Short answers, pulled from the story.
Ocean heat content (OHC) is the measure of energy absorbed and stored by the world's oceans. It matters because between 1971 and 2018 the oceans absorbed more than 90% of the excess heat generated by global warming, making OHC the primary indicator of how the planet's energy budget is changing.
Argo floats follow a 10-day cycle: each float descends to 1000 metres, drifts for nine days, then drops to 2000 metres and measures temperature, salinity, and pressure on its ascent to the surface, transmitting data via satellite relay. The network had expanded to nearly 4000 floats by 2020.
The five highest ocean heat observations at depths up to 2000 metres all occurred in the period 2020-2024. Global upper-2000 metre ocean heat content reached a new record in 2024, exceeding the 2023 value by 16 plus or minus 8 zettajoules, continuing a run of annual records spanning the past eight years.
Thermal expansion from ocean warming has accounted for 30-40% of global sea-level rise from 1900 to 2020. Breakup of the Thwaites Ice Shelf and its West Antarctic neighbours contributed about 10% of sea-level rise in 2020 as a separate but related consequence of warming waters.
A large-ensemble reanalysis published in 2024 estimated a 1961-2022 warming trend of 0.43 plus or minus 0.08 watts per square metre, with a statistically significant acceleration rate of 0.15 plus or minus 0.04 watts per square metre per decade.
Scientists say with very high confidence that increased ocean heat content in response to anthropogenic carbon dioxide emissions is essentially irreversible on human time scales. The net annual energy gain in the top 2000 metres from 2003 to 2018 averaged 9.3 zettajoules per year.