Growing impact of wildfire on western US water supply

Streamflow often increases after fire, but the persistence of this effect and its importance to present and future regional water resources are unclear. This paper addresses these knowledge gaps for the western United States (WUS), where annual forest fire area increased by more than 1,100% during 1984 to 2020. Among 72 forested basins across the WUS that burned between 1984 and 2019, the multibasin mean streamflow was significantly elevated by 0.19 SDs (P < 0.01) for an average of 6 water years postfire, compared to the range of results expected from climate alone. Significance is assessed by comparing prefire and postfire streamflow responses to climate and also to streamflow among 107 control basins that experienced little to no wildfire during the study period. The streamflow response scales with fire extent: among the 29 basins where >20% of forest area burned in a year, streamflow over the first 6 water years postfire increased by a multibasin average of 0.38 SDs, or 30%. Postfire streamflow increases were significant in all four seasons. Historical fire–climate relationships combined with climate model projections suggest that 2021 to 2050 will see repeated years when climate is more fire-conducive than in 2020, the year currently holding the modern record for WUS forest area burned. These findings center on relatively small, minimally managed basins, but our results suggest that burned areas will grow enough over the next 3 decades to enhance streamflow at regional scales. Wildfire is an emerging driver of runoff change that will increasingly alter climate impacts on water supplies and runoff-related risks.

Recent declines in soil moisture, streamflow, and reservoir storage signal the precariousness of water supplies in the western United States (WUS) and the urgency of managing associated risks (1, 2). Declining WUS water supplies are qualitatively consistent with modeled trends due to anthropogenic climate change (3, 4), but projections are uncertain due to not only climate but also the complexity of vegetation responses to climate change and associated disturbances such as wildfire (5–9). In addition to transpiration and interception, which directly divert moisture from runoff, vegetation also affects hydrology by shaping soil depth and structure and by modulating turbulent energy fluxes that alter snowpack and evaporation (10). In addition to direct effects on vegetation, wildfires can further affect streamflow by promoting water repellency and soil erosion (11–13). Given that the headwater areas of major WUS rivers are generally forested, altered forest cover or ecosystem water demand could potentially affect water resources at regional scales.In recent decades, the annual forest area burned in the WUS has risen rapidly, in step with climate trends toward warming and drying (14–20). In general, forest disturbances such as wildfire are known to temporarily enhance streamflow (21–23), although cases of postdisturbance streamflow declines, especially in arid areas, have also been documented (21, 24, 25). The likelihood that rapid increases in regional forest fire activity will continue (26, 27) suggests that wildfire may increasingly impact water resources in the water-limited WUS (6). Yet, the duration and seasonality of postdisturbance increases in runoff are unknown, raising the question of whether increased forest fire activity will meaningfully affect water availability in the WUS.Here we use stream gauge records from 179 river basins in the WUS to assess the strength, duration, and seasonality of postfire changes in streamflow and whether increasing forest fire activity is likely to have a detectable effect on regional streamflow.     

From the Cover: Control of the multimillennial wildfire size in boreal North America by spring climatic conditions

Wildfire activity in North American boreal forests increased during the last decades of the 20th century, partly owing to ongoing human-caused climatic changes. How these changes affect regional fire regimes (annual area burned, seasonality, and number, size, and severity of fires) remains uncertain as data available to explore fire–climate–vegetation interactions have limited temporal depth. Here we present a Holocene reconstruction of fire regime, combining lacustrine charcoal analyses with past drought and fire-season length simulations to elucidate the mechanisms linking long-term fire regime and climatic changes. We decomposed fire regime into fire frequency (FF) and biomass burned (BB) and recombined these into a new index to assess fire size (FS) fluctuations. Results indicated that an earlier termination of the fire season, due to decreasing summer radiative insolation and increasing precipitation over the last 7.0 ky, induced a sharp decrease in FF and BB ca. 3.0 kyBP toward the present. In contrast, a progressive increase of FS was recorded, which is most likely related to a gradual increase in temperatures during the spring fire season. Continuing climatic warming could lead to a change in the fire regime toward larger spring wildfires in eastern boreal North America.     

Field studies on Lyme disease in North America

The primary tick vector of Borrelia burgdorferi in eastern and central North America is Ixodes dammini; in western North America, Ixodes pacificus. Searching for the appropriate vector is the first step in determining whether a region is endemic and enzootic for the spirochete B burgdorferi, the etiological agent of Lyme disease, followed by examination of the ticks (questing or already attached to hosts) and wildlife for the spirochete. Questing ticks can be collected through a variety of methods. The two major animal hosts for I dammini are the white-footed mouse Peromyscus leucopus and the white-tailed deer Odocoileus virginianus. Sampling strategies should consider habitat and season. All three life stages of the vector tick should be located, indicating a self-sustaining population. Although B burgdorferi can be detected in many ways, there is no substitute for isolating the spirochete in Barbour-Stoenner-Kelly II medium for definitive proof of the presence of the Lyme disease spirochete.     

Effect of woody-plant encroachment on livestock production in North and South America

