By Andy May
The Holocene Thermal Maximum, also called the Holocene Thermal Optimum, occurred at different times in different parts of the world but generally between 10,000 BP and 4,000 BP. I use BP to indicate years before 2000. The world ocean was probably 0.7°C warmer than today 8,000 BP. This is remarkable because the ocean heat capacity is 1000 times larger than the atmosphere’s according to the IPCC and NOAA. Simple high school physics is all that is required to verify this, the calculation is described here. What this means is that if you heated the atmosphere to 1000°C and transferred all of that heat to the ocean, the ocean would only warm 1°C once the heat was well mixed. We can draw two conclusions from these facts. First, the world was much warmer 8,000 BP than today and the total heat stored in the atmosphere and in the oceans was much greater. That 0.7°C represents the heat required to warm the atmosphere to over 700°C. This would never happen, of course, ocean-atmosphere heat transfer processes would work to move heat from the ocean to the atmosphere and back again to keep temperatures moderate and stable.
The second conclusion is that there is no magic 2°C tipping point. Raising todays atmospheric temperature 2°C involves an insignificant amount of heat relative to the total ocean/atmosphere heat present only 8,000 years ago. If the oceans absorbed 2°C worth of atmospheric heat, the ocean temperature would only go up a trivial and unmeasurable 0.002°C.