Lacustrine sediment archives preserve continuous records of changes in basin-to regional-scale processes that reflect broader variability in climatic conditions. Here, we present a 1500-year sediment record of effective moisture (precipitation/evaporation; P/E) from glacially-formed Pope Lake in central Wisconsin that spans much of the Common Era (CE; i.e., the last 2 kyr), including the Current Warm Period (CWP; last 150 years), Medieval Climate Anomaly (MCA; ca. 950-1250 CE), and Little Ice Age (LIA; ca. 1400-1700 CE). A suite of sediment proxies constrained by 14C and 210Pb ages were analyzed at decadal resolution to investigate Common Era changes in Upper Midwest effective moisture in response to temperature and synoptic scale atmospheric variability. Lake water isotopes reconstructed using authigenic carbonate oxygen isotopes (518Ocal) from Pope Lake covary with results from other Midwestern closed-basin lakes, indicating that P/E was a significant control on the isotopic composition of lake water. Relatively lower 518Ocal values during the MCA suggest that the moisture availability was greater during the MCA than LIA, despite increased air temperatures. This is supported by low carbon/nitrogen (C/N) ratios and low terrestrial lithic contributions that reflect increased lake levels during the MCA. Reversals of these trends were observed during the cooler LIA. Comparisons of the Pope Lake record to synoptic scale forcings, such as the Pacific North American (PNA) pattern, suggest that shifts in regional P/E were consistent with high amplitude PNA-like variability that likely affected the source and seasonality of precipitation. The general warm/wet and cool/dry relationships respectively noted during the MCA and LIA underscore how global temperature anomalies may alter the balance of effective moisture in the Upper Midwest in relatively short succession. The Pope Lake sediment record presented here is an important step toward reconstructing the region's hydroclimatic history and may inform expectations of near-future climate variability in a region with few Late Holocene records of comparable length or resolution.

Northwest China is facing socio-environmental challenges linked to ongoing climatic warming. However, a scarcity of regional paleorecord syntheses limits our understanding of natural long-term climate variability in the region and hinders the contextualization of contemporary warming. Here, we present paleorecord syntheses for both summer and annual temperatures during the Holocene based on a range of lacustrine sediment records from northwest China, with consideration of chronological uncertainties within records. The syntheses show similar summer and annual temperature variations, including peak warmth at similar to 9000 years BP, followed by a 2000-year cooling trend, and stable temperatures thereafter. These variations may reflect inter-seasonal impact of summer insolation forcing through climate feedbacks (e.g., Arctic sea-ice cover) that are still not well represented in climate models. The early-Holocene peak warmth was >1.0 degrees C warmer than the 20th century mean warmth, but is expected to be exceeded during the 21st century even under a low emission scenario.

Paleoclimate evidence suggests that high-latitude ocean-atmosphere processes impact tropical South American temperature and precipitation dynamics during glacial cycles. Although variability in the North Atlantic has been implicated in tropical climate shifts during the latest Pleistocene, the links between high- and low-latitude ocean-atmosphere dynamics during late Marine Isotope Stage 3 (MIS 3) remain poorly resolved. Here, we use the Community Earth System Model (CESM2) to investigate how evolving boundary conditions, combined with meltwater events in the North Atlantic, may have affected South American climate at 30 ka BP, near the timing of Heinrich Event 3. Simulations without increased meltwater flux produce near-surface tropical cooling of similar to 1.7 degrees C and mid-tropospheric (524 hPa) cooling of similar to 2.4 degrees C relative to pre-industrial (PI) conditions, yielding temperatures similar to 0.2-0.4 degrees C warmer than those simulated for the Last Glacial Maximum. Increasing meltwater flux across regions of the North Atlantic perturbs Atlantic Meridional Overturning Circulation, and leads to tropical near-surface cooling (relative to PI) of similar to 1.9-2.4 degrees C and cooling of 2.5-3.2 degrees C near the alpine glacial limit. In addition, most meltwater experiments produce an intensified Intertropical Convergence Zone and a strengthened South American Summer Monsoon. While orbital forcing emerges as the dominant driver of latest Pleistocene tropical climate conditions, our results suggest that high-latitude processes can amplify low-latitude temperature and hydroclimate responses during meltwater events. As such, some tropical glaciers at similar to 30 ka BP were likely more extensive than during the LGM due to enhanced precipitation and modest alpine temperature changes.

