A Guide to Climate Tipping Points: The Oceans
Making sense of a world on edge
This article is part of a series:
- Introduction: What are tipping points?
- 1. The Arctic
- 2. The Antarctic
- 3. The Oceans
- 4. The Weather
- Reflection: Where to start?
3. The Oceans
In addition to local effects experienced in the Arctic and Antarctic, the oceans are heating and absorbing more CO2 on a global scale. This drives cascading effects on marine life and, to a lesser extent, methane deposits, both of which drive further emissions of greenhouse gases.
3.1 Weakening of the Marine Carbon Pump
How does the system normally function? The marine carbon pump, also known as the biological pump, is a food web that draws carbon from the atmosphere into the ocean and cycles it through marine life for up to thousands of years. A portion of the carbon returns to the atmosphere and another portion settles in the deep ocean, where it may remain for millions of years. The intermediate and deep ocean is believed to store 16 times more carbon than all soils and vegetation combined and draws down 17% of annual CO2 emissions.
How is it affected by climate change? The marine carbon pump is being disrupted by climate change in multiple ways. As the ocean absorbs more CO2 from the atmosphere, increasing concentrations of primary producer algae have been observed. However, rising temperatures are also causing the ocean to acidify, making algae less effective at storing carbon and making calcifying organisms like corals and certain phytoplankton less effective at maintaining their skeletons (see #3.2). Lastly, disruption to thermohaline circulation could reduce the extent to which primary producers at the surface receive essential nutrients and circulate carbon to deeper levels (see #1.3 and #2.2).
How does this affect the planet? Disruptions to the marine carbon pump could have far-reaching effects on the ocean food web, biodiversity, the ability of the ocean to draw down carbon, and the release of stored carbon.
What are the tipping points? Multiple potential tipping points have been predicted. One threshold could occur when primary producer populations shift from large diatoms to small-celled flagellates and cyanobacteria, diminishing energy transferred to the rest of the food web. Another would occur if the ocean becomes undersaturated in calcium carbonate, accelerating acidification and reducing the viability of calcifying organisms.
What is the timeline? The extent and timescale of these thresholds are uncertain. A tipping point for ocean acidification has been predicted at 450 ppm of atmospheric CO2 (2 °C global increase).
What can we do about it? Reducing CO2 in the atmosphere will lessen the multiple pressures on the marine carbon pump, as will the other strategies for reversing the slowdown of the ocean conveyor belt (see #1.3). To ease acidification, scientists are evaluating multiple additional proposals, including growing seagrasses, kelps, and shell beds in local hotspots, adding minerals like iron, limestone, or olivine to the ocean, boosting plankton growth, and chemically absorbing CO2.
3.2 Collapse of Coral Reefs
How does the system normally function? Coral reefs host thriving ecosystems. Not only are they the most biodiverse ecosystems per unit area, but the corals themselves are composite organisms comprised of an animal host and photosynthesizing algae. Coral reefs require specific temperature conditions, so they tend to exist within 30 degrees of the equator on either side.
How is it affected by climate change? Rising ocean temperatures, as well as increasing acidification due to absorption of CO2, force algae to leave their coral hosts. This is known as bleaching and, over time, causes coral to starve. Global-scale mass bleaching events associated with marine heatwaves occurred in 1998, 2010, and 2014–17. If favorable conditions return quickly, reefs can be restored.
How does this affect the planet? Damage to corals means damage to vast swaths of marine life: reefs host more than 25% of all marine fish species. They also protect coastlines from storms and erosions, which are likely to increase as a result of climate change.
What are the tipping points? Bleaching may be irreversible if coral skeletons are taken over by seaweeds. This state has shown to be reversible only in certain cases, such as coral showing strong genetic resilience or seaweed dying back due to heat or herbivorous fish. Recovery can take up to 15 years, but bleaching events now recur every 6 years.
What is the timeline? Scientists expect significant degradation to coral reefs by 2030. Under a 1.5 °C global increase, 90 percent of reefs may be at risk of degradation. At 2 °C, the rate increases to 98 percent.
What can we do about it? Preventing ocean temperature rise is critical. Scientists are experimenting with other ways to help the coral, including creating marine protected areas, breeding heat-tolerant “super coral,” and laying films of calcium carbonate on the ocean surface above reefs to shield them from the sun.
3.3 Release of Marine Methane Hydrates
How does the system normally function? Under high pressure and low temperature, methane can combine with water to form a solid known as a methane hydrate. An estimated 1,146 gigatons of methane is trapped in this inventory.
How is it affected by climate change? An increase in ocean bottom water temperature can dissociate the methane hydrate, melting the water and releasing methane as gas.
How does this affect the planet? A release of methane gas into the ocean could impact ocean oxygen levels and acidification (see #1.4 and #3.2). If the methane escapes to the atmosphere, it would increase the greenhouse gas concentration, accelerating global warming. Over a 20-year period, methane is 84 times more powerful than carbon dioxide in terms of its global warming potential, but it only lasts about 12 years, whereas carbon dioxide can last for thousands.
What are the tipping points? One study considers the conversion of marine methane hydrate into methane gas a “slow tipping point” though another says no particular thresholds have been identified, meaning its change is more gradual and continuous.
What is the timeline? The release of marine methane hydrates is not likely to contribute significantly to global warming in the near-term. An estimated 473 megatons could be released by 2100, though much of this is expected to be consumed by marine microbes. Releases are most likely in shallower coastal areas where increasing ocean temperatures are more likely to diffuse to the ocean floor.
What can we do about it? Though its effects do not present urgent concern, preventing a rise in ocean temperatures through a reduction in atmospheric greenhouse gas concentrations is the best way to mitigate the long-term release of methane hydrates.
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