Triple threat: The rising tide of climate change’s “deadly trio”

by Michael Roach

Climate change is without a doubt the most pressing issue of our time, a literally existential threat that transcends anything that may separate us. We hear a lot about extreme weather events, rising sea levels, and other destructive effects as they relate to us here on land. But how will the ocean, that vast wilderness that dominates nearly three-quarters of our planet’s surface, and its inhabitants be impacted as the climate continues to break down as a result of human activity? It turns out that our greatest ally in the fight against climate change may also be at greatest risk.

Increasing concentrations of certain gases in the atmosphere, particularly carbon dioxide (CO2), released through human activity such as the burning of fossil fuels, are changing the Earth’s climate [1]. In a phenomenon known as the “greenhouse effect”, these gases prevent heat from leaving the atmosphere and in turn cause the planet to warm [1]. It’s clear that both the concentration of CO2 and global average temperatures have been increasing over the past two centuries [2], [3], but these trends would undoubtedly be even more extreme if it weren’t for the vital role our ocean plays.

Not all CO2 ends up in the atmosphere. Natural carbon sinks, such as plants and the ocean, remove about half of all CO2 emitted through human activity [5]. In fact, the ocean has absorbed around one-third of all CO2 released over the past 200 years [6] and today holds around 60 times more carbon than the atmosphere itself [7]. Without the ocean, we would be in a much different place in the fight against climate change. But this help does not come without a price. As CO2 is absorbed into the ocean, it reacts chemically with the H2O (that is, the water) and produces carbonic acid (H2CO3) [6]. As a result, our ocean has become more acidic, a process known as acidification [6].

Ocean acidification is projected to have wide-ranging effects on marine life, but animals such as mollusks and corals are especially at risk in the face of this threat [8]. A knock-on effect of the production of carbonic acid is the reduced availability of calcium carbonate, a key building block that these animals use to construct their shells and skeletons [9]. With less of it available, these structures require more energy to form and maintain, diverting energy away from other crucial processes like reproduction and growth and putting the animals, and by extension the ecosystems that depend on them, at risk [10]. In severe cases, acidification can even cause these structures to dissolve entirely [11]. It is for these reasons that ocean acidification is sometimes called “osteoporosis of the sea” [6].

Despite the ocean’s role in absorbing a substantial amount of the CO2 released through human activity, concentrations in the atmosphere have continued to increase, trapping more heat and causing our planet to warm — by about 1.1°C, since 1850 [13]. But here, too, the ocean plays a significant part, as it has absorbed more than 90 percent of the excess heat generated by greenhouse gas emissions over the past 50 years [14]. Indeed, the ocean is a dynamic ally in the fight against climate change. But much like with its absorption of CO2, its capture of heat comes at a cost of its own, namely, as you might expect, increasing temperatures [14].

One especially infamous effect of ocean warming is the phenomenon of coral bleaching. As the ocean warms, corals expel from their tissues the microscopic algae with which they have a crucial symbiotic relationship for energy and sustenance, and, as a result, coral reefs lose their vibrant colors, turning white, and become vulnerable to disease, starvation, and other threats [15]. Bleached corals are not dead and may still recover, but their chances of survival are seriously threatened [15]. Since the 1950s, half of the world’s coral reefs have already been lost as a result of various climate change–related threats [16]. With the increasing frequency and intensity of ocean heatwaves, it is projected that the effects of warming, as well as acidification, is likely to result in the rapid decline and eventual loss of tropical coral reefs, home to 25 percent of all marine life [17], by 2050, unless warming is significantly curbed [18], [19].

Warming waters alone would be bad enough for life under the sea, but increasing temperatures are also one of the main contributors to a loss of oxygen, or deoxygenation. Water is not very effective at holding oxygen to begin with, and warmer water even less so [20]. What’s more is that as water heats up, it expands, becoming less dense; and this, combined with the melting of ice sheets in Greenland and Antarctica, adding freshwater to the salty seas, is resulting in a stratification of less dense water at the surface and denser water beneath, ultimately reducing the mixing of oxygen between the layers [14], [19].