We employ a semiempirical approach combining climate model simulations and observational temperatures to assess the likelihood of recent global temperature records. Monte Carlo simulations are used to generate global temperature series consistent with combined estimates of forced (anthropogenic + natural) and internal variability derived from observations and CMIP6 multimodel simulations. We find that the El Niño-boosted 2024 global temperature record had a ~12% likelihood of occurrence (a one-in-eight-year event), similar to the prior (also El Niño-boosted) record year 2016 (~14% likelihood). Of the records set during the past three decades, only 1998 is found to have been truly anomalous, with a ~2.5% likelihood of occurrence. Each of these records is found to have been nearly impossible in the absence of human-caused warming.

We present a 10,000-year carbonate oxygen isotope (518O) record from Shark Lake, Alberta, (50.8411 degrees N, 115.3983 degrees W; 1,857 m above sea level) that provides insight on past changes in precipitation seasonality, regional hydroclimate dynamics and atmospheric circulation. Shark Lake has a hydrologically open configuration, with water isotope values that mirror annual mean precipitation, and contains a continuous sediment sequence consisting largely of authigenic carbonate minerals precipitated from the water column. The sediment 518O record exhibits a shift from lower to higher-than-average 518O values from the middle to late Holocene circa 4,100 calendar years before present (yr BP). This transition generally aligns with hypothesized transformations in atmospheric circulation that ostensibly led to reduced winter precipitation in northwestern North America and heightened aridity in the southwest during the late Holocene. Monte Carlo cross-correlation analysis demonstrates that other precipitation 518O records from the Pacific Northwest have a significant positive correlation with the Shark Lake record, in contrast with records from the southern Rocky Mountains and central/eastern North America, which have a negative correlation with the Shark Lake record and others from the Northwest. We apply the previously established strong relationship between atmospheric circulation associated with the Pacific North American Pattern (PNA) and precipitation isotope values across North America to explain the marked coherency in precipitation 518O reconstructions. Collectively, the paleo-records suggest a middle to late Holocene transition in the mean state of atmospheric circulation over North America from one similar to the negative phase of the PNA to one comparable to the positive PNA phase, in response to non-linear changes in oceanatmosphere circulation driven by ice sheet decay and insolation forcing. This shift had substantial impacts on the source and isotopic composition of air masses traversing the continent and their interplay with circulation from the Gulf of Mexico as well as the seasonal amount and distributions of precipitation. Our results underscore the need for additional isotope-enabled climate model simulations and additional 518O records, especially from understudied regions across North America, for a comprehensive view of seasonal-scale hydroclimate variations and synoptic-scale atmospheric circulation pattern changes over the Holocene.

We investigate the characteristics of inter‐ and multidecadal temperature variability in the Community Earth System Model Last Millennium Ensemble through spatiotemporal spectral analysis of forced and internal fields. We find high spectral density in North Atlantic (NA) and global temperature that is concurrent with periods of high volcanic activity, suggesting a forced origin. There is no evidence in the ensemble of an internally‐generated and time‐persistent signal for Atlantic Multidecadal Variability (AMV), the dominant mode on those timescales. The spatial patterns of low‐frequency variability indicate activity throughout the North Pacific, where signals persist to a greater extent after the forced signal is removed, in contrast with the NA where only the subpolar region associated with deep water formation is active. Subtropical and tropical NA regions are strongly associated with forced responses, suggesting the canonical AMV pattern is comprised of both internal and forced components, with the latter being the main driver.