A large fraction of the world grasslands and savannas are undergoing a rapid shift from herbaceous to woody-plant dominance. This land-cover change is expected to lead to a loss in livestock production (LP), but the impacts of woody-plant encroachment on this crucial ecosystem service have not been assessed. We evaluate how tree cover (TC) has affected LP at large spatial scales in rangelands of contrasting social–economic characteristics in the United States and Argentina. Our models indicate that in areas of high productivity, a 1% increase in TC results in a reduction in LP ranging from 0.6 to 1.6 reproductive cows (Rc) per km². Mean LP in the United States is 27 Rc per km², so a 1% increase in TC results in a 2.5% decrease in mean LP. This effect is large considering that woody-plant cover has been described as increasing at 0.5% to 2% per y. On the contrary, in areas of low productivity, increased TC had a positive effect on LP. Our results also show that ecological factors account for a larger fraction of LP variability in Argentinean than in US rangelands. Differences in the relative importance of ecological versus nonecological drivers of LP in Argentina and the United States suggest that the valuation of ecosystem services between these two rangelands might be different. Current management strategies in Argentina are likely designed to maximize LP for various reasons we are unable to explore in this effort, whereas land managers in the United States may be optimizing multiple ecosystem services, including conservation or recreation, alongside LP.Grasslands, shrublands, and savannas, collectively termed “rangelands,” constitute about 50% of the Earth’s land surface (1). Although characterized by low yet highly variable annual rainfall, these areas provide 30–35% of terrestrial net primary productivity (NPP) (2), contain >30% of the world’s human population, and support the majority of the world’s livestock production (LP) (3, 4). Besides LP, rangelands also provide a variety of other ecosystem services, including fiber production, carbon sequestration, maintenance of the genetic library (conservation), and recreation (5).One of the most striking land-cover changes in rangelands worldwide over the past 150 y has been the proliferation of trees and shrubs at the expense of perennial grasses (6). In the United States, nonforest lands undergoing woody-plant encroachment are now estimated to cover up to 335 million ha (40% of the coterminous United States) (7) and the increase in woody cover ranges from 0.5% to 2% per y (8). The causes of this vegetation change are debated and the main potential drivers include intensification of livestock grazing, changes in climate and fire regimes, the introduction of nonnative woody species, and declines (natural and human induced) in the abundance of browsing animals (9–12). Historical increases in atmospheric nitrogen deposition and atmospheric carbon dioxide concentration have also been suggested to play a role (10, 11).Woody-plant encroachment has long been of concern to a broad range of stakeholders, from pastoralists to ranchers, because of the expected negative impact on LP (13). In response, brush management has been widely used to reduce the cover of encroaching woody-vegetation on both public and private lands. For example, the US Natural Resources Conservation Service spent US$127 million in brush management programs in the period 2005–2009, implemented on more than 1 million ha of rangeland (14). Despite claims about impact of woody-plant encroachment on LP and the large amounts of federal, state, and private spending on brush management, the impact of woody-plant encroachment on LP has seldom been quantified (15). Here our objectives are (i) to quantify how woody-plant encroachment affects LP at large spatial scales and (ii) to assess how this impact is modified under different ecological and social–economic conditions.We developed a general framework in which LP depends on NPP, woody-plant cover, and other nonbiological determinants. NPP sets the total amount of biomass and energy that is available to herbivores (16). The most common view on woody-plant encroachment is that encroachment diverts herbaceous productivity, on which cattle feed, to unpalatable woody-plant productivity, thus reducing potential energy intake (17–19). Thus, overall, primary production and woody-plant encroachment jointly determine the livestock carrying capacity of an ecosystem.Social and ecological factors interact to determine livestock rate. For example, Oesterheld et al. (20) assessed the relationship between NPP and LP in managed rangelands in Argentina, where management focuses on food production and found that the link between primary and secondary productivity was even tighter than in natural ecosystems. In these rangelands, management practices, such as providing water and minerals; regulating animal distribution; and reducing parasitism, predation, and diseases, resulted in stocking rates that were closely associated with NPP.We expect that in advanced industrial societies, where the production of goods (e.g., food by means of agriculture and ranching) plays a secondary role in the economy (21), landscapes will be managed to maximize multiple ecosystem services, and thus LP might be less driven by ecological drivers. Ecological factors, including NPP and woody-plant cover, determine potential stocking rates but actual stocking rates are modulated by manager’s decisions (22). In some cases, land managers overstock rangelands leading to degradation and desertification (23), whereas in other cases managers understock. The latter results from pursuing optimization of multiple ecosystem services of which food production is only one. Rangelands managed for multiple purposes and ecosystem services (24) seek provisioning of food, fiber, firewood, carbon sequestration, conservation, or recreation.Our hypotheses are (i) that overall LP decreases with woody-plant encroachment; (ii) the effect of woody-plant encroachment on LP is modulated by NPP, with a larger negative impact of woody-plant encroachment in those areas with higher NPP; and (iii) the role of ecological drivers [NPP and tree cover (TC)] on LP is larger in regions where the demand for ecosystem services is concentrated exclusively on food production.The scarcity of studies attempting to quantify the impact of woody-plant encroachment on LP reflects the difficulties of addressing this issue by means of conventional field approaches. An experimental approach necessitates monitoring the change in LP in a number of locations during the encroachment process, a process that might take decades (11). Our approach has been to explore how current rangeland LP varies at a regional scale along sites with different NPP and woody cover. We thus assessed the consequences of the process of woody-plant encroachment by evaluating the relationship between TC and LP at a given point in time across multiple locations. This approach of swapping time for space has been used to predict future trajectories of species in an ecological succession (25), and more recently, the expected change of organisms ranging from microbes (26) to trees (27) under a changing climate. We are aware of the limitations of this approach, mostly associated with the existence of lags that result in different models through space and time (28). Given the limitations of alternative options and the urgency of the problem, however, we consider our approach to be promising.To test our hypotheses, we collected information about woody-plant cover and primary productivity from remote-sensing sources and about LP from agricultural census data. Woody-plant encroachment occurs when there is an increase in the cover of trees or shrubs. The type of woody component depends on mean annual precipitation, arid systems invaded by shrubs, and mesic ecosystems invaded by trees. In our study areas, the transition between shrub and tree domains occurs approximately at 600 mm annual precipitation (Fig. S1). In the present work, we focused on encroachment of trees (i.e., areas >600 mm) because current remote-sensing tools assess TC with accuracy, but do not adequately estimate shrub cover (29), reducing our confidence to address this cover type. We aggregated data at the county level and combined remote-sensing and census data into a model that yields estimates of the impact of woody-plant cover on LP at large scales. To account for the effects of social–economic factors, we quantified the impact of TC on LP in two regions of the world that have extraordinary environmental similarity but contrasting social–economic characteristics (30, 31). The two regions are the US Central Grassland Region and the Argentinean Central Grassland. Both share similar temperature and precipitation gradients, yielding vegetation types that are remarkably similar (31) (Fig. 1). These environmental similarities contrast with large social–economic differences in the rural sector, specifically regarding LP (Fig. S2). During the last decades in the United States, there has been a reduction of people making a living from agriculture (40% reduction since 1980s) and a negative trend in the number of cattle in the region (22% reduction since the 1970s). At present, a large proportion of stakeholders in the United States are not full-time ranchers but maintain LP as a source of secondary income or for cultural or recreational reasons [US Department of Agriculture (USDA) Economic Research Service, www.ers.usda.gov; ref. 32]. In Argentina, although the relative importance of ranching has decreased due to the expansion of crop products, especially soybean, the reduction in the number of cattle has been much smaller (4% reduction since the 1970s; Fig. S2); beef is still the agricultural commodity with the largest output value (28% of the total agricultural production 2005–2007) (33). As a result, we expected stocking rates in Argentina to be closer to the NPP-derived carrying capacity of the system, and thus more tightly driven by ecological factors, than in the United States (20).Open in a separate windowFig. 1.Main environmental gradients (mean annual precipitation and mean annual temperature) in the US and Argentinean rangelands. Rangelands in this paper are defined as those areas encompassing the regular and regime mountain divisions of prairie, savanna, temperate and subtropical desert, and steppe, according to Bailey’s ecoregions (1). Within these areas, our work focused on those counties with mean annual precipitation values between 600 and 1,260 mm (Methods and Fig. 2). For both areas, national (bold lines) and county (thin lines) borders are shown. In the United States, state borders are also shown (bold lines).     