As a result of this warming and stratification, it is estimated that the ocean has lost 1–2 percent of its oxygen content since the mid-20th century [20], and this trend is expected to continue as long as warming is left unchecked. Low levels of oxygen can result in a host of problems for marine life, such as stunted growth, reduced reproduction, and even death [19]. Deoxygenation has received comparatively less attention as a climate issue [20], but it ranks alongside warming and acidification as a major threat to our world’s oceans [19].

It’s fitting, then, that these three issues are sometimes collectively referred to as the “deadly trio” of climate change [19]. All three result from or are worsened by, in one way or another, our continued emission of CO2 into the climate system, and together they’re altering the distribution of marine populations and harming sensitive species and ecosystems [19]. And what’s especially concerning is that studies suggest that at least one of these effects has been involved in most, if not all, of the five mass extinction events Earth has experienced thus far [19]. With acidification in particular reaching levels not seen in at least the last 300 million years [19], what happens next is anyone’s guess, but things do not look good.

The common denominator in all these issues is the emission of CO2 as well as other greenhouse gases. Significantly reduce our release of CO2 into the atmosphere, and we will be well on our way to reducing the downstream effects of warming, acidification, and deoxygenation. But this is, of course, much easier said than done. There is too much profit to be made on the part of corporations, and too little interest to upset those actors on the part of elected officials, to expect that things will change any time soon, at least not without great struggle.

But there is a lot that we as individuals can do in the meantime to play our part in the fight against climate change.

• Leave the car at home and instead walk, bike, or take public transportation as much as possible [21]. This may not be practical for everyone, and indeed this would represent a significant change for most workers in the United States [22]. But transportation is the single largest source of CO2 emissions in the U.S., accounting for about 38 percent of emissions, and the majority of that (nearly 60 percent) comes from personal vehicles [23]. Every trip taken in a more sustainable way is a positive step.

• Adopt a more plant-based diet [22]. Even just eliminating red meat like beef can significantly reduce your carbon footprint, since ruminant animals like cows release methane, another significant greenhouse gas and contributor to global warming, as part of their digestion [24]. What’s more, the clearing of land to raise the cows and also grow the crops needed to feed them is a leading driver of deforestation, which further contributes to greenhouse gas emissions [25].

• Avoid food waste [21]. The one-third of all food produced that is wasted or lost accounts for about 8–10 percent of global greenhouse gas emissions, as a result of the various stages in the supply chain like production, processing, and distribution [26]. Buying just as much as you need, finding creative ways to use scraps, and eating or composting any and all leftovers can significantly reduce your carbon footprint [27].

• And, of course, use your voice and exercise your right to vote to keep the pressure on politicians and elect candidates that take climate change seriously and support the initiatives and agreements that move us, on a systemic level, toward a greener, more sustainable future.

The challenges we face may seem daunting, but they are not insurmountable. What we’ve seen is that the ocean is resilient. The record of conservation efforts to date shows that, when given the time and space, marine life can bounce back [28]. It may not be easy, but it is well within our power to achieve it. When we come together and set our sights on a better future, we can change the world.

[1] NASA Science. The causes of climate change. https://science.nasa.gov/climate-change/causes/ (2024).

[2] EPA. Climate change indicators: Atmospheric concentrations of greenhouse gases. https://19january2021snapshot.epa.gov/climate-indicators/climate-change-indicators-atmospheric-concentrations-greenhouse-gases_.html (2020).

[3] Lindsey, R. & Dahlman, L. Climate change: global temperature. Climate.gov. https://www.climate.gov/news-features/understanding-climate/climate-change-global-temperature (2025).

[5] Lindsey, R. Climate change: atmospheric carbon dioxide. Climate.gov. https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide (2025).

[6] NOAA Fisheries. Understanding ocean acidification. https://www.fisheries.noaa.gov/insight/understanding-ocean-acidification [no date].

