Abstract: We used pollen and high-resolution charcoal analysis of lake sediment to reconstruct a 7600 yr vegetation and fire history from Anthony Lake, located in the Blue Mountains of northeastern Oregon. From 7300 to 6300 cal yr BP, the forest was composed primarily of Populus , and fire was common, indicating warm, dry conditions. From 6300 to 3000 cal yr BP, Populus declined as Pinus and Picea increased in abundance and fire became less frequent, suggesting a shift to cooler, wetter conditions. From 3000 cal yr BP to present, modern-day forests composed of Pinus and Abies developed, and from 1650 cal yr BP to present, fires increased. We utilized the modern climate-analogue approach to explain the potential synoptic climatological processes associated with regional fire. The results indicate that years with high fire occurrence experience positive 500 mb height anomalies centered over the Great Basin, with anomalous southerly component of flow delivering dry air into the region and with associated sinking motions to further suppress precipitation. It is possible that such conditions became more common over the last 1650 cal yr BP, supporting an increase in fire despite the shift to more mesic conditions.

Abstract: Abstract. This paper is the first of a series of four GMD papers on the PMIP4-CMIP6 experiments. Part 2 (Otto-Bliesner et al., 2017) gives details about the two PMIP4-CMIP6 interglacial experiments, Part 3 (Jungclaus et al., 2017) about the last millennium experiment, and Part 4 (Kageyama et al., 2017) about the Last Glacial Maximum experiment. The mid-Pliocene Warm Period experiment is part of the Pliocene Model Intercomparison Project (PlioMIP) – Phase 2, detailed in Haywood et al. (2016).The goal of the Paleoclimate Modelling Intercomparison Project (PMIP) is to understand the response of the climate system to different climate forcings for documented climatic states very different from the present and historical climates. Through comparison with observations of the environmental impact of these climate changes, or with climate reconstructions based on physical, chemical, or biological records, PMIP also addresses the issue of how well state-of-the-art numerical models simulate climate change. Climate models are usually developed using the present and historical climates as references, but climate projections show that future climates will lie well outside these conditions. Palaeoclimates very different from these reference states therefore provide stringent tests for state-of-the-art models and a way to assess whether their sensitivity to forcings is compatible with palaeoclimatic evidence. Simulations of five different periods have been designed to address the objectives of the sixth phase of the Coupled Model Intercomparison Project (CMIP6): the millennium prior to the industrial epoch (CMIP6 name: past1000); the mid-Holocene, 6000 years ago (midHolocene); the Last Glacial Maximum, 21 000 years ago (lgm); the Last Interglacial, 127 000 years ago (lig127k); and the mid-Pliocene Warm Period, 3.2 million years ago (midPliocene-eoi400). These climatic periods are well documented by palaeoclimatic and palaeoenvironmental records, with climate and environmental changes relevant for the study and projection of future climate changes. This paper describes the motivation for the choice of these periods and the design of the numerical experiments and database requests, with a focus on their novel features compared to the experiments performed in previous phases of PMIP and CMIP. It also outlines the analysis plan that takes advantage of the comparisons of the results across periods and across CMIP6 in collaboration with other MIPs.

