Background
The authors write that "instrumental records show that the increase in ocean heat content (OHC) accounts for ~90% of the expected warming of [the] Earth, thereby demonstrating that OHC more reliably represents the response of Earth's energy budget to radiative perturbations than do surface temperatures." In addition, they note that "subsurface water masses are directly ventilated in the high-latitude oceans, where they average the highly variable surface conditions, thereby providing an integrated measure of long-term trends."

What was done
Taking advantage of these facts, Rosenthal et al. (2013) used Mg/Ca measurements of the benthic foraminifer Hyalinea balthica obtained from "a suite of sediment cores along bathymetric transects in the Makassar Strait and Flores Sea in Indonesia to document changes in the temperature of western equatorial Pacific subsurface and intermediate water masses throughout the Holocene [0 to 10 thousand years before the present (ky B.P.)]," while noting, in this regard, that "this region is well suited to reconstruct Pacific OHC, as thermocline and 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."

What was learned
As the three researchers write in their paper's abstract, "water masses linked to North Pacific and Antarctic intermediate waters were warmer by 2.1 ± 0.4°C and 1.5 ± 0.4°C, respectively, during the middle Holocene Thermal Maximum [HTM] than over the past century," and that "both water masses were ~0.9°C warmer during the Medieval Warm Period [MWP] than during the Little Ice Age and ~0.65°C warmer than in recent decades."

What it means
In addition to providing further evidence for the greater warmth of the Medieval Warm Period compared to that of the Current Warm Period, Rosenthal et al. write that "the inferred similarity in temperature anomalies at both hemispheres is consistent with recent evidence from Antarctica (Orsi et al., 2012), thereby supporting the idea that the HTM, MWP and LIA were global events.

Reference
Orsi, A.J., Cornuelle, B.D. and Severinghaus, J.P. 2012. Little Ice Age cold interval in West Antarctica: Evidence from borehole temperature at the West Antarctic Ice Sheet (WAIS) Divide. Geophysical Research Letters 39: 1-7.