The impact of latitudinal variations in the Intertropical Convergence Zone (ITCZ) on northern Andean hydroclimate during the Medieval Climate Anomaly (MCA; 950-1,150 CE) and Little Ice Age (LIA; 1,300-1,850 CE) is uncertain. Synthesis of two new lacustrine paleoclimate records from the Eastern Colombian Andes with existing circum-Andean records shows that effective moisture anomalies were synchronous and in phase across the tropical Andes during the last millennium. During the MCA, when the ITCZ was shifted northward, topographically controlled responses in the northern Andes to vigorous atmospheric convection resulted in low precipitation and high evaporation, while precipitation was also reduced in the southern tropical Andes. During the LIA, precipitation decreased in the northern Andes as the ITCZ migrated southward but was offset by cooling that lowered evaporation, establishing high effective moisture. In the southern tropical Andes, the southward ITCZ position simultaneously strengthened precipitation, increasing effective moisture. MCA-like responses to continued warming trends could similarly reduce northern Andean precipitation while increasing evaporation, thereby lowering effective moisture and possibly reducing water resource availability. The position of the tropical rain belt has been proposed as a leading control on Andean hydroclimate during the Medieval Climate Anomaly (MCA; 950-1,150 CE) and Little Ice Age (LIA; 1,300-1,850 CE), producing opposite hydroclimate conditions in the northern and southern tropical Andes. Using two new paleoclimate records from the eastern Colombian Andes and existing records from the northern and southern tropical Andes, we show that tropical Andean hydroclimate responses were similar, with generally warmer and drier conditions during the MCA and cooler and wetter conditions during the LIA. Whereas southern tropical Andean responses were in line with variations in the tropical rain belt, northern tropical Andean responses were not. For the northern Andes, we propose that slow-moving, low-energy convective precipitation during the MCA was blocked by the eastern Andes, resulting in reduced precipitation at elevation and in the interior Andes while warming increased evaporation. Wet conditions in the northern Andes during the LIA were the result of cool atmospheric temperatures that reduced evaporation and increased effective moisture despite low precipitation. This suggests topography and temperature played important roles in the northern Andean hydroclimate and that further warming could reduce effective moisture through topographic precipitation blocking and increased evaporation. Tropical Andean hydroclimate variability was synchronous and in-phase between the northern and southern hemispheres during the last millennium Orographic influences on atmospheric convection-controlled precipitation in the northern tropical Andes during the last millennium Mean annual temperatures controlled effective moisture in the northern tropical Andes vis-& agrave;-vis evaporation

High-resolution paleoclimatic and paleoecologic datasets from a small lake in eastern Washington (USA) help elucidate Holocene environmental dynamics in the interior Pacific Northwest. Round Lake lies near the ecotone between sagebrush steppe and dry pine forest, making it highly sensitive to changes in precipitation-evaporation (P-E) balance. We present isotopic, sedimentological, and paleoecological data from a single sedimentary sequence analyzed at sufficient resolution to detect sub-decadal climatic variability. During the early Holocene (11,000-8500 cal yr BP), when summer insolation was higher than present, Round Lake experienced persistent aridity that led to reduced forest cover and small, probably frequent surface fires. Sub-centennial hydroclimate fluctuations during this period were muted, as indicated by low multi-decadal variability in delta 18Ocarbonate records from the region. Oxygen isotopes indicate increased cool-season moisture during the middle Holocene (85005300 BP), but stratigraphic evidence suggests intermediate and variable lake levels. The deposition of the Mazama tephra (ca. 7600 cal yr BP) immediately preceded to a protracted expansion of steppe at the expense of forest. Increased hydroclimate variability and fire activity following ash deposition likely restricted conifer taxa even when elevated cool-season moisture would have facilitated their growth. The late Holocene was marked by persistently high lake levels, as indicated by the lithostratigraphic and pollen records. Comparatively large multidecadal variations in precipitation, which peaked during the last millennium, were also prominent features of late-Holocene hydroclimate, and may have been related to the strengthening of the El Nino Southern Oscillation and Pacific Decadal Oscillation. Pinus and Pseudotsuga show a 60-100 year lag in their response to hydroclimatic variations inferred from the oxygen isotope records. These results highlight how millennial-todecadal scale hydroclimate conditions, along with abrupt landscape change, influence ecology in a waterstressed region of western North America.

