Most severe effects of the climate crisis have been hidden in the ocean’s depths

World View: This summer’s sea temperatures are off the charts. But the ocean can no longer store the energy caused by climate change

Oceans make up 70 per cent of the Earth’s surface. They have absorbed 90 per cent of the global warming imbalance and 25 per cent of the responsible greenhouse gases. Research reports this summer tell how the warmest ever measured ocean temperatures are affecting sea level rises, coastal inundation, increased floods and drought cycles, plankton and fish behaviour, acidification, bleached corals, intensification of cyclones causing forest fires, and melting of Arctic and Antarctic ice.

According to Matthew England, a climate scientist at the University of New South Wales, these new measurements are off the charts. “The oceans have stored the problem,” he says, “but it’s coming back to bite us”. Between 1971 and 2018 the oceans stored the equivalent of energy produced by 25 billion Hiroshima bombs.

Warming oceans also expand, adding to melting ice’s pressure on global sea levels. But the surface water holds and produces less oxygen. Warming air also holds more water, increasing rainfall volumes – but allowing rain to fall increasingly back into the oceans rather than on land, adding to droughts.

Photography by the European Space Agency’s Copernicus project reported last month that global non-polar sea surface temperatures had reached a record 20.96 degrees. Other measurements recorded particularly high temperatures in Atlantic seas off Britain and Ireland. They affect weather patterns in unexpected ways.


But the predominant influence on the low pressure weather events in Ireland and northern Europe this summer are in the west to east movements of the atmospheric jet streams 10km above us. As John Sweeney explained in this newspaper, the associated depressions are linked to high-pressure anticyclone systems bringing searing heat across the Mediterranean, Central Asia, China, Japan and the US.

The five blockages causing these unusually prolonged events are in turn linked to contrasting heat levels between equatorial and Arctic temperatures. Since the Arctic has been heating at four times global levels this year the usual pattern is disrupted because the jet streams are sluggish and not driven by the normally higher difference between these regional temperatures. Sweeney concludes: “Some of the events of summer 2023 would be unthinkable in the absence of greenhouse gas loading of the atmosphere. Countries not taking the hard decisions to reduce their emissions radically can no longer simply be labelled as ‘laggards’. Rather they are complicit actors in the furtherance of a dystopian future.”

These changes contributing to actual or potential climate breakdown are best understood holistically. Atmospheric shifts reflect their continuing relationship with ocean conditions and this tightly coupled system works at multiple levels in both spheres. Just as scientists think about higher and lower levels of atmospheric change the same applies between surface, middle and deep ocean levels.

Some of the major possible tipping points of sudden irreversible change are heralded in research on the deepest ocean levels. A study of the Atlantic Meridional Overturning Circulation (AMOC) by scientists in Copenhagen published last month used long-term data on sea surface temperatures south of Greenland as a proxy for flows of water through this ocean. The circulation brings warmer water from the Caribbean to the Arctic where it sinks and travels back with colder water at depth to the equator over about 1,000 years.

Since AMOC gives us the Gulf Stream, which keeps Ireland mild compared with its much colder comparative latitudes in northern Canada, we should pay the closest attention to what could be happening. Most other studies conclude the system is slowing because of less salinity near Greenland due to melting ice there. The Copenhagen study models change statistically to suggest the Gulf Stream could end drastically between 2025 and 2095.

In the last month, a satellite photography study of ocean surfaces tells us they are changing colour from blue to green, due to changing phytoplankton behaviour under the influence of climate change involving photosynthetic change in these micro-organisms. Some 56 per cent of low-altitude oceans have become greener in the last 20 years.

Forty per cent of the Irish population lives within 5km of the sea. The impact of these oceanic changes on Ireland is tracked in reports from the Irish Marine Institute. Graphics published by the institute show dramatic warming of sea surface temperatures from the 1980s to the 2000s. This year’s report summarises the latest trends comprehensively and makes proposals for policymakers, including on fish behaviour.

Louisburgh in Co Mayo is a good place from which to understand and observe these oceanic changes. Locals and visitors were gratified to hear about improving conditions for salmon and sea trout populations of the local Carrownisky and Bunowen rivers from fisheries scientist Ken Whelan. He showed how co-operative local action can make a real difference in restoring a balance with nature – even when confronted by such vast forces.