From the Yeager et. al. paper:
“Global satellite observations show the sea surface temperature (SST) increasing since the 1970s in all ocean basins, while the net air-sea heat flux, Q, decreases.”
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While it certainly could be the case that “natural variability” dominates this trend, with the inference stated by the researchers that “diminished ocean cooling due to vertical ocean processes” is the direct cause, what one would really have to at least consider is the potential of other not completely specified dynamics. Specifically, what possible role that increasing greenhouse gases in the atmosphere might play, and how these increases could impact “diminished ocean cooling”, while at the same time giving higher SST’s. If one is concerned with the mechanisms whereby the air-sea flux Q has been decreasing, rather than look at some “natural variability” in the oceans, why not begin with something we know is changing in air side of Q, and thus consider that the rapid increase in greenhouse gases might indeed result in higher SST’s, while at the same time, reducing the rate of heat flux from ocean to atmosphere by alteration of the thermal gradient across the ocean skin layer. This reduction in the thermal gradient, with higher temperatures at the very top of the skin (i.e. SSTs), would result in a reduction in Q. At least this might be a plausible place for further research, rather than just tossing it into the “natural variability” basket.