Accurate modelling and prediction of the local to continental-scale hydroclimate response to global warming is essential given the strong impact of hydroclimate on ecosystem functioning, crop yields, water resources, and economic security. However, uncertainty in hydroclimate projections remains large, in part due to the short length of instrumental measurements available with which to assess climate models. Here we present a spatial reconstruction of hydroclimate variability over the past twelve centuries across the Northern Hemisphere derived from a network of 196 at least millennium-long proxy records. We use this reconstruction to place recent hydrological changes and future precipitation scenarios in a long-term context of spatially resolved and temporally persistent hydroclimate patterns. We find a larger percentage of land area with relatively wetter conditions in the ninth to eleventh and the twentieth centuries, whereas drier conditions are more widespread between the twelfth and nineteenth centuries. Our reconstruction reveals that prominent seesaw patterns of alternating moisture regimes observed in instrumental data across the Mediterranean, western USA, and China have operated consistently over the past twelve centuries. Using an updated compilation of 128 temperature proxy records, we assess the relationship between the reconstructed centennial-scale Northern Hemisphere hydroclimate and temperature variability. Even though dry and wet conditions occurred over extensive areas under both warm and cold climate regimes, a statistically significant co-variability of hydroclimate and temperature is evident for particular regions. We compare the reconstructed hydroclimate anomalies with coupled atmosphere-ocean general circulation model simulations and find reasonable agreement during pre-industrial times. However, the intensification of the twentieth-century-mean hydroclimate anomalies in the simulations, as compared to previous centuries, is not supported by our new multi-proxy reconstruction. This finding suggests that much work remains before we can model hydroclimate variability accurately, and highlights the importance of using palaeoclimate data to place recent and predicted hydroclimate changes in a millennium-long context.

Observations from 1980 to 2013 of 20 aerosol constituents, ozone and mercury at Alert, Canada (82.50°N, 62.35°W), were analyzed for trends and dominant factors of the Arctic haze during winter and spring. Trends reflect changing emissions in Eurasia, the main source region for surface pollution in the high Arctic. SO42−, H+, NH4+, K+, Cu, Ni, Pb, Zn, nonsoil V, nonsoil Mn, and equivalent black carbon decreased between 23% and 80% as emissions declined rapidly in northern Eurasia during the early 1990s. NO3− increased by 20% as aerosol acidity declined. Metals were linked to emissions from smelting and fossil fuel combustion. In winter, ozone increased by 5% over 23 years, consistent with other observations and global modeling. Twelve PMF factors emerged for the dark period (November to February) and 13 for the light period (March to May). Eleven PMF factors are common to both dark and light, a twelfth factor was associated with sulfate in the dark and nitrate in the light, and the thirteenth (light period) was related to ozone and gaseous mercury depletion near Alert. IODINE and NITRATE factors, important for Arctic chemistry, changed with sunlight. In the light, 50% of all NO3− was on the NITRATE factor, while in the dark, most was associated with MODIFIED SEA SALT and equivalent black carbon. In the dark (light), 90% (28%) of iodine were found on the factor IODINE and 58% associated with SEA‐SALT and MODIFIED SEA‐SALT. These results help in understanding the role of atmospheric chemistry in weather and climate processes.

We analyse the spatio-temporal patterns of temperature variability over Northern Hemisphere land areas, on centennial time-scales, for the last 12 centuries using an unprecedentedly large network of temperature-sensitive proxy records. Geographically widespread positive temperature anomalies are observed from the 9th to 11th centuries, similar in extent and magnitude to the 20th century mean. A dominance of widespread negative anomalies is observed from the 16th to 18th centuries. Though we find the amplitude and spatial extent of the 20th century warming is within the range of natural variability over the last 12 centuries, we also find that the rate of warming from the 19th to the 20th century is unprecedented in the context of the last 1200 yr. The positive Northern Hemisphere temperature change from the 19th to the 20th century is clearly the largest between any two consecutive centuries in the past 12 centuries. These results remain robust even after removing a significant number of proxies in various tests of robustness showing that the choice of proxies has no particular influence on the overall conclusions of this study

Abstract. We undertake a study in two parts, where the overall aim is to quantitatively compare results from climate proxy data with results from several climate model simulations from the Paleoclimate Modelling Intercomparison Project for the mid-Holocene period and the pre-industrial, conditions for the pan-arctic region, north of 60 • N. In this first paper, we survey the available published local temperature and precipitation proxy records. We also discuss and quantifiy some uncertainties in the estimated difference in climate between the two periods as recorded in the available data. The spatial distribution of available published local proxies has a marked geographical bias towards land areas surrounding the North Atlantic sector, especially Fennoscandia. The majority of the reconstructions are terrestrial, and there is a large over-representation towards summer temperature records. The available reconstructions indicate that the northern high latitudes were warmer in both summer, winter and the in annual mean temperature at the mid-Holocene (6000 BP ± 500 yrs) compared to the pre-industrial period (1500 AD ± 500 yrs). For usage in the model-data comparisons (in Part 1), we estimate the calibration uncertainty and also the internal variability in the proxy records, to derive a combined minimum uncertainty in the reconstructed temperature change between the two periods. Often, the calibration uncertainty alone, at a certain site, exceeds the actual reconstructed climate change at the site level. In high-density regions, however, neighbouring records can be merged into a Correspondence to: H. S. Sundqvist (hanna.sundqvist@natgeo.su.se) composite record to increase the signal-to-noise ratio. The challenge of producing reliable inferred climate reconstructions for the Holocene cannot be underestimated, considering the fact that the estimated temperature and precipitation fluctuations during this period are in magnitude similar to, or lower than, the uncertainties the reconstructions. We advocate a more widespread practice of archiving proxy records as most of the potentially available reconstructions are not published in digital form.