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We assess the relationship between temperature and global sea-level (GSL) variability over the Common Era through a statistical metaanalysis of proxy relative sea-level reconstructions and tide-gauge data. GSL rose at 0.1 ± 0.1 mm/y (2σ) over 0–700 CE. A GSL fall of 0.2 ± 0.2 mm/y over 1000–1400 CE is associated with ∼0.2 °C global mean cooling. A significant GSL acceleration began in the 19th century and yielded a 20th century rise that is extremely likely (probability P ≥ 0.95) faster than during any of the previous 27 centuries. A semiempirical model calibrated against the GSL reconstruction indicates that, in the absence of anthropogenic climate change, it is extremely likely (P = 0.95) that 20th century GSL would have risen by less than 51% of the observed 13.8 ± 1.5 cm. The new semiempirical model largely reconciles previous differences between semiempirical 21st century GSL projections and the process model-based projections summarized in the Intergovernmental Panel on Climate Change’s Fifth Assessment Report.

The spatial context is critical when assessing present-day climate anomalies, attributing them to potential forcings and making statements regarding their frequency and severity in a long-term perspective. Recent international initiatives have expanded the number of high-quality proxy-records and developed new statistical reconstruction methods. These advances allow more rigorous regional past temperature reconstructions and, in turn, the possibility of evaluating climate models on policy-relevant, spatio-temporal scales. Here we provide a new proxy-based, annually-resolved, spatial reconstruction of the European summer (June-August) temperature fields back to 755 CE based on Bayesian hierarchical modelling (BHM), together with estimates of the European mean temperature variation since 138 BCE based on BHM and composite-plus-scaling (CPS). Our reconstructions compare well with independent instrumental and proxy-based temperature estimates, but suggest a larger amplitude in summer temperature variability than previously reported. Both CPS and BHM reconstructions indicate that the mean 20th century European summer temperature was not significantly different from some earlier centuries, including the 1st, 2nd, 8th and 10th centuries CE. The 1st century (in BHM also the 10th century) may even have been slightly warmer than the 20th century, but the difference is not statistically significant. Comparing each 50 yr period with the 1951-2000 period reveals a similar pattern. Recent summers, however, have been unusually warm in the context of the last two millennia and there are no 30 yr periods in either reconstruction that exceed the mean average European summer temperature of the last 3 decades (1986-2015 CE). A comparison with an ensemble of climate model simulations suggests that the reconstructed European summer temperature variability over the period 850-2000 CE reflects changes in both internal variability and external forcing on multi-decadal time-scales. For pan-European temperatures we find slightly better agreement between the reconstruction and the model simulations with high-end estimates for total solar irradiance. Temperature differences between the medieval period, the recent period and the Little Ice Age are larger in the reconstructions than the simulations. This may indicate inflated variability of the reconstructions, a lack of sensitivity and processes to changes in external forcing on the simulated European climate and or an underestimation of internal variability on centennial and longer time scales.

More than two decades ago, my coauthors, Raymond Bradley and Malcolm Hughes, and I published the now iconic “hockey stick” curve. It was a simple graph, derived from large-scale networks of diverse climate proxy (“multiproxy”) data such as tree rings, ice cores, corals, and lake sediments, that captured the unprecedented nature of the warming taking place today. It became a focal point in the debate over human-caused climate change and what to do about it. Yet, the apparent simplicity of the hockey stick curve betrays the dynamicism and complexity of the climate history of past centuries and how it can inform our understanding of human-caused climate change and its impacts. In this article, I discuss the lessons we can learn from studying paleoclimate records and climate model simulations of the “Common Era,” the period of the past two millennia during which the “signal” of human-caused warming has risen dramatically from the background of natural variability.