High-amplitude quasi-stationary atmospheric Rossby waves with zonal wave numbers 6-8 associated with the phenomenon of quasi-resonant amplification (QRA) have been linked to persistent summer extreme weather events in the Northern Hemisphere. QRA is not well-resolved in current generation climate models, therefore, necessitating an alternative approach to assessing their behavior. Using a previously-developed fingerprint-based semi-empirical approach, we project future occurrence of QRA events based on a QRA index derived from the zonally averaged surface temperature field, comparing results from CMIP 5 and 6 (Coupled Model Intercomparison Project). There is a general agreement among models, with most simulations projecting substantial increase in QRA index. Larger increases are found among CMIP6-SSP5-8.5 (42 models, 46 realizations), with 85% of models displaying a positive trend, as compared with 60% of CMIP5-RCP8.5 (33 models, 75 realizations), with a reduced spread among CMIP6-SSP5-8.5 models. CMIP6-SSP3-7.0 (23 models, 26 realizations) simulations display qualitatively similar behavior to CMIP6-SSP5-8.5, indicating a substantial increase in QRA events under business-as-usual emissions scenarios, and the results hold regardless of the increase in climate sensitivity in CMIP6. Projected aerosol reductions in CMIP6-SSP3-7.0-lowNTCF (5 models, 16 realizations) lead to halting effect in QRA index and Arctic Amplification during the 1st half of the twenty-first century. Our analysis suggests that anthropogenic warming will likely lead to an even more substantial increase in QRA events (and associated summer weather extremes) than indicated by past analyses.

Variability in the source and seasonality of precipitation in the midcontinental United States during the Holocene was investigated using isotopic and sedimentological data from Martin Lake, northeastern Indiana, USA. Between 7100 and 4000 years before present (yr BP; present = 1950 CE), high S 18 O cal and S 13 C cal values with low variability indicate that moisture was predominantly derived from subtropical, southerly sources and delivered primarily during the warm season. Mean state shifts toward lower S 18 O cal and S 13 C cal occurred at ca. 4000 and 2550 yr BP, respectively, indicating an increase in northerly-sourced cold-season precipitation during the Late Holocene (i.e., the past 4200 years) and a subsequent reduction in warm season duration after 2550 yr BP. Record low %lithics from ca. 5000 to 4000 yr BP indicates major reductions in warm-season rain storms, consistent with regional evidence of drought at this time. An increase in the amplitude of centennial-scale variability in S 18 O cal , S 13 C cal , and %lithics after 1900 yr BP indicates greater precipitation source variability during the Common Era. During this interval, precipitation fluctuated between southerly-sourced, convective rainstorms when the Northern Hemisphere (NH) was warm (e.g., during the Medieval Climate Anomaly; 700-1000 - 1000 yr BP) and northerly-sourced rain and snow when the NH was cool (e.g., during the Little Ice Age; 150-550 - 550 yr BP). These trends, especially the change at ca. 4000 yr BP, are consistent with other North American paleoclimate records that collectively suggest a continental-scale shift in precipitation seasonality during the Middle to Late Holocene transition as the tropical Pacific Ocean transitioned from La Nina-like a-like conditions to a more El Nino-like o-like mean state. Concurrent NH cooling and persistent El Nino-like o-like conditions during the Late Holocene would have favored a southerly polar front jet stream with enhanced ridge and trough atmospheric circulation over North America - conditions resembling the positive mode of the Pacific-North American teleconnection (PNA). This would have increased interactions between high-latitude and subtropical airmasses over the midcontinent, increasing the proportion of northerly precipitation with low S 18 O delivered during the cold season (i.e., snowfall) and during extended periods with +PNA-like atmospheric circulation (e.g., the Little Ice Age).

Abstract We present oxygen isotope and charcoal accumulation records from two lakes in eastern Washington that have sufficient temporal resolution to quantitatively compare with tree‐ring records and meteorological data. Hydroclimate reconstructions from tree‐rings and lake sediments show close correspondence after accounting for seasonal‐ to centennial‐ scale temporal sensitivities. Carbonate δ18O measurements from Castor and Round lakes reveal that the Medieval Climate Anomaly (MCA) experienced wetter November‐March conditions than the Little Ice Age (LIA). Charcoal records from Castor, Round, and nearby lakes show elevated fire activity during the LIA compared to the MCA. Increased multidecadal hydroclimate variability after 1250 CE is evident in proxy records throughout western North America. In the Upper Columbia River Basin, multidecadal wet periods during the LIA may have enhanced fuel loads that burned in subsequent dry periods. A notable decline in biomass burning occurred with Euro‐American settlement in the late nineteenth century.

