Climate change is profoundly altering our oceans and marine ecosystems. Some of these changes are happening quickly and are potentially irreversible. Many are taking place silently and unnoticed.
In recent years, tipping points – thresholds where a small change could push a system into a completely new state – have increasingly become a focus for the climate research community.
However, these are typically thought of in terms of unlikely changes with huge global ramifications – often referred to as “low probability, high impact” events. Examples include the slowdown of the Atlantic Meridional Overturning Circulation and the rapid disintegration of the West Antarctic ice sheet.
In a new paper, published in the Proceedings of the National Academy of Sciences, my co-authors and I instead focus on the potential for what we call “high probability, high impact” tipping points caused by the cumulative impact of warming, acidification and deoxygenation.
We present the challenge of dealing with these imminent and long-lasting changes in the Earth system, and discuss options for mitigation and management measures to avoid crossing these tipping points.
Warming, acidification and deoxygenation
The ocean is a giant reservoir of heat and carbon. Since the beginning of the industrial revolution, the oceans have taken up around 30-40% of the carbon dioxide (CO2) and 93% of the heat added to the atmosphere through human activity.
Without ocean uptake, the scale of atmospheric warming would already be much larger. But this comes with a high cost in the form of ocean warming, acidification – where the alkaline ocean becomes more acidic – and deoxygenation – where the oxygen content of the ocean falls.
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Acidification
Besides being the primary driver of global warming, CO2 also changes ocean chemistry, causing the acidification of seawater. Many marine organisms have shells or skeletal structures made of mineral forms particularly vulnerable to ocean acidification. A well-known example are pteropods – free-swimming sea snails and sea slugs – that live in the upper 10 metres of the ocean, which are a keystone species in the marine food web.
Currently observed acidification conditions are already unprecedented within the last 65m years, and are projected to continue and aggravate for many centuries even with the reduction of carbon emissions to net-zero.
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Helena Martins, CarbonBrief, 23 February 2021. Full article.
