The insidious changes occurring in the oceans across the world are striking, and will influence the future direction and impacts of global warming on life across the planet.
This is not surprising noting that the top 2000 metres of their waters absorbed more than 90% of the excess heat generated by the greenhouse effect since 1900. In doing so, the oceans prevented a calamitous increase in the Earth’s temperature from the greenhouse effect.
One estimate is that, were the heat stored down to 2000 m. to have been released, it would have increased the temperature of the lower atmosphere by an incredible 36 degrees C, and not the +1.1 degrees that has been recorded since 1900. Temperature increases greater than +5C could threaten life on Earth as we know it.
Equally as striking however, is that the ocean heat content grew by 40% in the period between 2014 and 2018.(Cheng). Further, it appears that there was yet another large surge in its heat energy during 2019 and 2020.
The cumulative heat gain is increasing the frequency and intensity of marine heat waves at the ocean surface. These changes are depleting critical coastal ecosystems such as mangroves, sea grasses and salt marshes in tropical and subtropical zones. They are not only critical breeding grounds for many species of marine life, but are also estimated to have sequestered more than three billion tonnes of carbon dioxide. (WMO, 2019)
Australia’s Great Barrier Reef, in particular, is witnessing more frequent and severe bleaching events that are progressively challenging its capacity to recover. (Hughes). Early 2020 saw the third mass bleaching event that was more destructive than those that occurred in 2016 and 2017, which were equal to the worst ever recorded.
The second global impact of the warmer water is the progressive rise in the base height of local seas levels in a number of regions. Water expands as it warms, and consistent with this, the average base height of the world’s seas rose by 16 cm over the past century.
But the pace of the rise is accelerating as the ice mass in the polar regions and the world’s glaciers melts, and in fact, the rate of the increase doubled after 1993. This has heightened the concerns held by many countries not only about the progressive inundation by their local seas into their coastal lands, but also the destruction being caused by the extreme storm surges that are being driven by the warmer and elevated oceans.
And finally, the accumulation of carbon dioxide in the ocean waters is causing significant changes to their chemistry. Wide areas of deoxygenation are causing episodes of marine life mortality; and the acidification of the waters caused by the dissolved carbon dioxide is weakening the ability of molluscs, corals, and krill to build and maintain their calcium-based exoskeleton and internal skeletal structure.
Krill, for example, which are microscopic crustaceans that exist in vast numbers at the bottom of the oceans’ food chain, present a special challenge. Preliminary estimates suggest that their mass could diminish within the next few decades at a rate that will threaten the biodiversity of the oceans, including the survival of the whales who consume them by the tonne daily.
Some impacts of the extreme weather events being caused by climate change on the terrestrial environment may be amenable to mitigation, if action is taken early enough to contain further carbon dioxide emissions.
Unfortunately though, it appears that there is already sufficient heat and carbon dioxide stored in the oceans to guarantee that their deterioration will continue, even if all greenhouse gas emissions were to cease tomorrow.
In the worst case, inadequately controlled emissions could generate sufficient energy in the oceans to witness a sea level rise by 1.0 metre in 2100. This would be calamitous, because major cities such as Shanghai, Tokyo, San Francisco, London and New York could be submerged decades beforehand.
The task facing civilisation may now be less about how to prevent these ocean impacts, and more to adapt to the new harsher global environment they are certain to underwrite.
Updated 17 August 2021
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