The forcing uncertainties and lack of observational measurements in the top-to-bottom global ocean preclude an assessment that modern warmth is due to anthropogenic activities.
Key points from a new paper (Gebbie, 2021):
• 93% of the changes to the Earth’s energy budget, manifested as warming of the Earth system, are expressed in the global ocean. Just 1% of global warming is atmospheric.
• Even with the advent of “quasi-global” temperature sampling of the ocean since 2005 (ARGO), these floats (pictured) “do not measure below 2,000-m depth.” This means that temperature changes in “approximately half the ocean’s volume” are still not being measured today.
• To detect the effects of anthropogenic forcing, it would require energy budget imbalance measurement precision of 0.1 W/m² at the top of the atmosphere (TOA). Uncertainty in the forcing changes affecting climate is ±4 W/m², meaning that uncertainty is about 80 times greater than an anthropogenic signal detection.
• Past changes in global ocean heat content, such as the last deglaciation, have been 20 times larger than modern changes.
• Ocean heat storage during the Medieval Warm Period (Medieval Climate Anomaly, or MCA) was much greater than modern. Modern global ocean heat uptake is “just one-third” of what is required to reach the levels attained during Medieval times.
One final point. Dr. Gebbie asserts that approximately 15% of modern global warming (ocean) can be attributed to geothermal heat fluxes through the seafloor that “persistently heat the ocean.”
Interestingly, he also assesses that the value attained for geothermal heating of the ocean, 87 mW/m², is similar to that which is required to end a glacial period (melt ice sheets) and transition into an interglacial.
Considering the ocean bottom waters warmed up 2°C from 19,000 to 17,000 years ago about 1,000 years before the surface warmed (and CO2 began rising) (Stott et al., 2007), and that Arctic bottom waters were 6-10°C warmer than today at the beginning of the Holocene about 10,000 years ago (Beierlein et al., 2015), geothermal heat fluxes could potentially explain a large portion of glacial-interglacial transitions – as well as millennial-scale global ocean temperature changes.
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