Ice and ocean constraints on early human migrations into North America along the Pacific coast

Founding populations of the first Americans likely occupied parts of Beringia during the Last Glacial Maximum (LGM). The timing, pathways, and modes of their southward transit remain unknown, but blockage of the interior route by North American ice sheets between ~26 and 14 cal kyr BP (ka) favors a coastal route during this period. Using models and paleoceanographic data from the North Pacific, we identify climatically favorable intervals when humans could have plausibly traversed the Cordilleran coastal corridor during the terminal Pleistocene. Model simulations suggest that northward coastal currents strengthened during the LGM and at times of enhanced freshwater input, making southward transit by boat more difficult. Repeated Cordilleran glacial-calving events would have further challenged coastal transit on land and at sea. Following these events, ice-free coastal areas opened and seasonal sea ice was present along the Alaskan margin until at least 15 ka. Given evidence for humans south of the ice sheets by 16 ka and possibly earlier, we posit that early people may have taken advantage of winter sea ice that connected islands and coastal refugia. Marine ice-edge habitats offer a rich food supply and traversing coastal sea ice could have mitigated the difficulty of traveling southward in watercraft or on land over glaciers. We identify 24.5 to 22 ka and 16.4 to 14.8 ka as environmentally favorable time periods for coastal migration, when climate conditions provided both winter sea ice and ice-free summer conditions that facilitated year-round marine resource diversity and multiple modes of mobility along the North Pacific coast.

