2,000-year-long temperature and hydrology reconstructions from the Indo-Pacific warm pool

Most high-resolution climate reconstructions for the past millennia have focused on Northern Hemisphere land records. Ocean reconstructions have to date been rare and, critically, have missed the most recent centuries, preventing a comparison with the observational records. A new reconstruction of sea surface temperatures for the Indo-Pacific warm pool now provides a decadally resolved record that spans the last two millennia and overlaps the instrumental record, enabling both a direct calibration of proxy data to the instrumental record and an evaluation of past changes in the context of twentieth century trends. The data show that while recent decades have been anomalously warm, they are statistically indistinguishable from temperatures prevailing during the Medieval Warm Period from around AD 1000 to AD 1250 Temperature reconstructions for the surface of the Northern Hemisphere are based largely on terrestrial records from extra-tropical or high-elevation sites, despite the fact that global average surface temperature changes closely follow those of the global tropics, which are 75 per cent ocean. Here, a decadally resolved continuous sea surface temperature is reconstructed for the Indo-Pacific warm pool that spans the past 2,000 years, allowing for comparison with observations. Northern Hemisphere surface temperature reconstructions suggest that 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 (refs 1, 2). These temperature reconstructions are based largely on terrestrial records from extra-tropical or high-elevation sites; however, global average surface temperature changes closely follow those of the global tropics3, which are 75% ocean. In particular, the tropical Indo-Pacific warm pool (IPWP) represents a major heat reservoir that both influences global atmospheric circulation4 and responds to remote northern high-latitude forcings5,6. Here we present a decadally resolved continuous sea surface temperature (SST) reconstruction from the IPWP that spans the past two millennia and 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. Our record from the Makassar Strait, Indonesia, exhibits trends that are similar to a recent Northern Hemisphere temperature reconstruction2. Reconstructed SST was, however, within error of modern values from about ad 1000 to ad 1250, towards the end of the Medieval Warm Period. 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. A companion reconstruction of δ18O of sea water—a sea surface salinity and hydrology indicator—indicates a tight coupling with the East Asian monsoon system and remote control of IPWP hydrology on centennial–millennial timescales, rather than a dominant influence from local SST variation.