[7] Rohr, T. et al. Oceans absorb 30% of our emissions, driven by a huge carbon pump. Tiny marine animals are key to working out its climate impacts. The Conversation. https://theconversation.com/oceans-absorb-30-of-our-emissions-driven-by-a-huge-carbon-pump-tiny-marine-animals-are-key-to-working-out-its-climate-impacts-207219 (2023).

[8] Kroeker, K. J. et al. Impacts of ocean acidification on marine organisms: quantifying sensitivities and interaction with warming. Global Change Biology 19, 1884–1896.

[9] Woods Hole Oceanographic Institution. Ocean acidification. https://www.whoi.edu/ocean-learning-hub/ocean-topics/how-the-ocean-works/ocean-chemistry/ocean-acidification/ [no date].

[10] Natural History Museum. How does ocean acidification affect marine life? https://www.nhm.ac.uk/discover/quick-questions/how-does-ocean-acidification-affect-marine-life.html [no date].

[11] EPA. Effects of ocean and coastal acidification on marine life. https://www.epa.gov/ocean-acidification/effects-ocean-and-coastal-acidification-marine-life [no date].

[12] NOAA Fisheries. Understanding ocean acidification. https://www.fisheries.noaa.gov/insight/understanding-ocean-acidification [no date].

[13] IPCC. Climate change widespread, rapid, and intensifying – IPCC. https://www.ipcc.ch/2021/08/09/ar6-wg1-20210809-pr/ (2021).

[14] Venegas, R. M. et al. Three decades of ocean warming impacts on marine ecosystems: A review and perspective. Deep Sea Research Part II: Topical Studies in Oceanography 212, 1–19 (2023).

[15] Great Barrier Reef Foundation. Coral bleaching. https://www.barrierreef.org/the-reef/threats/coral-bleaching [no date].

[16] Wetzel, C. The Planet Has Lost Half of Its Coral Reefs Since 1950. Smithsonian Magazine. https://www.smithsonianmag.com/science-nature/the-planet-has-lost-half-of-coral-reefs-since-1950-180978701/ (2021).

[17] NOAA Fisheries. Corals. https://www.fisheries.noaa.gov/corals [no date].

[18] Yves-Marie, B. et al. A rapidly closing window for coral persistence under global warming. Nature Communications 16, 1–15 (2025).

[19] Bijma, J. et al. Climate change and the oceans – What does the future hold? Marine Pollution Bulletin 74, 495–505 (2013).

[20] Limburg, K. E. et al. Ocean deoxygenation: A primer. One Earth 2, 24–29 (2020).

[21] UNEP. 10 ways you can help fight the climate crisis. https://www.unep.org/news-and-stories/story/10-ways-you-can-help-fight-climate-crisis (2022).

[22] Florida, R. The great divide in how Americans commute to work. Bloomberg. https://www.bloomberg.com/news/articles/2019-01-22/how-americans-commute-to-work-in-maps (2019).

[23] CBO. Emissions of carbon dioxide in the transportation sector. https://www.cbo.gov/publication/58861 (2022).

[24] Waite, R. et al. 6 pressing questions about beef and climate change, answered. World Resources Institute. https://www.wri.org/insights/6-pressing-questions-about-beef-and-climate-change-answered (2022).

[25] Union of Concerned Scientists. Beef cattle. https://www.ucs.org/resources/beef-cattle (2016).

[26] Londre, R. 7 ways to reduce food waste in your kitchen. Mayo Clinic Health System. https://www.mayoclinichealthsystem.org/hometown-health/speaking-of-health/7-ways-to-reduce-food-waste-in-your-kitchen%5C (2021).

[27] FAO. Food wastage footprint & climate change. https://www.fao.org/fileadmin/templates/nr/sustainability_pathways/docs/FWF_and_climate_change.pdf [no date].

[28] Duarte, C. M. et al. Rebuilding marine life. Nature 580, 39–51 (2020).