Human dispersal pathways from Beringia into North America continue to be debated. Prevailing ideas include a coastal route and an interior route via an ice-free corridor between the Laurentide and Cordilleran ice sheets (1–6). The Laurentide and Cordilleran ice sheets merged during the Last Glacial Maximum (LGM) (7), closing the ice-free inland corridor between ~26 ± 1 ka (Fig. 1 and ref. 8) and 13.8 ± 0.5 ka (ref. 9). Archaeological sites south of the ice sheets in North America during this time frame (10–15) thus require either a coastal route, or entry through the interior prior to the LGM. A pre-LGM migration scenario is at odds with apparent genetic divergence between Siberian and Beringian populations between about 25 to 24 ka (95% CI 21 to 28 ka; ref. 16) and an inferred “Beringian Standstill” in migration until 18 to 16 ka (16–19). Was this biogeographical pause due to favorable conditions in Beringia, glacial bottlenecks that prevented southward transit along the coast, or a combination of both? How did Beringians make the arduous journey along the Pacific Coast corridor – by land, sea, or ice? Was the coastal route effectively blocked throughout the LGM, or were there intervals when passage was more or less possible? Building on recent evidence for multiple intervals of Cordilleran ice retreat within the last ice age (20), we evaluate these scenarios and define relatively benign climatic intervals when human migration along the Cordilleran coast may have been most feasible.Open in a separate windowFig. 1.Map of coastlines and ice extent at various time periods A) 32.5 ka, B) 27.5 ka, C) 25 ka, D) 15 ka during the late Pleistocene, showing possible migration pathways at each stage. Relative sea level (RSL) and ice sheet topography are from (8) and are interpolated and applied to the ETOPO01 bathymetry grid (21). Post LGM glacial ice evolution is unknown for Siberia, though some ice sheets were likely present during these time periods. North American archaeological sites (black dots) are shown that have median dates for initial human occupation that fall within ± 1 ka of the various time slices shown (for a full list of archaeological site data and references, including those that fall outside of the time/space domains shown here, see SI Appendix, Table S1 and Dataset S1); sites with controversial evidence for human presence are denoted with question marks. White dashed line on panel (C) shows the estimated extent of winter sea ice during the LGM, based on (22). Seasonal sea ice was present along the Alaskan coastal corridor to varying degrees during all the periods shown, but the spatial extent is not as well defined for the other intervals. Sediment cores identified in panel (A) are for the various proxy datasets shown in Figs. 3 and ​and55.Despite evidence for older archaeological sites farther inland, thus far, there is no definitive evidence of human occupation along the Pacific Coast of North America prior to ~13.8 ka (23). The absence of earlier coastal sites may reflect submergence of former occupation sites by rising postglacial sea level, exacerbated locally by relaxation of a subsiding glacio-isostatic forebulge (3, 24). Other factors may also have limited the viability of a coastal transit at certain times. The most obvious obstacle is ice cover on land, with large outlet glaciers emanating from the Alaska Peninsula and Southeast Alaska terminating in the ocean. Heavily crevassed ice streams would have been difficult or impossible to cross on land and dangerous at sea, potentially preventing passage for migrating groups of people.The strength of the cyclonic Alaska Coastal Current (ACC) also may have partially impeded southward movement for seafarers, as this current flows northward against the direction of migration (25). The ACC is driven by wind and Coriolis forcing and strengthened by coastal freshwater inputs (26) (Fig. 2). Royer and Finney (25) hypothesized that southward migration was impeded by freshwater input and rapid sea-level rise that accelerated coastal currents during global Meltwater Pulse 1a (MWP1a: 14.65 to 14.30 ka; ref. 27), effectively assuming that local freshwater inputs tracked global-average sea-level rise. Testing this hypothesis requires reconstruction of regional ice retreat and the resulting reduction of coastal salinity from regional meltwater flux, along with quantitative modeling of coastal current strength, issues we address here.Open in a separate windowFig. 2.Simulations of ocean currents in the Northeast Pacific under different climate and sea level conditions: Modern climate state (A), LGM climate state, with sea level −120 m below modern (B), LGM boundary conditions with an increased freshwater flux (C), and intermediate sea level (−75 m), as would have occurred during the mid-deglacial period (D). Mean annual surface ocean velocity shows a strengthening of the cyclonic Alaska Current during the LGM relative to modern conditions, as well as a contraction of the shelf area on which the ACC flows. Boundary currents flow in a cyclonic (anticlockwise) direction.The extent of land ice, both along the coastal corridor and inland route, has been widely debated over many decades (4, 28–30). However, the assessment of ice in the marine environment—such as the extent of sea ice and icebergs, and their impact on human migration—has received less attention. Evidence from ice-rafted debris (IRD) in marine sediments shows that the seaward edge of the Cordilleran Ice Sheet (CIS) and its outlet glaciers was extremely variable and subject to repeated abrupt retreats onto land or into silled fjords during the late Pleistocene (referred to as “Siku Events”; ref. 20). Sea ice formed in the subarctic North Pacific through much of this interval (22, 31), which may have impacted boat transit and altered marine resource composition and availability during certain months of the year. Today, land-fast sea ice provides a relatively unobstructed and flat surface as a platform for travel between otherwise inaccessible high Arctic communities, typically in winter or spring (32, 33). In addition to ease of movement, sea ice facilitates hunting of marine mammals near the ice edge and sub-ice intertidal shellfishing; both are important food resources in the Arctic winter (32). With the seasonal melting of sea ice, kelp forest habitats can provide important marine resources in summer (34, 35). Reconstructions of North Pacific sea ice are essential to building a clearer picture of the conditions that coastal people in the North Pacific would have contended with during the glacial and deglacial periods.To help address these issues, we present records of sea-ice variations based on the %C_37:4 proxy (36) and synthesize previously published reconstructions of sea ice, sea-surface temperature (SST), salinity, and IRD from marine sediment cores in the North Pacific (Fig. 1). Together, these paleoenvironmental data help discern major climate and oceanographic changes that may have facilitated or impeded human migration during the terminal Pleistocene. We present model results from a high-resolution (1/6°) eddy-permitting general circulation model (MITgcm) and a lower resolution model (GENMOM) to evaluate changes in current velocity of the Alaska Current system between glacial and modern climate states, as well as in response to increases in regional freshwater discharges and intermediate sea level conditions. We compare paleo-SST reconstructions from the North Pacific with simulated SST from the transient deglacial simulation in iTRACE (37) for major climate intervals between the LGM and early Holocene. These paleoenvironmental reconstructions and models suggest possible time intervals when southward dispersal along the Northwest Coast was most feasible for people and provide insight into factors that may have influenced subsequent coastal habitability.     

Adverse effects of acid rain on the distribution of the Wood Thrush Hylocichla mustelina in North America

Research into population declines of North American bird species has mainly focused on the fragmentation of habitat on the breeding or wintering grounds [Robinson, S. K., Thompson, F. R., Donovan, T. M., Whitehead, D. R. & Faaborg, J. (1995) Science 267, 1987-1990]. In contrast, research into declines of European species has mainly focused on intensification of agriculture [Donald, P. F., Green, R. E. & Heath, M. F. (2001) Proc. R. Soc. London Ser. B 268, 25-29] and the role played by the atmospheric deposition of pollutants, in particular, acid rain [Graveland, J. (1998) Environ. Rev. 6, 41-54]. However, despite widespread unexplained declines of bird populations in regions of heavy wet acid ion deposition [Sauer, J. R., Hines, J. E. & Fallon, J. (2001) The North American Breeding Bird Survey Results and Analysis 1966-2000 (Patuxent Wildlife Research Center, Laurel, MD)], no North American studies have presented evidence linking such widespread terrestrial bird declines to acid rain. To address the question of the role played by acid rain in population declines of eastern North American songbird species, we combine data from several sources. We use a multiple logistic regression model to test for adverse effects of acid rain on the Wood Thrush, while controlling for regional abundance, landscape-level habitat fragmentation, elevation, soil pH, and vegetation. We show a strong, highly significant, negative effect of acid rain on the predicted probability of breeding by this species, and interactions with elevation, low pH soils, and habitat fragmentation that worsen these negative effects. Our results suggest an important role for acid rain in recent declines of some birds breeding in the eastern United States, particularly in high elevation zones with low pH soils, and show the need to consider other large-scale influences, in addition to habitat fragmentation, when addressing bird population declines.     

