Global warming and the oceans: a unique threat

The insidious changes occurring in the oceans across the world are striking, and could progressively dominate the future direction and impacts of global warming on life across the planet.

Although there was been little public interest in the topic, the reality is that the top 2000 metres of their waters absorbed more than 90% of the excess heat generated by the greenhouse effect since 1900. And in doing so, the oceans have prevented a calamitous increase in the Earth’s temperature from the greenhouse effect.

One estimate is that, were the heat that has been stored down to 2000m. to have been released, it would have increased the temperature of the lower atmosphere by an incredible 36 degrees C, and not the +1.2 degrees that has been recorded since 1880. Temperature increases greater than +5C could eliminate 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 level 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 the 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 accelerated recently as the ice mass in the polar regions and the world’s glaciers started to melt, and in fact the rate of the increase doubled after 1993.  This has heightened the concerns held by many countries about the the progressive inundation by their local seas into their coastal lands, as well as about the damage being caused by the extreme storm surges that are being driven by the warmer and elevated oceans.

In addition however, the risk profile has been raised by the finding published in late 2018 that the the ice loss in the Western Antarctic ice-sheet is far greater than it was considered to be.  The impact of this on the world’s oceans will be reported on by the United Nations in 2021.

And finally, the accumulation of carbon dioxide in the ocean waters is causing significant changes to their chemistry. A key change is the creation of carbonic acid when the gas dissolves in water.  This weakens 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 and serve 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 those whales who consume them by the tonne daily.

Aspects of the worst impacts of climate change on the terrestrial environment are amenable to mitigation should action be taken early enough to contain further carbon dioxide emissions. Unfortunately though,  it appears that there is already sufficient heat stored in the oceans to guarantee that the changes discussed will occur, even if all greenhouse gas emissions were to cease tomorrow.

The task facing civilisation is now less about how to prevent these ocean impacts as it is to adapt to the new global environment they are certain to underwrite.

In the worst case, inadequately controlled emissions could generate sufficient energy in the oceans to witness average sea level rise by 1.3 metres in 2100. This would be calamitous,  because major cities such as Shanghai, Tokyo, San Francisco, London and New York could be submerged well beforehand.

Geoff Noonan

Updated 11 May 2020


 Bamber J.L et al  Ice sheet contributions to future sea-level rise from structured expert judgment;  PNAS  June 4, 2019  vol. 116 no. 23 11195–11200;

Cheng L. et al.  11 Jan 2019,  How fast are the oceans warming?   Science   Vol. 363, Issue 6423, pp. 128-129

Intergovernmental Panel on Climate Change (IPCC)  September 2019  Special Report (SPM) on the Ocean and Cryosphere in a Changing Climate.  United Nations

Intergovernmental Panel on Climate Change (IPCC) Special Report: Global Warming of 1.5 °C (IPCC SR15)   October 2018, United Nations ;

International Union for Concerned Scientists  () September 2016,  Explaining ocean warming: Causes, scale, effects and consequences

National Oceanic and Atmospheric Administration (NOAA):, Washington DC.

Rignot,E, et al,22 January 2019   Four decades of Antarctic Ice Sheet mass balance from 1979–2017  PNAS  116 (4) 1095-1103

Steffen Wil,l et al   14 August 2018:  Trajectories of the Earth System in the Anthropocene.   PNAS  2018 115 (33) 8252-8259;

United Nations Environment Program (UNEP),  13 March 2019; .  3-50C rise is now “locked-in” for the Arctic;, Nairobi, Kenya

World Meteorological Organisation (WMO)  January 2020,  Statement on the State of the Global Climate in 2019,  Geneva.

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