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ocean heat content

When people discuss global warming, they usually mean surface air temperature — the number that rises and falls with El Niño and La Niña cycles and makes headlines each time a new annual record is set or narrowly missed. That figure captures only a small fraction of the extra energy the planet has retained since the industrial era began. Well over 90 per cent of that energy has gone into the ocean, and the resulting measurement — ocean heat content — has continued climbing with a consistency that surface temperature records do not show.

• The upper ocean (roughly the top 700 metres) has warmed measurably across every ocean basin monitored.
• Deeper ocean layers are also absorbing heat, though the signal there is smaller and takes longer to detect with confidence.
• Because the trend is driven by long-term energy accumulation rather than short-term weather variability, it has shown fewer of the flat or cooling years sometimes seen in surface records.
• Warmer ocean water expands in volume, making thermal expansion one of the largest contributors to observed sea level rise, alongside land ice melt.