The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean

Clouds developing in a polluted environment tend to have more numerous but smaller droplets. This property may lead to suppression of precipitation and longer cloud lifetime. Absorption of incoming solar radiation by aerosols, however, can reduce the cloud cover. The net aerosol effect on clouds is currently the largest uncertainty in evaluating climate forcing. Using large statistics of 1-km resolution MODIS (Moderate Resolution Imaging Spectroradiometer) satellite data, we study the aerosol effect on shallow water clouds, separately in four regions of the Atlantic Ocean, for June through August 2002: marine aerosol (30 degrees S-20 degrees S), smoke (20 degrees S-5 degrees N), mineral dust (5 degrees N-25 degrees N), and pollution aerosols (30 degrees N- 60 degrees N). All four aerosol types affect the cloud droplet size. We also find that the coverage of shallow clouds increases in all of the cases by 0.2-0.4 from clean to polluted, smoky, or dusty conditions. Covariability analysis with meteorological parameters associates most of this change to aerosol, for each of the four regions and 3 months studied. In our opinion, there is low probability that the net aerosol effect can be explained by coincidental, unresolved, changes in meteorological conditions that also accumulate aerosol, or errors in the data, although further in situ measurements and model developments are needed to fully understand the processes. The radiative effect at the top of the atmosphere incurred by the aerosol effect on the shallow clouds and solar radiation is -11 +/- 3 W/m2 for the 3 months studied; 2/3 of it is due to the aerosol-induced cloud changes, and 1/3 is due to aerosol direct radiative effect.     

INAUGURAL ARTICLE by a Recently Elected Academy Member:Paleoindian large mammal hunters on the plains of North America

From ≈11,200 to 8,000 years ago, the Great Plains of North America were populated by small Paleoindian hunting groups with well developed weaponry and the expertise to successfully hunt large mammals, especially mammoths and bison. Mammoths became extinct on the Plains by 11,000 years ago, and, although paleoecological conditions were worsening, their demise may have been hastened by human predation. After this, the main target of the Plains Paleoindian hunters consisted of subspecies of bison, Bison antiquus and Bison occidentalis. As bison populations gradually diminished, apparently because of worsening ecological conditions, by ≈8,000 years ago, human subsistence was forced into a greater dependence on small animal and plant foods. Human paleoecology studies of the Paleoindian time period rely heavily on multidisciplinary efforts. Geomorphologists, botanists, soil scientists, palynologists, biologists, and other specialists aid archaeologists in data recovery and analysis, although, with few exceptions, their contributions are derived from the fringes rather than the mainstream of their disciplines.     

The influence of latent viral infection on rate of cognitive decline over 4 years

OBJECTIVES: To examine whether cytomegalovirus (CMV) and herpes simplex virus type-1 (HSV-1) are associated with cognitive decline over a 4-year period and to assess whether C-reactive protein (CRP) modifies these relationships. DESIGN: Prospective cohort study over a 4-year period. SETTING: Community-dwelling elderly population. PARTICIPANTS: The sample was a subset (1,204/1,789) of participants in the Sacramento Area Latino Study on Aging (SALSA) aged 60 to 100. MEASUREMENTS: Participants were screened annually over a 4-year period for cognitive function and episodic memory. Cognitive function was assessed using the modified Mini-Mental State Examination, and episodic memory was assessed using a word list-learning test of delayed recall. Baseline serum samples were assayed for levels of immunoglobulin G antibodies to CMV and HSV-1 and for levels of CRP. RESULTS: There was a significantly higher rate of cognitive decline over the 4-year period in subjects with the highest CMV antibody levels at baseline than in individuals with the lowest levels (beta=-0.053, standard error =0.018; P=.003), after controlling for age, sex, education, income, and chronic health conditions. There was no association between HSV-1 antibody levels and cognitive decline. CRP did not modify the relationship between viral antibody levels and cognitive decline. CONCLUSION: This is the first study to show that individuals with higher levels of antibody to CMV experience a more-rapid rate of cognitive decline than those with lower levels. Understanding the mechanisms by which CMV influences cognition may aid development of intervention strategies targeting infection, viral reactivation, and immune response over the life course.     

Management of von Willebrand disease: a survey on current clinical practice from the haemophilia centres of North America.

The optimal treatment of patients with von Willebrand's disease (vWD) remains to be defined. Moreover, it has not been firmly established which, if any, commonly measured parameters of von Willebrand factor (vWF) protein in the plasma are useful in guiding treatment. To better understand what guidelines physicians follow in the management of vWD, we surveyed 194 North American physicians who are members of the Hemophilia Research Society. Ninety-nine per cent of responding physicians depend on factor VIII (FVIII):C, vWF:RCo activity and vWF:AG to diagnose vWD, while only 49% use the bleeding time. The minimal goals of treatment for patients undergoing major surgery/trauma or central nervous system haemorrhage were FVIII:C and vWF:RCo activity greater than 80% while levels of more than 50% for minor surgery and dental extractions were considered adequate. Treatment of vWD was based on the type of vWD with type 1 patients being treated most often with desmopressin acetate (DDAVP) alone, types 2A and 2B patients with a combination of DDAVP and a vWF-containing FVIII product, type 3 patients with vWF-containing concentrate. Viral infections, including human immunodeficiency virus, hepatitis A, B and C viruses, and parvovirus have been seen in vWD and the efficacy of viral attenuation processes is a major criterion for the selection of treatment by physicians. Based on this survey, prospective studies need to be designed to address the clinical efficacy, safety and predictive value of laboratory monitoring of patients with vWD.     

Prevalent influence of systolic over pulse pressure on left ventricular mass in essential hypertension.