Abstract: To assess the relative importance of climate and humans in burning over recent millennia, we reconstruct fire history from three independent sources, specifically, historical, fire-scar, and charcoal data. We then compare the results to data on climate and population. We also construct a statistical model to determine the extent to which climate alone can predict biomass burning. The historical data were obtained from the United States Department of Agriculture Forest Service estimates of the extent, use, and destruction of original saw timber (i.e., trees older than 50 y in 1630 CE) from 1630 to 1940 CE ( 2 ). Fire-scar data ( n  = 359) were obtained from the International Multiproxy Paleofire Database, which provides annually resolved data on fires from around 1400 CE to present (primarily from dry interior ponderosa pine forests). We estimate temporal trends in fire by using the proportion of site records with scars for each year. Sedimentary charcoal accumulation rates from 69 records are used to identify decadal-to centennial-scale trends in biomass burning over the past three millennia, and a subset ( n  = 41) of records are further analyzed to reconstruct fire-episode frequency over this interval. Temperature, drought, and population changes are used to understand the changes in forest fire activity, while temperature and drought are used in a statistical regression model (Generalized Additive Model or GAM) to explain the variability in biomass burning during the past 1,500 y. Anthropogenic impacts on fire occur against the backdrop of climate variability. There have been marked human influences on western wildfires since Euro-American settlement, including increased ignitions (e.g., from forest clearance, agriculture, logging, and railroads), and fire exclusion (e.g., from landscape fragmentation, grazing, and suppression). Other significant impacts on vegetation and fire occurred indirectly, such as changes in plant succession pathways (e.g., when shrublands previously maintained by frequent fire converted to forest after fires were regularly excluded) and the introduction of nonnative species. Indigenous fire use prior to settlement varied in intensity, extent, and persistence, and probably varied with season, migration, and cultural and technological developments. Climate is generally considered to be the primary control of contemporary fire regimes in the western United States ( 1 ). Climate influences fire primarily through variations in temperature and precipitation (and thus available moisture). Increasing temperature leads to more fires; however, this is only true if sufficient vegetation is present to support the spread of fire. Precipitation is also linked to fire—there must be sufficient moisture to support continuous vegetation on the landscape, but not so much that fuels never become dry enough to burn. Forest fires in the western United States have been increasing in extent for several decades, prompting much research into the causes and consequences of such changes. The overall level of fire activity in a given place is governed by processes relating to climate, people, and vegetation that operate over decades and centuries; yet, most fire research is based on much shorter time scales. Given that future climate change is expected to drive fire activity well above its historical range of variability, a long-term perspective provides essential context to current changes. We use sedimentary charcoal accumulation rates to construct baseline levels of burning for the past 3,000 y in the American West; we then compare this record to independent fire-history data obtained from historical records and fire scars. We also create a statistical model, based only on independent temperature and drought reconstructions, that predicts 85% of the variability in biomass burnt (thought to reflect area burnt) prior to the 1800s, before human and ecological influences became dominant. Large shifts in biomass burning since the 1800s are not unprecedented, but their causes and effects differ greatly from climate-driven shifts in the past. Fire regimes are currently in disequilibrium with the climate, due to the opposing forces of fire exclusion practices (e.g., grazing and fire suppression) and global warming; consequently, a large “fire deficit” exists. The 20th Century Fire Deficit in the Western United States. Observed and predicted changes in biomass burning diverge in the late 1800s, despite increasing temperature and drought (Fig. 4 A ). Observed biomass burning, fire scars, charcoal-based fire frequencies, and human-caused fires decline to levels similar to the levels during the LIA. In contrast, predicted biomass burning rises from 1880 CE to present, which is consistent with increased temperature and drought. This pattern indicates that nonclimatic factors became the dominant control of fires around 1880 CE. The decline in fires during the 20th century may be explained by multiple factors. In the late 1800s, widespread domestic livestock grazing reduced grassy fuel loads, compacted soils, and greatly reduced fire frequencies. By 1900 CE, the western frontier had largely closed, and intentionally set fires probably declined due to changing attitudes and policies towards fire. In addition, landscape fragmentation from trail and road building limited the spread of fire. Furthermore, after the 1940s, fire suppression became highly effective, preventing the spread of many forest fires. However, ecological factors also played a role, as the number of young stands and aspen stands, which are resistant to burning, increased after logging and previous extensive burning. Consequently, a fire deficit now exists and has been growing throughout the 20th century, pushing fire regimes into disequilibrium with climate. Hence, while current