Editor's commentsExperimental observations show that the global temperature rise has slowed since 2000. 90% of the heat absorbed from the sun is stored in the oceans, most in the top 700 meters but there is evidence that deep waters are also warming. This implies that the total heat stored in the ocean called ocean heat content (OHC) is a more reliable measure of how Earth’s energy budget responds to radiative changes than are surface temperatures. This article is an attempt to estimate the change in the OHC of the Pacific over the past 10,000 years by measuring an intermediate water temperature proxy in the shells of a particular plankton species deposited on the ocean floor near Indonesia. A critical assumption is that the sediment record near Indonesia is representative of most of the Pacific. Based on this assumption the observations in this article suggests that the current rate of change in OHC in the Pacific is the most rapid in the past 10,000 years.
They also suggest that small changes in high-latitude climate due to changes in the solar radiation impinging on the Earth are efficiently transferred to the ocean’s interior, and that over time this can affect the Earth's climate. This is an interesting finding because although changes in the solar radiation impinging on the Earth (radiative forcing) is thought to initiate macro climate changes such as the glacial-interglacial cycles, this must be suplemented by other mechanisms such as ice sheet dynamics, heat transport by ocean currents and biological cycles to fully explain Earth's major climate cycles.



Experimental observations show that the global temperature rise has slowed since 2000. The trend for January 1999 to December 2008 is +0.07±0.07°C per decade, much less than the 0.18°C per decade recorded between 1979 and 2005. Instrumental records show that the increase in ocean heat content (OHC) accounts for ~90% of the expected warming of Earth. This implies that OHC is a more reliable measure of how Earth’s energy budget responds to radiative changes than are surface temperatures.

A recent study attributes the global temperature slowdown to an increase in ocean heat uptake and reports that most of the excess energy was absorbed in the top 700 m of the ocean, 65% of it in the tropical Pacific and Atlantic oceans.

Experimental observations of intermediate water temperatures


In this article the researchers use a suite of sediment cores from the Makassar Strait and Flores Sea in Indonesia to document changes in the temperature of western equatorial Pacific subsurface and intermediate water masses for the past 10 thousand years. The authors argue that this region is well suited to reconstruct the entire Pacific OHC, as intermediate water masses found here form in the mid- and high-latitudes of both the northern and southern Pacific Ocean and can be traced by their distinctive salinity and density as they flow toward the equator.

The authors use Mg/Ca measurements from a type of plankton that lives in the top several hundred meters as a proxy for reconstructing intermediate water temperatures (IWTs). The species of plankton they chose is ideally suited to track small temperature changes due to its high Mg/Ca-temperature sensitivity.

The magnesium/calcium ratio (Mg/Ca) is a paleothermometer ( temperature proxy). Magnesium (Mg) is incorporated into the calcite shells of plankton as a trace element. More is incorporated into the growing calcite at higher temperatures. Therefore a high Mg/Ca ratio implies a high temperature.


Pacific Ocean IWT over the past 10 000 years
Pacific Ocean IWT over the past 10 000 years


Comparison between reconstructions of surface and intermediate-water temperatures (IWT) for past 10,000 years
(A) Global (red) and 30°N to 90°N (green) surface temperatures anomalies (°C)
(B) 30°S to 90°S surface temperature anomalies (°C)
(C) Anomalies (°C) in IWT at 500 m
(D) Anomalies (°C) in IWT at 600 to 900 m
Shaded bands represent ±1 standard deviation.
A temperature anomaly is a difference in temperature relative to a reference temperature. In this figure all anomalies are calculated relative to the temperature at 1850 to 1880 CE.
Note the different temperature scales for surface temperatures (A and B-) and IWTs (C and D).


Their reconstructions show that IWTs at all depths were substantially warmer 10,500 to 6,000 years ago than in the past century.

Intermediate Water Temperature at 500 m
Period (before present)Temperature(°C)
10,500 to 9,000 BP~10°C
8,000 to 6,000 BP~10.7°C
100 BP~7.8°C


They found that that the North Pacific and Antarctic intermediate waters were warmer by 2.1 ± 0.4°C and 1.5 ± 0.4°C, respectively, during the thermal maximum (HTM) 6,000 to 7,000 years ago than in the 20th Century. Both water masses were about 0.9°C warmer during the Medieval Warm period (950 to 1250 CE) than during the Little Ice Age (1550 to 1850 CE) and ~0.65° warmer than currently.

Northern Pacific intermediate waters
PeriodTemperature relative to 20th C
HTM 6-7,000 years ago2.1 ± 0.4°C higher
MWP 920-1250 CE0.65°C higher


Antarctic intermediate waters
PeriodTemperature relative to 20th C
HTM 6-7,000 years ago1.5 ± 0.4°C higher
MWP 920-1250 CE0.65°C higher



Two time scales are used in this article. "Years ago" or Before Present (BP) years is a time scale used to specify when events in the past occurred typically when isotopic ratios are used for dating. Because the "present" time changes, standard practice is to use 1 January 1950. CE refers to common era or anno domini (AD).


Estimating Pacific Ocean Heat Content


A critical assumption is that the observations near Indonesia are representative of the entire Pacific. Therefore to reconstruct the OHC for the entire Pacific the authors consider three cases in which the observed IWT trends near Indonesia are applied to 25, 50, and 75% of the entire Pacific volume between 0 and 700 m.

The reconstructed OHC is compared with modern observations for the whole Pacific at the same depth range. The comparison suggests that
  • Pacific OHC was substantially higher during most of the past 10,000 years than between 2000 to 2010, except during the Little Ice Age.
  • The modern rate of change in Pacific OHC change is the highest observed in the past 10,000 years.

Pacific Ocean heat content over the past 10 000 years
Pacific Ocean heat content over the past 10 000 years



Changes in Pacific Ocean heat content for the past 10,000 years
(A) Reconstructed Pacific ocean heat content (OHC) changes for the top 700 meters for the early Holocene, mid-Holocene, Medieval Warm period, and Little Ice Age periods. Reconstructed anomalies are calculated relative to the reference period of 1965 to 1970 CE.
(B) Reconstructed rates of OHC change during the main transition periods. Reconstructed anomalies and rates are compared with modern observations for the 2000 to 2010 and 1955 to 2010 CE periods, respectively.
The middle line at each box represents an average estimate for 50% of the Pacific volume between 0 and 700 meters.
The top and bottom quartiles of the box represent 62.5% and 37.5% of the total volume in this depth interval, respectively.
The bottom whiskers represent 25% of the volume.
The top whisker denotes 75%.
The modern value is based on the entire Pacific volume for 0 to 700 meters.


Relationship between ocean heat content and surface temperature


The relationship with surface temperatures and ocean heat content is complicated. But although the exact mechanism is still not understood, based on their reconstructions the authors suggest that small changes in high-latitude climate due to radiative forcing are efficiently transferred to the ocean’s interior. Over a long time, the ocean’s interior builds up large heat anomalies that reflect and, more importantly, affect the global climate.

Science 1 November 2013: Vol. 342 no. 6158 pp. 617-621 DOI: 10.1126/science.1240837