A paleo-perspective on ocean heat content: Lessons from the Holocene and Common Era

Abstract

The ocean constitutes the largest heat reservoir in the Earth's energy budget and thus exerts a major influence on its climate. Instrumental observations show an increase in ocean heat content (OHC) associated with the increase in greenhouse emissions. Here we review proxy records of intermediate water temperatures from sediment cores in the equatorial Pacific and northeastern Atlantic Oceans, spanning 10,000 years beyond the instrumental record. These records suggests that intermediate waters were 1.5-2°C warmer during the Holocene Thermal Maximum than in the last century. Intermediate water masses cooled by 0.9°C from the Medieval Climate Anomaly to the Little Ice Age. These changes are significantly larger than the temperature anomalies documented in the instrumental record. The implied large perturbations in OHC and Earth's energy budget may seem at odds with very small radiative forcing anomalies throughout the Holocene and Common Era. We suggest that even very small radiative perturbations can change the latitudinal temperature gradient and strongly affect prevailing atmospheric wind systems and hence air-sea heat exchange. These dynamic processes provide an efficient mechanism to amplify small changes in insolation into relatively large changes in OHC. Over long time periods the ocean's interior acts like a capacitor and builds up large (positive and negative) heat anomalies that can mitigate or amplify small radiative perturbations as seen in the Holocene trend and Common Era anomalies, respectively. Evidently the ocean's interior is more sensitive to small external forcings than the global surface ocean because of the high sensitivity of heat exchange in the high-latitudes to climate variations.

Atlantic Warming Since the Little Ice Age

“…This importance of the deep layer is due to the long timescale of the Little Ice Age cooling trend, where the deep Atlantic had enough time to fully respond to the surface. Our findings highlight how the memory of the ocean preserves the signal of temperature variability in intermediate and deep waters, as previously inferred from paleoceanographic observations over the Holocene (Rosenthal et al, 2017).…”

Early Holocene Thermal Maximum recorded by branched tetraethers and pollen in Western Europe (Massif Central, France)

“…At the scale of the northern hemisphere, the temperature evolution at St Front is also consistent with Iceland mean annual temperature obtained from brGDGTs (Moossen et al, 2015), the Greenland temperature record based on combined nitrogen and argon isotope analyses ( Fig. 7E; Kobashi et al, 2017), the North Atlantic SST based on Mg/Ca ratios (Rosenthal et al, 2017), the southwestern Chinese temperature obtained from brGDGTs (Ning et al, 2019) and the northern hemisphere temperatures synthesis from Marcott et al (2013) (Fig. 7B).…”

Temperature Evolution of the Indo‐Pacific Warm Pool Over the Holocene and the Last Deglaciation

“…Intermediate water temperatures in the Indonesian Throughflow region derived from North Pacific and southern sourced intermediate waters also show a similar long‐term cooling over the Holocene (Rosenthal et al, ). The Holocene cooling deeper in the water column was potentially driven by high‐latitude processes (Kalansky et al, ; Rosenthal et al, ), and consequently regionally expressed in the SST of the IPWP by upward mixing and thermal diffusion processes (e.g., Richards et al, ). It is therefore possible that the consistent Holocene cooling in the southern regions, most noticeably in the Timor Sea region, was a result of increased upward mixing of the cold waters via water mass transformation within the ITF region (i.e., the Banda Sea).…”