High-amplitude quasi-stationary atmospheric Rossby waves with zonal wave numbers 6 to 8 associated with the phenomenon of quasi-resonant amplification (QRA) have been linked to persistent summer extreme weather events in the Northern Hemisphere. We project future occurrence of QRA events based on an index derived from the zonally averaged surface temperature field, comparing results from CMIP5 and CMIP6 (Coupled Model Intercomparison Projects) climate projections. Under the scenarios analyzed, there is a general agreement among models, with most simulations projecting a substantial increase in QRA index. Larger increases are found among CMIP6-SSP585 (42 models, 46 realizations) models with 85% of models displaying a positive trend, as compared with as compared with 60% of CMIP5-RCP85 (35 models, 75 realizations), and a reduced spread among SSP585 models. The CMIP6-SSP370 (24 models, 28 realizations) simulations display qualitatively similar behavior to SSP585, indicating a substantial increase in QRA events under business-as-usual emissions scenarios. Our analysis suggests that anthropogenic warming will likely lead to an even more substantial increase in QRA events (and associated summer weather extremes) than our previous analysis of CMIP5 simulations.

Continued global warming is expected to result in reduced precipitation and a drier climate in Central America. Projections of future changes are highly uncertain, however, due to the spatial resolution limitations of models and insufficient observational data coverage across space and time. Paleoclimate proxy data are therefore critical for understanding regional climate responses during times of global climate reorganization. Here we present two lake-sediment based records of precipitation variability in Guatemala along with a synthesis of Central American hydroclimate records spanning the last millennium (800-2000 CE). The synthesis reveals that regional climate changes have been strikingly heterogeneous, even over relatively short distances. Our analysis further suggests that shifts in the mean position of the Intertropical Convergence Zone, which have been invoked by numerous studies to explain variability in Central American and circum-Caribbean proxy records, cannot alone explain the observed pattern of hydroclimate variability. Instead, interactions between several ocean-atmosphere processes and their disparate influences across variable topography appear to have resulted in complex precipitation responses. These complexities highlight the difficulty of reconstructing past precipitation changes across Central America and point to the need for additional paleo-record development and analysis before the relationships between external forcing and hydroclimate change can be robustly determined. Such efforts should help anchor model-based predictions of future responses to continued global warming.

Cyanobacterial blooms are increasing in frequency, duration, and severity globally in freshwater ecosystems. The Laurentian Great Lakes are prone to toxin-producing cyanobacterial blooms and have experienced annually recurring blooms. Because of its oligotrophic nature, Lake Superior has been relatively free of bloom occurrences. However, in recent years, Dolichospermum blooms have occurred with increasing frequency, especially in the western arm. During a Dolichospermum bloom in 2018, opportunistic samples were collected from the offshore bloom and investigated with shotgun metagenomics. We identified a near-complete Dolichospermum genome that is highly similar to genomes from cultures recovered in Lakes Erie and Ontario. The genomes from the Laurentian Great Lakes are typified by their putative ability to produce a suite of secondary metabolites like anabaenopeptin, but not toxins like microcystin. Additionally, we recovered a Dolichospermum lemmermannii 16S rRNA gene from the bloom and using datasets collected from the epilimnion and sediments in Lake Superior show this organism is ubiquitous and that several strains may exist. While there is much to learn about Lake Superior cyanobacterial bloom development and triggers, understanding this organism is endemic to the region, what its genome is capable of and that specific strains may have provenance within the lake provides a distinct ecological basis for understanding and working towards a predictive framework for future blooms.