levels of large-scale biomass burning ( 1 ) remain within the realm of natural variability during the past 1,000 y, if levels of burning were to come into equilibrium with climate, they would exceed the natural range of variability experienced in at least the last 3,000 y. Three independent fire-history reconstructions for the western United States show that there have been large changes in wildfires since the 1800s. In earlier periods, changes of this scale were driven by climate; in the past 200 y, human behavior has played a much larger role. Fire suppression practices have greatly reduced fire, whereas global warming has increased the probability of fire. A widening gap, or fire deficit, therefore exists between actual levels of burning and expected levels of burning given current climate conditions. Recent increases in catastrophic wildfires in the West are an indication of this deficit, and suggest that current fire suppression practices are unsustainable. Fires are projected to increase even further in coming decades, and may require reevaluation of fire management policies and potential investment of additional resources. Climate Controls Fire Until the Beginning of Euro-American Settlement of the West. A statistical model, calibrated using data from 500 to 1800 CE, predicts multidecadal-to-centennial changes in biomass burning from temperature and drought area indices. Climate explains most of the multidecadal-to century-scale variations of biomass burning. Temperature alone accounts for half of the total variance of biomass burning, while drought area explains about 1/3 of overall variance. Downward trends were recorded for temperature, drought, biomass burning, fire frequency (see full text), and predicted biomass burning during the past two millennia until the Settlement Era ( Fig. P1 A – E ). In contrast, the human population gradually increased until ca. 500 y ago, when Native American populations began to collapse after European contact ( Fig. P1 F ). The population began to recover around 350 y ago, and then rose dramatically since settlement. Biomass burning was high when climate changed rapidly at the beginning of the Medieval Climate Anomaly (MCA), which was around 1000 CE when both temperature and drought were also high. Fire activity was high again at the onset of the Little Ice Age (LIA), around 1400 CE, when drought was high. However, as the LIA progressed, biomass burning declined substantially, which coincided with large declines in temperature, drought, and population ( Fig. P1 A , D and E ). Fig. P1. ( A ) Smoothed and standardized 25-year (gray) and 100-year (red) trend line through standardized biomass burning records ( n  = 69) along with predicted biomass burning based on a GAM (black dashed line) fit to the 100-year charcoal values. ( B ) Smoothed proportions of dendrochronological sites recording fire scars. ( C ) Estimated historical sawtimber affected by lightning- and human-caused fires in the western United States ( 2 ) ( D ) Smoothed gridded temperature anomalies for the western United States ( 3 ). ( E ) Smoothed Palmer Drought Severity Index for the western United States ( 4 ). ( F ) Population estimates for the western United States ( 5 ). All smoothed curves are plotted with 95% bootstrap confidence intervals. The transition from the LIA into the Settlement Era is marked by a sharp increase in burning, as evidenced by historical records, fire-scar, and (observed and predicted) charcoal data. The increase in fire activity, which reached its maximum ca. 1850–1870 CE (Fig. 4 C ), is consistent with increased ignitions from land clearance, logging, agriculture, and railroads during settlement, and also with increasing temperature and drought, which reached a maximum between 1700 and 1900 CE ( Fig. P1 A – E ). Thus, climate and humans acted synergistically to increase fire.

Abstract: Abstract. Many ecosystem processes that influence Earth system feedbacks – vegetation growth, water and nutrient cycling, disturbance regimes – are strongly influenced by multidecadal- to millennial-scale climate variations that cannot be directly observed. Paleoclimate records provide information about these variations, forming the basis of our understanding and modeling of them. Fossil pollen records are abundant in the NE US, but cannot simultaneously provide information about paleoclimate and past vegetation in a modeling context because this leads to circular logic. If pollen data are used to constrain past vegetation changes, then the remaining paleoclimate archives in the northeastern US (NE US) are quite limited. Nonetheless, a growing number of diverse reconstructions have been developed but have not yet been examined together. Here we conduct a systematic review, assessment, and comparison of paleotemperature and paleohydrological proxies from the NE US for the last 3000 years. Regional temperature reconstructions (primarily summer) show a long-term cooling trend (1000 BCE–1700 CE) consistent with hemispheric-scale reconstructions, while hydroclimate data show gradually wetter conditions through the present day. Multiple proxies suggest that a prolonged, widespread drought occurred between 550 and 750 CE. Dry conditions are also evident during the Medieval Climate Anomaly, which was warmer and drier than the Little Ice Age and drier than today. There is some evidence for an acceleration of the longer-term wetting trend in the NE US during the past century; coupled with an abrupt shift from decreasing to increasing temperatures in the past century, these changes could have wide-ranging implications for species distributions, ecosystem dynamics, and extreme weather events. More work is needed to gather paleoclimate data in the NE US to make inter-proxy comparisons and to improve estimates of uncertainty in reconstructions.

