A 2000-Year Temperature History of the Indo-Pacific Warm Pool

In reference to the claims of Jansen et al. (2007) and Mann et al. (2008) that Northern Hemisphere surface temperature reconstructions indicate "the late twentieth century was warmer than any other time during the past 500 years and possibly any time during the past 1,300 years," authors Oppo et al. (2009) state that these temperature reconstructions may not be as representative of the planet as a whole as they are typically made out to be, because they "are based largely on terrestrial records from extra-tropical or high-elevation sites," while "global average surface temperature changes closely follow those of the global tropics, which are 75% ocean."

In an effort to remedy this less-than-desirable situation, Oppo et al. derived a "continuous sea surface temperature (SST) reconstruction from the IPWP [Indo-Pacific Warm Pool]," which they describe as "the largest reservoir of warm surface water on the Earth and the main source of heat for the global atmosphere." This temperature history -- which was based on δ¹⁸O and Mg/Ca data obtained from samples of the planktonic foraminifera Globigerinoides ruber found in two gravity cores, a nearby multi-core (all at 3°53'S, 119°27'E), and a piston core (at 5°12'S, 117°29'E) recovered from the Makassar Strait on the Sulawesi margin -- spans the past two millennia and, more importantly, as they describe it, "overlaps the instrumental record, enabling both a direct comparison of proxy data to the instrumental record and an evaluation of past changes in the context of twentieth century trends." So what did they find?

The three U.S. scientists report that their SST reconstruction "shows cooler temperatures between about AD 400 and AD 950 [the Dark Ages Cold Period] than during much of the so-called Medieval Warm Period (about AD 900-1300)." Of this latter period, they say "reconstructed SSTs were warmest from AD 1000 to AD 1250," when "SSTs within error of modern SSTs occurred in the IPWP," as also was the case "during brief periods of the first millennium AD," when the Roman Warm Period held sway. They also report that "SSTs during the Little Ice Age (approximately AD 1550-1850) were variable, and ~0.5 to 1°C colder than modern values during the coldest intervals."

Based on a globally significant SST history, "enabling both a direct comparison of proxy data to the instrumental record and an evaluation of past changes in the context of twentieth century trends," there is now substantial evidence that throughout portions of both the Roman and Medieval Warm Periods, SSTs in the Indo-Pacific Warm Pool were essentially equivalent to those of "the late twentieth century," indicating -- once again -- that there is nothing unusual, unnatural or unprecedented about current air temperatures in this critically important region of the globe.

Additional References
Jansen, E. et al. 2007. In: Solomon, S. et al. (Eds.). Climate Change 2007: The Physical Science Basis. Cambridge University Press, Cambridge, UK, pp. 466-482.

Mann, M.E. et al. 2008. Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia. Proceedings of the National Academy of Sciences, USA 105: 13,252-13,257.