BACKGROUND: Elevated pulse pressure, an index of increased large artery stiffness, has been associated with increased left ventricular mass. It is unknown whether this relation is independent or mediated by other blood pressure components. METHODS AND RESULTS: We examined data in 2545 untreated hypertensive subjects (45% women) who underwent echocardiography and 24-h ambulatory blood pressure monitoring. Left ventricular mass increased with all blood pressure components and all associations were closer with ambulatory than with office blood pressure. In a multiple regression analysis, after adjustment for the significant association with age, gender, body weight and duration of hypertension, the proportion of variability of left ventricular mass explained by systolic blood pressure was greater than that explained by other blood pressure components. When different blood pressure components were forced into the same model, the same degree of left ventricular mass variability was accounted for by models including 24-h systolic blood pressure alone, or 24-h mean blood pressure plus 24-h pulse pressure, or 24-h diastolic blood pressure plus 24-h pulse pressure. When 24-h systolic blood pressure and 24-h pulse pressure were forced into the same model, 24-h pulse pressure lost statistical significance. CONCLUSIONS: The association between pulse pressure and left ventricular mass is explained by systolic blood pressure, which is the main pressure determinant of left ventricular mass in essential hypertension.     

The influence of age and sex on nutritional parameters in subjects aged 60 years and over

The nutritional assessment of the elderly shows several interpretative difficulties due to the lack of standard parameters. Moreover chronic age-related diseases can interfere with the physiological nutritional status. Anthropometric (triceps skinfold, arm muscle area, total body muscle mass, fat mass and Body Mass Index (BMI)), biochemical (serum prealbumin, transferrin, ceruloplasmin, total protein and albumin) and immunological (serum lymphocytes) parameters were measured in 583 out-patients aged 60 years or over selected on the basis of clinical and biochemical criteria and with BMI /= 75) for each sex. The F-test analysis for all anthropometric parameters except BMI showed significant differences with respect to age (P < 0.05) and sex (P < 0.05). Among biochemical parameters, prealbumin showed a significant difference for age (P < 0.05) and sex (P < 0.05) (males, 30.3 +/- 8.2; females, 29.1 +/- 7.5) while ceruloplasmin showed a significant difference for sex only (P < 0.05) (males, 40.9 +/- 9.3; females, 43.8 +/- 8.2). When the biochemical mean values obtained in this study were compared with those utilized in the daily routine of the hospital central laboratory, ceruloplasmin and prealbumin resulted in significantly higher (P < 0.05) while total protein and albumin were significantly lower values (P < 0.05).     

From the Cover: Quantifying contributions of natural variability and anthropogenic forcings on increased fire weather risk over the western United States

Previous studies have identified a recent increase in wildfire activity in the western United States (WUS). However, the extent to which this trend is due to weather pattern changes dominated by natural variability versus anthropogenic warming has been unclear. Using an ensemble constructed flow analogue approach, we have employed observations to estimate vapor pressure deficit (VPD), the leading meteorological variable that controls wildfires, associated with different atmospheric circulation patterns. Our results show that for the period 1979 to 2020, variation in the atmospheric circulation explains, on average, only 32% of the observed VPD trend of 0.48 ± 0.25 hPa/decade (95% CI) over the WUS during the warm season (May to September). The remaining 68% of the upward VPD trend is likely due to anthropogenic warming. The ensemble simulations of climate models participating in the sixth phase of the Coupled Model Intercomparison Project suggest that anthropogenic forcing explains an even larger fraction of the observed VPD trend (88%) for the same period and region. These models and observational estimates likely provide a lower and an upper bound on the true impact of anthropogenic warming on the VPD trend over the WUS. During August 2020, when the August Complex “Gigafire” occurred in the WUS, anthropogenic warming likely explains 50% of the unprecedented high VPD anomalies.

