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. Bob Tisdale shows while we have good Argo float data from 0-2000 meters, these depths only include about one-half of the volume of the oceans. NASA has shown that the water below 2000 meters has shown no detectable warming. The key point is that the oceans will mitigate any atmospheric warming, man-made or not. Direct infrared radiation from greenhouse gases probably warms the oceans a little, but direct solar radiation does most of the work. Longer term ocean/atmospheric heat transfer (like evaporation, ENSO and other processes) do transfer a lot of heat from the atmosphere to the oceans and back again to stabilize the system. The graph below (Figure 1), made from data from the NOAA web site, shows that the world ocean average temperature has gone up only 0.1°C in the last 60 years, less than the error in the data. The precision of the ARGO thermometers is very good, +-0.002°C, but the accuracy is only 0.5°C over most of the world ocean.
There seems to be general agreement that the cause of the Holocene Thermal Maximum is the Earth’s precession cycle. As described in Michael Bender’s book “Paleoclimate,” a part of the Princeton Primers on Climate series:
“The orientation of Earth’s spin axis has changed over the past 10 Kyr so that northern summers now occur when Earth is farthest from the sun, whereas at 10 Ka [10,000 BP] they occurred when Earth was closest to the sun. Northern summertime insolation reached a maximum at about 10 Ka and has declined to the present, when it is near the minimum.”
Bender has determined that remnants of the Laurentide Ice Sheet (LIS) from the last glacial period contributed to cooling many northern areas and delaying their warm periods. These areas, like Western Europe had their climate optimum between 4,000 BP and 7,000 BP. In Germany and Scandinavia mean annual temperatures were warmest 6,000 BP to 7,000 BP and they have since fallen 2.5°C. In Alaska and western Canada the thermal maximum occurred 4,000 years before it occurred in northeast Canada and again the LIS was to blame.
The world ocean is all connected and currents distribute heat from one area to another. While the ocean is never at thermal equilibrium, over long periods (hundreds of years) heat can be redistributed all over the world. Rosenthal, et al, 2013 chose an area in Indonesia that is well located to reconstruct past Pacific Ocean heat content. They used a suite of sediment cores from the sea floor of the Makassar Strait and the Flores Sea (see Figure 2 below) to perform the reconstruction. These areas are major conduits for the exchange of water between the Pacific and the Indian Oceans and collectively they are referred to as the “Indonesian Throughflow.” The shallower current (0-200 meters) is from the North Pacific Ocean and the deeper current gets a large contribution from the Banda Sea and the South Pacific. The study uses both Magnesium/Calcium ratios and Oxygen isotope ratios in Foraminifera to reconstruct the temperatures of the past. Because the Foraminifera studied live at different depths reconstructions of both the surface temperatures and the intermediate depth temperatures were possible.
Figure 3 is taken from a portion of Figure 2 in Rosenthal, et al. 2013. In graph A the green curve is the reconstructed average surface water temperature for 30°N to 90°N latitude and the red curve is the reconstructed global average surface water temperature. In graph C the Northern Hemisphere (30°N to 90°N latitude) reconstructed average intermediate water temperature for a depth of 500 meters is plotted. Both plots show that Northern Hemisphere ocean temperatures between 9,000 BP and 7,000 BP were 2.5°C+-0.4°C warmer than the late 20th Century. Global Ocean temperatures are estimated to be 0.7°C warmer than in the late 20th Century.
In addition to warming the northern oceans, the Earth’s precession cycle also moves the “ITCZ” or the Intertropical Convergence Zone according to Michael Bender’s “Paleoclimate.” The ITCZ is a zone of warm rising air and high precipitation. This zone follows the sun, so when the Earth was closest to sun in the northern summer 10,000 BP the ITCZ was farther north and the Northern Hemisphere tropics received more rain. Currently the ITCZ is roughly centered on the equator (5.3°S to 7.2°N). In Africa, as everywhere on land, it moves a lot from summer to winter. However, it stays south of the Sahel, about 15°N. This process contributed to the Sahara region becoming a desert roughly 5,000 BP as the ITCZ moved south. In China, An, et al, have found that the peak monsoon precipitation event moved from northern China 10,000 BP to Southern China 3,000 BP. This suggests that China has been getting progressively cooler and drier over the last 10,000 years.
Climate and climate change are long term processes. Looking long term, it is clear the Earth and especially the Northern Hemisphere are cooler today than 7,000 BP and we are in a cooling trend. For a longer perspective see my previous post “Climate and Human Civilization over the last 18,000 Years.” Might this trend be changing and might it be due to man’s influence? Perhaps, at least in part. But, it is very premature to predict a disaster based on a shaky 150 year surface temperature record. Particularly when the record does not agree with existing atmospheric balloon and satellite datasets that are of arguably better quality. Further, it is clear that the enormous heat capacity of the world ocean will dampen any radical atmospheric temperature changes. Basically, there is nothing to worry about, no radical action is required.
One more point, the media and the climate alarmists like to say that the reason atmospheric temperatures are not rising with the increase in carbon dioxide is that the extra heat is “hiding” in the deep ocean. It has to be the deep ocean because measurements of shallow ocean temperatures have not shown any excess warming. Certainly, as we have seen, most of the heat is going into the oceans. But, the temperature rise caused by that transfer is very small. Since heat only moves from a warmer object to a cooler object, the heat will never exit from the ocean until the atmospheric temperature drops. At that point we will want that heat. If indeed, carbon dioxide is causing more heat to be trapped in the atmosphere, the oceans are the perfect place for it to go.