Science1 publisher2 min readPublished
University of Bonn ocean-heat maps diverge from climate models by double-digit percentages in some basins
University of Bonn geodesists mapped ocean warming basin by basin and found gaps from climate model simulations in the double-digit percentage range. The comparison covers the recent past, so whether regional projections carry the same error is still untested.
The Scientist · Science desk
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What happened
- Summed across all oceans, the Bonn reconstruction largely agrees with climate models, so the gaps appear only when the ocean is split into basins.
- Data from the GRACE gravity satellites let the team subtract sea-level rise from melting ice, leaving the expansion of warming water as a full-depth thermometer.
- Over the past 20 years, the Pacific, Atlantic and Indian oceans each absorbed about one-third of the additional heat.
- Earth is absorbing more heat than it radiates to space, and about 90 percent of that surplus ends up in the oceans.
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Why it matters
- constraint A model that matches total ocean heat can still be off in individual basins. If some basins miss by double digits, the global match is not enough to trust it for regional sea-level, marine heat-wave or El Nino work.
- decision Groups that turn sea-level rise into ocean heat using one global expansion coefficient now have a published reason to redo the conversion region by region.
- exposure The monthly maps need altimetry, gravity satellites and buoys working together. Without the gravity missions, rise from warming cannot be separated from rise from melting ice, and the authors want that funding to depend less on the United States.
- capability Monthly, full-depth heat maps by basin give modellers an observed target for testing how their simulations move heat between oceans.
The explainer gives the size of the basin gaps only as "double-digit" percentages [5]. The Bonn team wrote it themselves for phys.org, and it does not say which basins, in which direction, against which models or over what period [16]. The paper, by Bernd Uebbing and colleagues, appeared in Earth System Science Data [16].
When the total agrees and the basins do not, the question becomes where heat goes after the ocean absorbs it [4][5]. The Indian Ocean took up an unusually large amount from 2005 to 2015 [6]. The authors read the roughly even three-way split of the past two decades as a sign of heat moving between oceans [7]. Uptake and redistribution "can change dramatically within just a few years," they wrote, for causes not yet fully understood [13].
Two readings fit that record. In one, the models move heat between basins wrongly. In the other, they move it about right but in different years from the real ocean, and a gap built up over a decade could shrink as the record lengthens. The satellite record is short, with data more limited before 1993 [15]. I think the double-digit figure is best treated as a measure of how closely models track the recent past in each basin.
The thermometer is sea level. Radar altimeters measure it very accurately, and warming water expands enough to raise it by a few millimetres a year [8]. Once the GRACE correction removes the meltwater contribution, what remains is expansion averaged over the whole water column [3][9]. Drifting buoys cover the upper layers, though their coverage is incomplete [2]. Comparing the buoy record with the full-depth figure gives an estimate of how the heat splits by depth [10].
To turn millimetres into heat, you need to know how much seawater expands per degree. The team says it is the first to calculate that coefficient for different regions [11]. "This value was usually assumed to be the same everywhere in related studies, which can lead to significant errors," the researchers wrote [12].
What to watch
- Whether the full paper's basin-level gaps, with their direction and the models compared, fall outside the reconstruction's own uncertainty.
- The planned extension to decades before 1993 using historical measurements, which would show whether the basin gaps persist over a longer record.
- Results once data from the GRACE-C and NGGM satellites are added to the reconstruction.