The report has been written by Lijing Cheng (International Center for Climate and Environmental Studies, Beijing, China), John Abraham (University of St Thomas, St Paul, USA), Zeke Hausfather (University of California at Berkeley, USA), and Kevin E. Trenberth (National Center for Atmospheric Research, Boulder, USA).
The authors state that about 93% of the energy imbalance in the Earth’s climate system, caused by rising concentrations of greenhouse gases, accumulates in the oceans. This is referred to as the ocean heat content (OHC). This phenomenon reduces the rate at which surface temperatures would otherwise rise. OHC is well suited for detecting the effect of human influxes on the climate as it is less sensitive to internal variability than surface temperature measurements.
Recent measurements show a rapid warming of the oceans over the last few decades. This has resulted in increased rainfall intensity, rising sea levels, damage to coral reefs, reduced ocean oxygen levels, and a reduction in ice sheets, glaciers and icecaps in the polar regions.
The analysis of ocean data is a complicated process and the methodology has received much attention since the publication of the Intergovernmental Panel on Climate Change 5th Report (AR5) in 2013. Methods used to accurately record and interpret temperatures at different ocean depths have been significantly improved in recent years.
Recent OHC measurements show highly consistent changes since the late 1950s, with warming during the period 1971-2010 larger than that reported in AR5. The figure below shows the increasing heat content of the oceans at shallow (0 – 700 m) and deeper (700- 2,000m) depths from 1940 onwards.
The authors state that recent results indicating a slow down of increases in the global mean surface temperatures are at least in part due to a redistribution of heat within the climate system from the earth’s surface into the oceans.
It is concluded that the steady rise in OHC shows that the Earth is clearly warming, with expected increases in the climate effects already experienced.