The reconstruction of global annual mean temperatures made by the PAGES 2k Consortium in 2019 represents one of the most influential sequences of global climate variability over the Common Era. However, it is still not clear how the reconstruction can be influenced by the selection of reconstruction methods and the selection of proxy records with different temporal resolutions over different regions. We adopt a widely used Composite-Plus-Scale method to elucidate the effects of the selection of the proxy records on temperature reconstruction. To ensure the uniformity of data, different types of proxy records spanning the past ∼2000 years from the PAGES 2k proxy network were used to investigate the potential effects of proxy selection in hemispheric and global temperature reconstructions during the past two millennia. The long-term trends, spectral characteristics, and volcanic responses of the annual temperatures were studied based on the reconstructions. Our results reveal a significant cooling trend in the global annual mean temperature using both tree-ring and non-tree-ring records during the 1–1850 CE period, and show that the cooling exhibits a stronger trend in the Southern Hemisphere (SH) than that in the Northern Hemisphere (NH). Yet, the long-term trends vary according to different combinations of proxy records. Different reconstructions based on different types of proxies also exhibit different features in terms of volcanic responses and spectral properties. Tree-ring-based temperature reconstructions show stronger cooling responses to tropical volcanic eruptions, while non-tree-ring-based reconstructions suggest less robust volcanic responses, which may be related to dating uncertainties and low temporal resolution of the proxies. Tree-ring width records tend to preserve a substantial proportion of high-frequency (<200 years) variability, whereas non-tree-ring proxies tend to capture a larger fraction of low-frequency (>200 years) variations. Efforts are needed to reduce uncertainties of the temperature reconstruction over the Common Era associated with the insufficient spatiotemporal coverage of the current proxy network, especially for the first millennium and for the SH and tropics, also to develop statistical methods and to improve the signal strength and constrain uncertainties in existing proxy records.

We compare hydrogen isotopic measurements of long-chain leaf-wax n-alkanes (δ2Hw; C27, C29, and C31) from Martin Lake, Indiana, USA, with a calcite-based reconstruction of the oxygen isotopic composition of precipitation (δ18Op) from the same lake. We observe stable and high δ2Hw during the Common Era (last 2000 years), which we interpret as growing-season precipitation originating mainly from the Gulf of Mexico and Atlantic. During the Little Ice Age (LIA; 1200–1850 CE), δ2Hw values increased by 3–8 ‰, concomitant with a significant decrease in δ18Op values by up to 12.5 ‰. Multiple proxy records for this time indicate persistent growing-season drought. We interpret these relatively high δ2Hw values, as compared to the δ18Op values, as a signal of low relative humidity that resulted in an 2H enrichment in plant source water resulting in high δ2H values through enhanced plant water and/or soil evaporation. These results support the occurrence of low humidity conditions during the LIA in the midcontinental USA that also contributed to the marked decline of regional pre-Columbian Mississippian populations.

Tree growth trends can affect the interpretation of the response of tree-ring proxies (especially tree-ring width) to climate in the low-frequency band, which in turn may limit quantitative understanding of centennial-scale climate variability. As such, it is difficult to determine if long-term trends in tree-ring measurements are caused by age-dependent growth effects or climate. Here, a trend similarity ranking method is proposed to define the range of tree growth effects on tree-ring width chronologies. This method quantifies the inner and outer boundaries of the tree growth effect following two extreme standardization methods: curve fitting standardization and regional curve standardization. The trend similarity ranking method classifies and detrends tree-ring measurements according to the ranking similarity between the regional growth curve and their long-term trends through curve fitting. This standardization process mainly affects the secular trend in tree-ring chronologies, and has no effect on their inter-annual to multi-decadal variations. Applications of this technique to the Yamal and Torneträsk tree-ring width datasets and the maximum latewood density dataset from northern Scandinavia reveals that multi-centennial and millennial-scale temperature variations in the three regions provide substantial positive contributions to the linear warming trends in the instrumental period, and that the summer warming rate during the 20th century is not unprecedented over the past two millennia in any of the three regions.

