International audienceKnowledge of natural climate variability is essential to better constrain the uncertainties in projections of twenty-first-century climate change 1–5. The past 2,000 years (2 kyr) have emerged as a critical interval in this endeavour, with sufficient length to characterize natural decadal-to-centennial scale change, known external climate forcings 6 and with distinctive patterns of spatiotemporal temperature variations 7. However, reconstructions for the full 2 kyr interval are not available for the global ocean, a primary heat reservoir 8 and an important regulator of global climate on longer timescales 9–11. Here we present a global ocean sea surface temperature (SST) synthesis (Ocean2k SST synthesis) spanning the Common Era, which shows a cooling trend that is similar, within uncertainty, to that simulated by realistically forced climate models for the past millennium. We use the simulations to identify the climate forcing(s) consistent with reconstructed SST variations during the past millennium. The oceans mediate the response of global climate to natural and anthropogenic forcings. Yet for the past 2,000 years — a key interval for understanding the present and future climate response to these forcings — global sea surface temperature changes and the underlying driving mechanisms are poorly constrained. Here we present a global synthesis of sea surface temperatures for the Common Era (ce) derived from 57 individual marine reconstructions that meet strict quality control criteria. We observe a cooling trend from 1 to 1800 ce that is robust against explicit tests for potential biases in the reconstructions. Between 801 and 1800 ce, the surface cooling trend is qualitatively consistent with an independent synthesis of terrestrial temperature reconstructions, and with a sea surface temperature composite derived from an ensemble of climate model simulations using best estimates of past external radiative forcings. Climate simulations using single and cumulative forcings suggest that the ocean surface cooling trend from 801 to 1800 ce is not primarily a response to orbital forcing but arises from a high frequency of explosive volcanism. Our results show that repeated clusters of volcanic eruptions can induce a net negative radiative forcing that results in a centennial and global scale cooling trend via a decline in mixed-layer oceanic heat content

Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850–2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python.

SignificanceA causality between millennial-scale climate cycles and the replacement of Neanderthals by modern humans in Europe has tentatively been suggested. However, that replacement was diachronous and occurred over several such cycles. A poorly constrained continental paleoclimate framework has hindered identification of any inherent causality. Speleothems from the Carpathians reveal that, between 44,000 and 40,000 years ago, a sequence of stadials with severely cold and arid conditions caused successive regional Neanderthal depopulation intervals across Europe and facilitated staggered repopulation by modern humans. Repetitive depopulation–repopulation cycles may have facilitated multiple genetic turnover in Europe between 44,000 and 34,000 years ago.

Water resources in western North America depend on winter precipitation, yet our knowledge of its sensitivity to climate change remains limited. Similarly, understanding the potential for future loss of winter snow pack requires a longer perspective on natural climate variability. Here we use stable isotopes from a speleothem in southwestern Oregon to reconstruct winter climate change for much of the past 13,000 years. We find that on millennial time scales there were abrupt transitions between warm-dry and cold-wet regimes. Temperature and precipitation changes on multi-decadal to century timescales are consistent with ocean-atmosphere interactions that arise from mechanisms similar to the Pacific Decadal Oscillation. Extreme cold-wet and warm-dry events that punctuated the Holocene appear to be sensitive to solar forcing, possibly through the influence of the equatorial Pacific on the winter storm tracks reaching the US Pacific Northwest region.

The extent to which climate change causes significant societal disruption remains controversial. An important example is the decline of the Akkadian Empire in northern Mesopotamia ∼4.2 ka, for which the existence of a coincident climate event is still uncertain. Here we present an Iranian stalagmite record spanning 5.2 ka to 3.7 ka, dated with 25 U/Th ages that provide an average age uncertainty of 31 y (1σ). We find two periods of increased Mg/Ca, beginning abruptly at 4.51 and 4.26 ka, and lasting 110 and 290 y, respectively. Each of these periods coincides with slower vertical stalagmite growth and a gradual increase in stable oxygen isotope ratios. The periods of high Mg/Ca are explained by periods of increased dust flux sourced from the Mesopotamia region, and the abrupt onset of this dustiness indicates threshold behavior in response to aridity. This interpretation is consistent with existing marine and terrestrial records from the broad region, which also suggest that the later, longer event beginning at 4.26 ka is of greater regional extent and/or amplitude. The chronological precision and high resolution of our record indicates that there is no significant difference, at decadal level, between the start date of the second, larger dust event and the timing of North Mesopotamia settlement abandonment, and furthermore reveals striking similarity between the total duration of the second dust event and settlement abandonment. The Iranian record demonstrates this region’s threshold behavior in dust production, and its ability to maintain this climate state for multiple centuries naturally.

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Between 5 and 4 thousand years ago, crippling megadroughts led to the disruption of ancient civilizations across parts of Africa and Asia, yet the extent of these climate extremes in mainland Southeast Asia (MSEA) has never been defined. This is despite archeological evidence showing a shift in human settlement patterns across the region during this period. We report evidence from stalagmite climate records indicating a major decrease of monsoon rainfall in MSEA during the mid- to late Holocene, coincident with African monsoon failure during the end of the Green Sahara. Through a set of modeling experiments, we show that reduced vegetation and increased dust loads during the Green Sahara termination shifted the Walker circulation eastward and cooled the Indian Ocean, causing a reduction in monsoon rainfall in MSEA. Our results indicate that vegetation-dust climate feedbacks from Sahara drying may have been the catalyst for societal shifts in MSEA via ocean-atmospheric teleconnections.