Sylvia Earle, John Baxter and Dan Laffoley – While the world is finally turning its attention to the problem of carbon emissions, we are missing something big.
While it’s positive that the recently concluded climate discussions at COP25 in Madrid, Spain, finally recognised the ocean as a key part of the climate system, solving the climate crisis is not just about radically reducing carbon dioxide in the air. We also need to sustain the level of oxygen in the ocean. In fact, the two are inextricably linked.
Human activities are driving this life-giving gas out of the aquatic world that provides 90 per cent of the available living space for species on Earth. This increasingly widespread phenomenon is being termed by scientists ‘ocean deoxygenation’.
The scale of oxygen decline in the open ocean – and the link between carbon emissions and oxygen decline in both the open ocean and coastal waters – has been recognised so recently that few know it is a problem. And even fewer recognise it with the seriousness and significance it warrants.
Of the two main causes of deoxygenation, one is increasingly well understood: the loss of oxygen caused by nutrient run-off from land and sewage, and the deposition of nitrogen from the burning of fossil fuels. Such nutrient enrichment induces excessive growth of algae which ultimately results in oxygen depletion of the water body.
This has resulted in marine areas where oxygen levels are now so low that life struggles to exist. Of the 700 known “dead zones”, 500 have formed in estuaries and other coastal water bodies since the 1960s.
The other, more recently recognised cause is the heating of the ocean due to climate change. Over 90 per cent of the excess heat resulting from the enhanced greenhouse effect in the atmosphere has ended up in the ocean, supercharging it as the largest thermal reservoir on the planet. All that heat coupled with climate change-driven changes in weather, winds and currents, is driving progressive ocean deoxygenation across great swathes of our watery world.
Ocean heating reduces the amount of oxygen seawater can hold and reduces the replenishment of ocean oxygen that normally occurs through vertical mixing.