The western United States (WUS) is prone to large wildfires, over 90% of which occur in the warm season (May to September) according to the Monitoring Trends in Burn Severity (MTBS) database (1). The year 2020 was a record-breaking fire season in the history of the WUS, especially in the coastal states of California, Oregon, and Washington. Many recent studies of fire behavior in the WUS have indicated warm season increases in the area burned by fires, fire frequency and intensity, and fire season length (2–12). Analysis of the MTBS data in Fig. 1 A and B shows that the average warm season burned area in the WUS during 2001 to 2018 was about 3.35 million acres, nearly double (+98%) that of the previous period of 1984 to 2000 (1.69 million acres). According to the National Interagency Fire Center (NIFC) report, the area burned by wildfire during the 2020 warm season reached 8.8 million acres (13, 14), more than five times the average during 1984 to 2000. This rapid increase in burned area has been observed across most of the WUS except in Wyoming. It has been linked to more extreme fire weather risk, largely due to high vapor pressure deficit (VPD) (10, 15, 16). In the warm season, the number of days per year with high VPD (defined as days with VPD larger than the 90th percentile value of VPD in the climatological period of 1979 to 2010) increased by 94% during 2001 to 2018 relative to 1984 to 2000 (Fig. 1 C and D).Open in a separate windowFig. 1.(A) Annual mean burned areas (10⁵ acres/yr) in the warm season during the period 1984 to 2000. Results for the average of each state are given by shading and with a numerical value. The averaged burned areas over the whole WUS are shown in the Lower Left corners. (B) Same as A but for the period of 2001 to 2018. The percentage changes of burned areas relative to those of the 1984 to 2000 period are shown below the annual mean burned areas. (C) Average days with high VPD (percentile VPD′ over 90% in a year) for the 1984 to 2000 period. (D) Same as B but for the averaged days with high VPD.Many factors and their complex interactions can contribute to increased fire activity. In addition to an increase in VPD or evaporative demand due to warming, fire behavior is also affected by ignition sources (17), forest management (18), tree mortality from bark beetles (19), earlier and reduced springtime snowmelt (7), reduced summer precipitation (20), cloud shading (21), vegetation cover (22), fog frequency (23), live fuel moisture content (23–25), and increase in fire-prone wind patterns (26, 27). Recently, there has been intense interest in the issue of how anthropogenic warming may impact fire behavior. Several studies have used climate model simulations to assess the impact of anthropogenic forcing on increased fire activity in the WUS (10) and in other regions (28, 29). The contribution of anthropogenic climate change is often estimated by the linear trend or long-term low-pass filtered time series of the fire indices. The contribution of atmospheric internal variability is approximated by considering the detrended fire index time series (16, 30) or by comparing historical simulations with realistic anthropogenic forcings to simulations with natural climate forcings only (28).The VPD at synoptic to decadal time scales is closely related to atmospheric circulation patterns (31–34). Winds from hot inland areas and subsidence associated with high surface pressure systems generate hot and dry air, leading to high VPD values. However, few studies have evaluated the influence of natural internal climate variability on multidecadal changes in VPD. This is in part because of the difficulty of partitioning observed temporal variations into internally generated and externally forced components (35). Partitioning these components is more straightforward in climate models. Large initial condition ensembles are particularly useful for this purpose (36). One problem, however, is that many models may inadequately represent regional patterns of internal variability, especially over the WUS (37). Using climate model simulations to estimate the impact of natural variability of the atmospheric circulation on VPD is therefore challenging and subject to large uncertainties.As a consequence, it has been unclear whether the observed change in VPD since 1979 exceeds the VPD change that can be explained by internal variability alone. To address this issue, and to better quantify the relative contributions of internal variability and external forcing (particularly anthropogenic forcing) to the observed increase in fire weather in the WUS, we consider an observation-based flow analogue approach (38–40). This approach characterizes VPD values based on their distribution for a given atmospheric circulation pattern (e.g., geopotential height at 500 hPa, Z500) constructed from a suite of similar circulation patterns during a climatological period (e.g., 1979 to 2010).Different flow analogue approaches have been reported in the literature. Our analysis shows that the choice of approach and the choice of observational dataset (the reanalyses listed in SI Appendix, Table S1) can affect our flow analogue estimates. This is why we introduce an ensemble constructed flow analogue scheme. In this approach, multiple analogue schemes are constructed. Their interquartile range (IQR) is used to account for uncertainties in analogue VPD estimates arising from the choice of approach and observational dataset (see Methods). In addition, we also evaluate VPD trends in multimodel ensembles of simulations provided by the sixth phase of the Coupled Model Intercomparison Project (CMIP6; reference SI Appendix, Table S2). Our analysis of CMIP6 simulations yields a model-based estimate of the forced component of VPD changes, thus providing an independent check on our observational attribution of VPD trends.Historical Trends of Fire Weather Risk.How has the WUS warm season fire weather risk (as represented by VPD) increased since the beginning of the satellite era in 1979? Previous studies have already shown an increase of VPD over a large area of the United States (15, 16, 32). For example, Abatzoglou and Williams (10) estimated the VPD trend over 1979 to 2015 to be 1.73 σ per 37 y (0.47 σ/decade). Here, we extend the analysis period from 2015 to 2020 using gridded surface meteorological (gridMET) (41) observations and show the linear trend in the time series of warm season mean VPD anomaly for the WUS (VPD′; Methods). Fig. 2A indicates that the warm season mean VPD′ over the WUS has increased significantly (P < 0.01) by 0.48 ± 0.25 hPa/decade (95% CI). After normalizing the trend by the SD (σ = 0.93 hPa) of the detrended VPD (see Methods) during the climatological period of 1979 to 2010, it is equivalent to 0.52 ± 0.27 σ/decade. This trend is close to the previously estimated VPD trend during 1979 to 2015 (10). The trend of increasing VPD is significant across most of the WUS, except for the northeastern WUS and part of Washington state (Fig. 2D). Further analysis reveals that these VPD trends are generally robust to different choices of method used for estimating the slope of a regression line (SI Appendix, Table S3).Open in a separate windowFig. 2.