The spatiotemporal patterns and underlying causes of climate variations spanning the mid-to late-Holocene in the North Atlantic region are largely unsettled. Here, we present a decadally resolved carbonate oxygen isotope (delta O-18) record spanning the last similar to 5000 calibrated years before present (cal yr BP) from Norman's Pond, a small, hydrologically open lake located in west-central Newfoundland, Canada. Stable isotope data from regional lakes, rivers, and precipitation samples indicate the delta O-18 record primarily reflects changes in the integrated, annual delta O-18 of precipitation, which is largely controlled by local atmospheric temperature and to a lesser extent, changes in the seasonality of precipitation. The record exhibits a general trend of decreasing delta O-18 values over the last -5000 cal yr BP that is consistent with gradually declining Boreal summer insolation and long-term cooling. Large decadal to centennial timescale fluctuations in delta O-18 values occurred during the last millennium, with persistent low delta O-18 values during the early Medieval Climate Anomaly (MCA, 950 to 1250 CE) and more variable and low values across the Little Ice Age (LIA, 1450 to 1850 CE). Relatively low delta O-18 values during the early MCA at similar to 950 cal yr BP (1000 CE) suggest colder temperatures in Newfoundland and/or an increase in coldseason or decrease in warm-season precipitation across the island. MCA delta O-18 values are lower than those of the LIA, revealing that climate conditions in Newfoundland were variable and differed from the broader North Atlantic region. For example, the shift to very low delta O-18 during the early MCA is coincident with a transition to colder sea surface temperatures (SSTs) in the Labrador Sea region, which was potentially triggered by melting of the Greenland Ice Sheet and alpine glaciers and ice caps in/around Baffin Bay, and subsequent delivery of cold waters further south via the Labrador Current. The early MCA interval of inferred cooling is coincident with Norse settlement at L'Anse aux Meadows in northern Newfoundland at similar to 929 cal yr BP (similar to 1021 CE). Subsequent low and variable 8 18 0 during the LIA suggests regional shifts in the seasonality of precipitation and/or cooling, consistent with other terrestrial climate records from the region. Proxy evidence from Norman's Pond therefore reinforces the idea of substantial spatiotemporal variability in climatic change in the broader North Atlantic region since at least the mid-Holocene, and in particular during the MCA, when cooler conditions prevailed in Newfoundland as a result of lower offshore sea surface temperatures that were likely driven by greater meltwater delivery via the Labrador current. (C) 2022 Elsevier Ltd. All rights reserved.

We use an ensemble of simulations of a coupled model (NCAR Community Earth System Model) driven by natural radiative forcing estimates over the pre‐industrial past millennium to test the efficacy of methods designed to remove forced variability from proxy‐based climate reconstructions and estimate residual internal variability (e.g., a putative “Atlantic Multidecadal Oscillation”). Within the framework of these experiments, the forced component of surface temperature change can be estimated accurately from the ensemble mean, and the internal variability of each of the independent realizations can be accurately assessed by subtracting off that estimate. We show in this case, where the true internal variability is known, that regression‐based methods of removing the forced component from proxy reconstructions will, in the presence of uncertainties in the underlying natural radiative forcing, fail to yield accurate estimates thereof, incorrectly attributing unresolved forced features (and multidecadal spectral peaks associated with them) to internal variability.

Uncertainty about the influence of anthropogenic radiative forcing on the position and strength of convective rainfall in the Intertropical Convergence Zone (ITCZ) inhibits our ability to project future tropical hydroclimate change in a warmer world. Paleoclimatic and modeling data inform on the timescales and mechanisms of ITCZ variability; yet a comprehensive, long-term perspective remains elusive. Here, we quantify the evolution of neotropical hydroclimate over the preindustrial past millennium (850 to 1850 CE) using a synthesis of 48 paleo-records, accounting for uncertainties in paleo-archive age models. We show that an interhemispheric pattern of precipitation antiphasing occurred on multicentury timescales in response to changes in natural radiative forcing. The conventionally defined “Little Ice Age” (1450 to 1850 CE) was marked by a clear shift toward wetter conditions in the southern neotropics and a less distinct and spatiotemporally complex transition toward drier conditions in the northern neotropics. This pattern of hydroclimatic change is consistent with results from climate model simulations indicating that a relative cooling of the Northern Hemisphere caused a southward shift in the thermal equator across the Atlantic basin and a southerly displacement of the ITCZ in the tropical Americas, with volcanic forcing as the principal driver. These findings are at odds with proxy-based reconstructions of ITCZ behavior in the western Pacific basin, where changes in ITCZ width and intensity, rather than mean position, appear to have driven hydroclimate transitions over the last millennium. This reinforces the idea that ITCZ responses to external forcing are region specific, complicating projections of the tropical precipitation response to global warming.