Authors of a new paper published in the journal Science (Gebbie and Huybers, 2019) insist “the deep ocean ultimately plays a leading role in the planetary heat budget.” The global deep ocean has much less heat today than it had during both the Medieval Warm Period and the Little Ice Age.
Image Source: Gebbie and Huybers, 2019
A Bottom-Up Heat Flux?
The deep ocean may warm hundreds (to thousands) of years before hemispheric surface temperatures and CO2 concentrations do (Stott et al., 2007).
Image Source: Stott et al., 2007
This bottom-up hemispheric-scale heat flux – independent of CO2-forcing – may occur for land area as well.
“The increase of carbon dioxide concentrations occurred 2–3 thousands of years later than the heat flux increase and synchronously with temperature response.” (Demezhko and Gornostaeva, 2015)
“GST [ground surface temperature] and SHF [surface heat flux] histories differ substantially in shape and chronology. Heat flux changes ahead of temperature changes by 500–1000 years.” (Demezhko et al., 2017)
“During the Last Glacial Maximum 26–19 thousand years ago (ka), a vast ice sheet stretched over North America [Clark et al., 2009]. In subsequent millennia, as climate warmed and this ice sheet decayed, large volumes of meltwater flooded to the oceans [Tarasov and Peltier, 2006; Wickert, 2016]. This period, known as the ‘last deglaciation’, included episodes of abrupt climate change, such as the Bølling warming [~14.7–14.5 ka], when Northern Hemisphere temperatures increased by 4–5°C in just a few decades [Lea et al., 2003; Buizert et al., 2014], coinciding with a 12–22 m sea level rise in less than 340 years [5.3 meters per century] (Meltwater Pulse 1a (MWP1a)) [Deschamps et al., 2012].” (Ivanovic et al., 2017)
Deep ocean heat leads surface temperature change yet today?
A new paper indicates that the deep ocean in the Pacific has continued cooling in recent decades, extending the long-term cooling trend that commenced after the warmer-than-today Medieval Warm Period ended.
Other authors (Wunsch and Heimbach, 2014) have also documented a global-scale deep ocean (below 2,000 meters) cooling trend within the last few decades.
“About 52% of the ocean lies below 2000 m and about 18% below 3600 m. … A very weak long-term [1993-2011] cooling is seen over the bulk of the rest of the ocean below that depth [2,000 meters] including the entirety of the Pacific and Indian Oceans, along with the eastern Atlantic basin.” (Wunsch and Heimbach, 2014)
Image Source: (Wunsch and Heimbach, 2014)
Little Ice Age conditions may still dominate in the deep ocean despite the dramatic rise in CO2 concentrations during the last few hundred years — from about 280 ppm during the late 1700s to well over 400 ppm today.
The global ocean below 2000 meters may actually be colder today than during the 18th century.
The Little Ice Age and 20th-century deep Pacific cooling
“The ongoing deep Pacific is cooling, which revises Earth’s overall heat budget since 1750 downward by 35%.”
“In the deep Pacific, we find basin-wide cooling ranging from 0.02° to 0.08°C at depths between 1600 and 2800 m that is also statistically significant. The basic pattern of Atlantic warming and deep-Pacific cooling diagnosed from the observations is consistent with our model results, although the observations indicate stronger cooling trends in the Pacific.”
“These basin-wide average trends are used to relax the assumption of globally uniform changes in surface conditions and to constrain regional temperature histories for 14 distinct regions over the Common Era by a control theory method. The result, referred to as OPT-0015, fits the observed vertical structure of Pacific cooling and Atlantic warming. Global surface changes still explain the basic Atlantic-Pacific difference in OPT-0015, but greater Southern Ocean cooling between 600 and 1600 CE leads to greater rates of cooling in the deep Pacific over recent centuries.”
“OPT-0015 indicates that the upper 2000 m of the ocean has been gaining heat since the 1700s, but that one-fourth of this heat uptake was mined from the deeper ocean. This upper-lower distinction is most pronounced in the Pacific since 1750, where cooling below 2000 m offsets more than one-third of the heat gain above 2000 m.”
“Finally, we note that OPT-0015 indicates that ocean heat content was larger during the Medieval Warm Period than at present, not because surface temperature was greater, but because the deep ocean had a longer time to adjust to surface anomalies. Over multicentennial time scales, changes in upper and deep ocean heat content have similar ranges, underscoring how the deep ocean ultimately plays a leading role in the planetary heat budget.”
Image Source: Gebbie and Huybers, 2019
A lack of long-term CO2→OHC correlation
It may be worth a closer look at the graph of global ocean heat content (OHC, 0 m-bottom) during the last 2,000 years from Gebbie and Huybers (2019).
Image Source (bottom graph, heavily annotated): Gebbie and Huybers, 2019
It is interesting to note the multiple centennial-scale warming and cooling trends during the last two millennia that exceed the rate and amplitude of the ocean heat changes that have occurred since 1950, or since atmospheric CO2 concentrations began rising dramatically.
For example, despite the very modest associated changes in atmospheric CO2 concentrations (< 5 ppm), it appears that both the 1850-1875 and 1925-1945 global warming periods in the 0-700 m layer exceeded the rate and amplitude of the heat content changes since 1950.
As the global oceans rapidly warmed and cooled in the centuries preceding modern times (i.e., the Medieval Warm Period and Little Ice Age), the corresponding CO2 concentrations were remarkably stable, neither rising with the warming or falling with the cooling.
Considering 93% of the Earth’s heat changes are expressed in the global ocean, and that just 1% of global warming is said to be reflected in surface air temperatures (IPCC, 2013), the lack of conspicuous correlation between ocean heat content and CO2 during the last 2,000 years would seem to undermine claims that atmospheric CO2 concentration changes drive zero-to-bottom global ocean warming.