Indonesian and Pacific Intermediate Water temperature and heat content over the last 10,000 years

Along with my colleagues Yair Rosenthal (Rutgers) and Delia Oppo (Woods Hole) we used the Mg/Ca of the benthic foraminfera Hyalinea balthica from Indonesian sediment cores to document that water masses linked to North Pacific and Antarctic intermediate waters were 2.1±0.4 and 1.5±0.4°C warmer, respectively, during the middle Holocene Thermal Maximum (~10,000 to 8,000 years BP) than in the last century.  The results of this study also indicate that Intermediate water masses in the Pacific were ~ 0.9°C warmer during the Medieval Warm Period (MWP) than during the Little Ice Age (LIA) and ~0.65° warmer than in recent decades. Thus intermediate waters in the Pacific have undergone a long term cooling trend that has recently reversed possibly due to the current imbalance in the planetary energy budget. Intermediate waters in the Pacific are sourced from the surface at high latitudes in the north and south Pacific where they uptake and release heat from the atmosphere and are currently working to mediate the rise in atmospheric temperatures. The changes in intermediate water temperatures (IWT) during the MWP and LIA, are particularly interesting and imply that the heat uptake by Pacific intermediate waters has varied substantially over the last ~2,000 years. 

ABOVE: Temperature anomaly reconstructions for the Common Era (CE) relative to the modern data (note the age scale is in years CE with present on the right).  (A) ∆SST from the Makassar Straits (orange; based on (26) compared with Northern Hemisphere temperature anomalies (27, 28); (B) Compiled IWT anomalies based on Indonesian records spanning the ~500-900 water depth (for individual records see Fig. S7).  Shaded band is ±1 SD. (This is Figure 3 from Rosenthal et al. 2013).

ABOVE: Cores Sites in Indonesia

ABOVE: Comparison between Holocene records of (A) global (red) and 30-90°N (green) surface temperature anomalies, (B) 30-90°S surface temperature anomalies (see ref 24 in Rosenthal et al. 2013); and (C) changes in Pacific intermediate water temperature (IWT) as measured at 500m and (D) 600-900m in Indonesia. All anomalies are calculated relative to temperature at 1850-1880CS. Shaded bands are +/- 1 SD. Note the different temperature scales (this is Figure 2 from Rosenthal et al., 2013).

The benthic foraminifera Hyalinea balthica.  Mg/Ca measurements of fossil specimens of this bottom dwelling calcareous protozan were used to make the intermediate water temperature reconstructions shown below.

ABOVE: A. Reconstructed anomalies in Pacific OHC in the 0-700m depth interval for the early-Holocene, mid-Holocene, MWP and LIA periods . Reconstructed anomalies are calculated relative to the reference period of 1965-1970 CE (15).  B. Reconstructed rates of OHC change during the main transition periods.  Reconstructed anomalies and rates are compared with modern observations for the 2000-2010 CE and 1955-2010 CE periods, respectively (5).  The middle line at each box represents an average estimate for 50% of the Pacific volume between 0 and 700m whereas the top and bottom quartiles of the box represent 62.5 and 37.5% of the total volume in this depth interval, respectively. Bottom whisker represents 25% and top 75% of the volume.  The modern value is based on the entire Pacific volume for 0-700m. (This is Figure 4 from Rosenthal et al. 2013)

Non-specialist Summary:

    Along with lead author Yair Rosenthal (Rutgers) and co-author (Delia Oppo) we published a paper in Science discussing the results of our research into reconstructing Pacific Intermediate water temperatures over the last 10,000 years.

To set the stage to discuss our new results we must first discuss the instrumental record of intermediate water temperatures in the ocean that extends back to 1955.

Oceanic intermediate waters refer to waters between approximately 500 and 1000m below the surface, so mid-depths. Previous work based on instrumental thermometer data has shown that these intermediate waters have been warming everywhere (in all ocean basins) since the late 1950s.  But we know little about changes in intermediate waters before this time.

    In our research we were able to reconstruct Pacific intermediate water temperatures using the Mg content of bottom dwelling fossils call foraminifera over the last 10,000 years. These single celled protozoans live in the mud and it turns out that their Mg chemistry serves as a paleo-thermometer.  The warmer the water, the higher the Mg concentration in the fossil shells.

    Our new results indicate that the intermediate waters in the Pacific have cooled ~ 2°C over the last 7,000 years.  The cooling trend was rather steady until approximately 1,500 years ago when temperatures rose a little during a time known in the northern Hemisphere, as the Medieval Warm Period (a time when there were vineyards in England). Following this relatively warm interval, intermediate water temperatures continued their long-term cooling trend reaching a minimum approximately 400 years ago (~1600AD) during what is known as the little Ice Age.  After this the sediment record shows that intermediate water temperatures stopped cooling and the trend reversed.

    Our results place the current warming of intermediate waters in temporal perspective and indicate that the present rate of heat gain in intermediate waters is ~15X that of any time in the last 7,000 years, based on the resolution of our sediment-based data.

    Our results also demonstrate that these century-scale climate events known as the Medieval Warm Period (a.k.a. Medieval Climate Anomaly) and Little Ice Age were global in scale and actually effected water temperatures between 500 and 1000m down in the western Pacific. Thus the deep ocean is sensitive to century-scale changes in Earth’s surface temperature.

    Intermediate waters in the Pacific and Atlantic actually form in mid-latitudes where surface water sinks after equilibrating with atmospheric temperature.  Thus our results also demonstrate that the ocean is taking up heat from the atmosphere much more efficiently that previously thought.

    The oceans are thus working to buffer the global climate system and the ongoing rise in atmospheric temperatures.Unfortunately we do not know what the down-stream effects of this heating will be and how much time this will buy us. There could be positive effects or negative effects. The heat will eventually come back out. In our view, there is a lot of  uncertainty and more work is needed to understand intermediate water formation and heat storage in the oceans.