Abstract: High-resolution charcoal and pollen analyses were used to reconstruct a 12,000-yr-long fire and vegetation history of the Tumalo Lake watershed and to examine the short-term effects that tephra deposition have on forest composition and fire regime. The record suggests that, from 12,000 to 9200 cal yr BP, the watershed was dominated by an open Pinus forest with Artemisia as a common understory species. Fire episodes occurred on average every 115 yr. Beginning around 9200 cal yr BP, and continuing to the present, Abies became more common while Artemisia declined, suggesting the development of a closed forest structure and a decrease in the frequency of fire episodes, occurring on average every 160 yr. High-resolution pollen analyses before and after the emplacement of three distinct tephra deposits in the watershed suggest that nonarboreal species were most affected by tephra events and that recovery of the vegetation community to previous conditions took between 40 and 100 yr. Changes in forest composition were not associated with tephra depositional events or changes in fire-episode frequency, implying that the regional climate is the more important control on long-term forest composition and structure of the vegetation in the Cascade Range.

Abstract: Abstract. Two interglacial epochs are included in the suite of Paleoclimate Modeling Intercomparison Project (PMIP4) simulations in the Coupled Model Intercomparison Project (CMIP6). The experimental protocols for simulations of the mid-Holocene (midHolocene, 6000 years before present) and the Last Interglacial (lig127k, 127 000 years before present) are described here. These equilibrium simulations are designed to examine the impact of changes in orbital forcing at times when atmospheric greenhouse gas levels were similar to those of the preindustrial period and the continental configurations were almost identical to modern ones. These simulations test our understanding of the interplay between radiative forcing and atmospheric circulation, and the connections among large-scale and regional climate changes giving rise to phenomena such as land–sea contrast and high-latitude amplification in temperature changes, and responses of the monsoons, as compared to today. They also provide an opportunity, through carefully designed additional sensitivity experiments, to quantify the strength of atmosphere, ocean, cryosphere, and land-surface feedbacks. Sensitivity experiments are proposed to investigate the role of freshwater forcing in triggering abrupt climate changes within interglacial epochs. These feedback experiments naturally lead to a focus on climate evolution during interglacial periods, which will be examined through transient experiments. Analyses of the sensitivity simulations will also focus on interactions between extratropical and tropical circulation, and the relationship between changes in mean climate state and climate variability on annual to multi-decadal timescales. The comparative abundance of paleoenvironmental data and of quantitative climate reconstructions for the Holocene and Last Interglacial make these two epochs ideal candidates for systematic evaluation of model performance, and such comparisons will shed new light on the importance of external feedbacks (e.g., vegetation, dust) and the ability of state-of-the-art models to simulate climate changes realistically.

Abstract: Understanding climatic influences on the rates and mechanisms of landscape erosion is an unresolved problem in Earth science that is important for quantifying soil formation rates, sediment and solute fluxes to oceans, and atmospheric CO 2 regulation by silicate weathering. Glaciated landscapes record the erosional legacy of glacial intervals through moraine deposits and U-shaped valleys, whereas more widespread unglaciated hillslopes and rivers lack obvious climate signatures, hampering mechanistic theory for how climate sets fluxes and form. Today, periglacial processes in high-elevation settings promote vigorous bedrock-to-regolith conversion and regolith transport, but the extent to which frost processes shaped vast swaths of low- to moderate-elevation terrain during past climate regimes is not well established. By combining a mechanistic frost weathering model with a regional Last Glacial Maximum (LGM) climate reconstruction derived from a paleo-Earth System Model, paleovegetation data, and a paleoerosion archive, we propose that frost-driven sediment production was pervasive during the LGM in our unglaciated Pacific Northwest study site, coincident with a 2.5 times increase in erosion relative to modern rates. Our findings provide a novel framework to quantify how climate modulates sediment production over glacial-interglacial cycles in mid-latitude unglaciated terrain.