Environment Counts | Observation of Pacific Ocean heat content over the past 10,000 years
Author: Geoff Zeiss – Published At: 2013-12-28 13:39 – (1151 Reads)
It has been argued that the total heat stored in the ocean is a more reliable measure of how Earthâ€™s energy budget responds to radiative changes than are surface temperatures. In this article the authors compiled high-resolution proxy records for ocean temperature and heat content (OHC) from sediment cores from the Makassar Strait and Flores Sea in Indonesia to extend observations of the Pacific OHC 10,000 years beyond the modern instrumental record. Assuming the record near Indonesia is representative of the entire Pacific, their results suggests that Pacific OHC was substantially higher during most of the past 10,000 years than in the past decade (2000 to 2010), except during the Little Ice Age. They also find that the rate of change of Pacific OHC between 1955 and 2010 is the highest in the past 10,000 years. The authors suggest that small changes in high-latitude climate due to radiative forcing are efficiently transferred to the oceanâ€™s interior. Over a long time, the oceanâ€™s interior builds up large heat anomalies that affect the global climate. Science 1 November 2013: Vol. 342 no. 6158 pp. 617-621 DOI: 10.1126/science.1240837
Experimental observations show that the global temperature rise has slowed since 2000. The trend for January 1999 to December 2008 is +0.07Â±0.07Â°C per decade, much less than the 0.18Â°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 Earthâ€™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 700â€‰m of the ocean, 65% of it in the tropical Pacific and Atlantic oceans.
Experimental observations of intermediate waterÂ 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.
Comparison between reconstructions of surface and intermediate-water temperatures (IWT) for past 10,000 years
(A) Global (red) and 30Â°N to 90Â°N (green) surface temperatures anomalies (Â°C)
(B) 30Â°S to 90Â°S surface temperature anomalies (Â°C)
(C) Anomalies (Â°C) in IWT at 500 m
(D) Anomalies (Â°C) in IWT at 600 to 900 m
Shaded bands represent Â±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 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(Â°C)|
|10,500 to 9,000 BP||~10Â°C|
|8,000 to 6,000 BP||~10.7Â°C|
They found that that the North Pacific and Antarctic intermediate waters were warmer by 2.1 Â± 0.4Â°C and 1.5 Â± 0.4Â°C, respectively, during the thermal maximum (HTM) 6,000 to 7,000 years ago than in the 20th Century. Both water masses were about 0.9Â°C warmer during the Medieval Warm period (950 to 1250 CE) than during the Little Ice Age (1550 to 1850 CE) and ~0.65Â° warmer than currently.
Northern Pacific intermediate waters
|Period||Temperature relative to 20th C|
|HTM 6-7,000 years ago||2.1 Â± 0.4Â°C higher|
|MWP 920-1250 CE||0.65Â°C higher|
Antarctic intermediate waters
|Period||Temperature relative to 20th C|
|HTM 6-7,000 years ago||1.5 Â± 0.4Â°C higher|
|MWP 920-1250 CE||0.65Â°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 HeatÂ 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.
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 surfaceÂ 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 oceanâ€™s interior. Over a long time, the oceanâ€™s interior builds up large heat anomalies that reflect and, more importantly, affect the global climate.