Past global climate changes had strong regional expression. To elucidate their spatio-temporal pattern, we reconstructed past temperatures for seven continental-scale regions during the past one to two millennia. The most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the nineteenth century. At multi-decadal to centennial scales, temperature variability shows distinctly different regional patterns, with more similarity within each hemisphere than between them. There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between AD 1580 and 1880, punctuated in some regions by warm decades during the eighteenth century. The transition to these colder conditions occurred earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere regions. Recent warming reversed the long-term cooling; during the period AD 1971-2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years

We present two reconstructions of annual average temperature over temperate North America: a tree-ring based reconstruction at decadal resolution (1200-1980 CE) and a pollen-based reconstruction at 30 year resolution that extends back to 480 CE. We maximized reconstruction length by using long but low-resolution pollen records and applied a three-tier calibration scheme for this purpose. The tree-ring-based reconstruction was calibrated against instrumental annual average temperatures on annual and decadal scale, it was then reduced to a lower resolution, and was used as a calibration target for the pollen-based reconstruction. Before the late-19th to the early-21st century, there are three prominent low-frequency periods in our extended reconstruction starting at 480 CE, notably the Dark Ages cool period (about 500-700 CE) and Little Ice Age (about 1200-1900 CE), and the warmer medieval climate anomaly (MCA; about 750-1100 CE). The 9th and the 11th century are the warmest centuries and they constitute the core of the MCA in our reconstruction, a period characterized by centennial-scale aridity in the North American West. These two warm peaks are slightly warmer than the baseline period , but nevertheless much cooler than temperate North American temperatures during the early-21st century.

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Snowpack in the Sierra Nevada Mountains accounts for around one-third of California’s water supply. Melting snow provides water into dry summer months characteristic of the region’s Mediterranean climate. As climate changes, understanding patterns of snowpack, snowmelt, and biological response is critical in this region of agricultural, recreational, and ecological value. Here we investigated the relationships between tree rings of montane conifer trees ( Tsuga mertensiana, Abies magnifica, Abies concolor, Calocedrus decurrens, Juniperus occidentalis, and Pinus ponderosa) and regional climate indices with the goal of reconstructing April 1 snow-water equivalent (SWE) in the North Fork American River watershed of the Sierra Nevada. Chronologies were positively correlated with April 1 SWE of the year prior to ring formation. Temporal trends in correlation between tree-ring chronologies and climate indices indicate strengthening tree growth response to climate over time. We developed a skillful, nested reconstruction for April 1 SWE, 1661–2013. Variability of the reconstruction is within the envelope of 20th and 21st-century variability; however, the 2015 record low snowpack is unprecedented in the tree-ring record, as in results from previous studies. Future research should focus on integrating modern snow sensor data into paleoclimate research and understanding mechanistic linkages between snow and tree growth response.

Since the original publication of this Progress Article, errors have been identified in the data set used for the Arctic temperature reconstruction. Accordingly, the following corrections have been made to the data set (see updated Supplementary Databases): (1) three records 1-3 have been removed, as well as sections of five records 4-8 that did not meet the authors' criteria for sensitivity to temperature; (2) the signs of the proxy temperature relation for two records 9,10 have been corrected; and (3) a 50-year offset in the ages of one record 11 has been corrected. Following these corrections, the period from 1941-1970 emerges as the second warmest 30-year period in the Arctic record, and 1971-2000 the third warmest, rather than the first and second warmest as reported in the original version. The ranked order of the best estimate of temperature indicates that the warmest 30-year period is centred on AD 395. No major conclusions have been affected by the corrections made to the Arctic data set including the conclusion that, during the period AD 1971-2000, the area-weighted average reconstructed temperature among regions was higher than any other time in nearly 1,400 years. Table 1 has been revised to reflect the updated values of the number of lake sediments (19, not 22) and the calibration r value (0.55, not 0.56). Figures 2 and 4b have been updated and replaced (also shown below) and the above corrections have been made in all online versions of the Progress Article. New versions of Supplementary Databases S1 and S2 have been uploaded online to reflect the updated version of the Arctic data set (v.1.1.1). A comparison of the original and revised Arctic data sets is available 12 . The Arctic data set, the reconstruction and its revision history are available at http://ncdc.noaa.gov/paleo/study/16973. The PAGES 2k data sets and any future updates are available from http://ncdc.noaa.gov/paleo/study/12621.The authors thank D. Divine, S. McIntyre and K. Seftigen, who helped improve the Arctic temperature reconstruction by finding errors in the data set.