019). Retrieved from https://www.iucn.org/resources/issues-briefs/coral-reefs-and-climate-change

[13] A breathless ocean: What we know about ocean deoxygenation | United Nations Educational, Scientific and Cultural Organization. (2019). Retrieved from http://www.unesco.org/new/en/media-services/single-view/news/a_breathless_ocean_what_we_know_about_ocean_deoxygenation/

[14] Another Threat to the Ocean: Deoxygenation. (2019). Retrieved from https://www.newsdeeply.com/oceans/articles/2017/07/05/another-threat-to-the-ocean-deoxygenation

[15] Ocean Deoxygenation. (2019). Retrieved from https://www.oceanscientists.org/index.php/topics/ocean-deoxygenation

[16]https://sustainabledevelopment.un.org/content/documents/5849The%20Ocean%20is%20Losing%20its%20Breath.pdf