How climate controls hurricane variability has critical implications for society is not well understood. In part, our understanding is hampered by the short and incomplete observational hurricane record. Here we present a synthesis of intense‐hurricane activity from the western North Atlantic over the past two millennia, which is supported by a new, exceptionally well‐resolved record from Salt Pond, Massachusetts (USA). At Salt Pond, three coarse grained event beds deposited in the historical interval are consistent with severe hurricanes in 1991 (Bob), 1675, and 1635 C.E., and provide modern analogs for 32 other prehistoric event beds. Two intervals of heightened frequency of event bed deposition between 1400 and 1675 C.E. (10 events) and 150 and 1150 C.E. (23 events), represent the local expression of coherent regional patterns in intense‐hurricane–induced event beds. Our synthesis indicates that much of the western North Atlantic appears to have been active between 250 and 1150 C.E., with high levels of activity persisting in the Caribbean and Gulf of Mexico until 1400 C.E. This interval was one with relatively warm sea surface temperatures (SSTs) in the main development region (MDR). A shift in activity to the North American east coast occurred ca. 1400 C.E., with more frequent severe hurricane strikes recorded from The Bahamas to New England between 1400 and 1675 C.E. A warm SST anomaly along the western North Atlantic, rather than within the MDR, likely contributed to the later active interval being restricted to the east coast. Key Points Significant variability in the frequency of intense‐hurricanes has occurred Prehistoric intense‐hurricane frequency often exceeded historic levels Regional sea‐surface temperature warming contributed to active intervals

We find that maximal decadal Northern Hemisphere warming increases as rapidly before as after the industrial revolution (0.86 °C decade−1 before 1880 and 0.60–0.68 °C decade−1 after 1880). However, whereas the number of decadal periods with large increase and decrease rates were about equal before 1880 (267 vs. 273), after 1880 there are more periods with high increase rates (35) than with high decrease rates (9). The same patterns hold for bi-decadal rates. However, for time windows greater than about 20 years, the slope in global warming with time becomes greater after 1880. After 1971, there is only one short 11 year period with negative slopes. This reflects the higher frequency of positive slopes during the industrial period caused by the contribution of greenhouse gases (GHG). Maximum temperature changes for detrended series were associated with the beginning and end of extreme warm or cold sub periods. They occurred throughout all of the Common Era. Because the detrended temperature series showed sign of a pacemaker mechanism (regular cycle periods) we suggest that ocean variabilities were a dominating mechanism for multidecadal temperature variability during the Common Era.

We employed the modern analog technique to quantitatively reconstruct temperature and precipitation over the past 2500 yr based on fossil pollen records from six high-elevation lakes in northern Colorado. Reconstructed annual temperatures for the study area did not deviate significantly from modern over the past 2500 yr despite hemispheric expressions of Medieval Climate Anomaly warmth and Little Ice Age cooling. Annual precipitation, however, shifted from lower than modern rates from 2500 to 1000 cal yr BP to higher than modern rates after 1000 cal yr BP, a greater than 100 mm increase in precipitation. Winter precipitation accounts for the majority of the change in annual precipitation, while summer precipitation rates did not change significantly over the past 2500 yr. The large change in winter precipitation rates from the first to second millennium of the Common Era is inferred from a shift in fossil pollen assemblages dominated by subalpine conifers, which have southern sites as modern analogs, to assemblages representing open subalpine vegetation with abundant Artemisia spp. (sagebrush), which have more northern modern analogs. The change helps to explain regional increases in lake levels and shifts in some isotopic and tree-ring data sets, highlighting the risk of large reductions in snowpack and water supplies in the Intermountain West.

Quantitative assessment of natural internal variability and externally forced responses of Northern Hemisphere (NH) temperatures is necessary for understanding and attributing climate change signals during past warm and cold periods. However, it remains a challenge to distinguish the robust internally generated variability from the observed variability. Here, large-ensemble (70 member) simulations, Energy Balance Model simulation, temperature ensemble reconstruction, and three dominant external forcings (volcanic, solar, and greenhouse gas) were combined to estimate the internal variability of NH summer (June-August) temperatures over the past 2000 years (1-2000 CE). Results indicate that the Medieval Climate Anomaly was predominantly attributed to centennial-scale internal oscillation, accounting for an estimated 104% of the warming anomaly. In contrast, the Current Warm Period is influenced mainly by external forcing, contributing up to 90% of the warming anomaly. Internal temperature variability offsets cooling by volcanic eruptions during the Late Antique Little Ice Age. These findings have important implications for the attribution of past climate variability and improvement of future climate projections.