To summarize, climate crisis is the defining issue of the century. Buildup of anthropogenic greenhouse gases (GHGs) in Earth’s atmosphere is driving global heating, while a convergence of global crises threatens to rupture key planetary boundaries. Although the human activities which drive these converging crises (for simplicity: the climate crisis) are diverse and complex, the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) breaks down the sources of anthropogenic GHG emissions into five high-level sectors. Similarly, the impacts of climate crisis – in their variety and complexity – are almost impossible for an individual to grasp, but so far, this sub-series has covered: global heating, Arctic amplification, heat waves, droughts and floods, disruption of oceanic and atmospheric patterns, and cryosphere collapse. In the absence of dramatic global climate action this decade, climate crisis will likely spiral out of control, rupturing key planetary boundaries and endangering the future of organized human life on Earth.
Today’s post will address, in a simple (so highly reductive) fashion, a complex topic – namely the reduction in dissolved oxygen content in Earth’s oceans. Ongoing ocean acidification was addressed in a previous post on planetary boundaries, and I suspect most readers will have come across the statistic that, to date, oceans have taken up ~90% of the additional heat that has been trapped within the bounds of Earth’s atmosphere since the onset of anthropogenic global heating. The oceans are getting both more acidic (though, for now, they remain mildly basic) and hotter. As the EPA (still a thing, sort of) puts it, an “inverse relationship […] exists between dissolved oxygen and temperature. As the temperature of the water increases, dissolved oxygen levels decrease.” So as the oceans warm, their oxygen content decreases. This poses some clear and, on their face, relatively obvious threats to living beings in the oceans and (on top of warming, acidification, changes in currents and major climate patterns, loss of sea ice, etc.) the stability of marine ecosystems.
In brief, here’s what AR5 includes on the matter:
For readers interested in going a little deeper (pun acknowledged) on a related topic, the June issue of Nature Climate Change had a good piece on the risk that, even under an ambitious mitigation pathway, deep ocean waters are likely to warm disproportionately over the remainder of the 21st century owing to heating that is already locked in.
Postscript: At my mom’s request/suggestion, I may start including an action item/potential solution in each post. These are “Primers” are meant, not to engender despair, but to provide a foundation of knowledge for action and to reinforce the urgency of taking coordinated global climate action at scale.