(A) Average time series of VPD′ from the gridMET dataset (solid line) and burned areas from the MTBS dataset (bars) for all warm season days. The VPD′ trend is the slope of the regressed line (dashed line) of the time series for all available years (1979 to 2020). The VPD′ trend for the shorter period 1984 to 2018 shows a similar result (SI Appendix, Fig. S1). (B and C) Same as A but for time series of e_s and e_a. (D–F) Trend map of these anomalies for the WUS (all warm season days). The absence of hatching denotes regions where the trends are significant at the P < 0.05 level.Fig. 2A also shows that the burned area in the warm season generally follows both the VPD trend and the variations in VPD on interannual to decadal time scales. The correlation coefficient between the burned area and VPD′ time series is 0.73 (P < 0.01). This indicates that VPD is the leading climatic control on the burned area over the WUS. Strong functional relationships between VPD and the burned area have been found in United States and other regions of the world (10, 15, 16, 28, 42). VPD′ associated with large fire events, defined by VPD′ averaged within the areas and during the days of the large fires, are systematically higher than those of all warm season days by about 3 hPa on average (SI Appendix, Fig. S1). The former shows a similar increase trend to the latter.To quantify the contributions of surface warming and drying to the VPD trend over 1979 to 2020, we evaluate the time series of saturated vapor pressure (e_s; Fig. 2B) and actual vapor pressure (e_a; Fig. 2C) of the surface air. Fig. 2B shows a significant (P < 0.01) trend of increasing e_s at a rate of 0.40 ± 0.20 hPa/decade (0.50 ± 0.25 σ/decade). In contrast, e_a decreases but does not show a significant negative trend (P = 0.12; Fig. 2C). Overall, the increase in e_s explains 82% of the total VPD trend, indicating that the increase in VPD over the WUS is largely due to warming (increase of e_s; Fig. 2 B and E). This is generally consistent with the findings of previous studies (10). The spatial distributions of the e_s and VPD trends are very similar (Fig. 2 D and E); the drying effect represented by the decrease of e_a accounts for 18% of the trend (Fig. 2C) and is only significant over parts of California, Nevada, and the Southwest (Fig. 2F).Contribution of Atmospheric Circulation Changes and Anthropogenic Warming to Increasing Fire Weather Risk. The relationship between hot and dry conditions and large-scale atmospheric circulation is well known. For example, Crimmins (31) found that 80% of the extreme fire weather days during late spring to early summer in the US Southwest were linked to the southwesterlies and anomalous high pressure systems over that region. While such a general characterization captures the averaged anomalous atmospheric circulation pattern associated with high VPD (SI Appendix, Fig. S2), there is a significant variation in the location, shape, and strength of the anomalous high associated with high VPD in different states of the WUS (SI Appendix, Fig. S3).To quantify the contribution of the atmospheric circulation changes to the observed changes of VPD′, we apply an ensemble constructed flow analogue method modified from previous flow analogue or dynamical adjustment approaches (38, 43). For simplicity, this method is referred to as the analogue method, and the estimated VPD′ associated with atmospheric circulation is hereafter referred to as the analogue VPD′. Full details are provided in the Methods section. We then determine the fraction of the observed increase in VPD in recent decades can be explained by a more frequent occurrence of circulation patterns that favor high VPD, that is, by the analogue VPD′. The underlying assumption here is that any change in the frequency of a “high VPD” atmospheric circulation pattern is due to internal variability alone. If the VPD associated with a specific circulation pattern is systematically higher in recent decades than in the past, then such systematic increases in VPD are likely due to anthropogenic warming and associated thermodynamic feedbacks, particularly if they are consistent with the VPD changes simulated by global climate models in response to anthropogenic forcing.Our results show that the daily analogue VPD′ explains a large fraction of the total variance of the observed VPD′ averaged over the WUS for all warm season days during 1979 to 2020 (R² = 77%), indicating that the analogue method successfully captures the influence of synoptic variations in circulation patterns on VPD.Fig. 3A shows the time series of the observed VPD′ compared to that of the VPD′ expected from the atmospheric circulation (i.e., the analogue VPD′) during the warm season of 2020. The 2020 warm season started with relatively mild weather conditions from May to early July. The observed VPD′ closely matches the analogue VPD′ during this period, suggesting that the observed VPD′ was mainly influenced by the variation of the atmospheric circulation. After early July, however, the observed VPD′ is higher than the analogue VPD′. This difference between observed VPD′ and analogue VPD′ is especially pronounced for the two extreme VPD′ spikes during the August Complex “Gigafire” (mid-August) and the California Creek fire (early September). These two fires were ranked No. 1 and No. 5 in California wildfire history at the time of writing (44). For August 2020 (Fig. 3B), the probability density function (PDF) of the observed VPD′ showed a strong shift toward high VPD′ relative to both its climatology and analogue VPD′. The mean value of the observed VPD′ in August 2020 was 4.9 hPa (∼2.1 σ) higher than that of the August climatology; the mean analogue VPD′ in August 2020 exceeded the climatological mean of the observed VPD′ by 2.3 hPa (∼1.0 σ). We conclude from this that the strong anomalous circulation condition can only explain about half of the exceptionally high VPD′ in August 2020.Open in a separate windowFig. 3.(A) VPD′ time series in 2020 warm season over the WUS from both observations (black line) and analogues (blue line for mean analogue; shading for IQR). Starting days of the August Complex fire and California Creek fire are labeled. Dashed horizontal lines are the warm season mean values. (B) PDF of August VPD′ for the observations from the climatological period of 1979 to 2010 (black curve), 2020 observations (red bars, shaded dark gray where they overlap with blue bars), and 2020 analogues (blue bars). The three vertical lines in each box plot represent the 25th, 50th, and 75th percentiles, the dot represents the mean value, and the whiskers extend to two SDs from the mean. (C) Map of Z500 (contours) and its standardized anomalies relative to 1979 to 2010 climatology (shading) averaged over four reanalysis datasets (the fifth generation of the European Centre for Medium-Range Weather Forecasts [ECMWF] atmospheric reanalysis [ERA5], the Modern Era Retrospective analysis for Research and Applications version 2 [MERRA-2], the National Centers for Environmental Prediction [NCEP] Climate Forecast System Reanalysis [CFSR], and the Japanese 55-y Reanalysis [JRA55]) on August 16, 2020, the start date of the August Complex fire. (D) Percentile VPD map on the same date as C, overlaid with the 95, 99, and 100% contours. (E) Same as D but for constructed analogue VPD map.On August 16, 2020 when the August Complex fire started, an extensive and strong anomalous high was centered over the Southwest and dominated the whole WUS (Fig. 3C); most values of VPD′ across the WUS ranked in the 99th or even the 100th percentile—that is, they were equal to or exceeded maximum VPD′ values observed in the same region (within a 31-d period centered on August 16) during the climatological period of 1979 to 2010 (Fig. 3D). While the analogue VPD′ on this day also show very high VPD conditions over the whole WUS, they were less extreme than the observed VPD′ (Fig. 3E; note that there are no contours of the 99th and 100th percentiles). In fact, averaged over the WUS, the analogue VPD′ could only account for ∼68% of the observed VPD′ for the August 16 event and even less (∼48%) for the September 4 event (Fig. 3A). Thus, the observed high VPD′ values during the 2020 warm fire season significantly exceeded VPD′ values that can be explained by the atmospheric circulation pattern.On the interannual time scale, Fig. 4A shows that the analogue and observed warm season mean VPD′ time series display very similar variations (R² = 68%). Since 2000, however, the observed VPD′ was systematically higher than the analogue VPD′. The trend of analogue warm season mean VPD′ is 0.15 ± 0.15 hPa/decade, explaining 32% of the observed VPD trend (0.48 ± 0.25 hPa/decade); the IQR of these trends for all 180 different analogue schemes (see Methods) is 0.13 to 0.20 hPa/decade, explaining 27 to 42% of the observed VPD trend. The residual VPD trend (observed minus analogue) is 0.33 ± 0.16 hPa/decade, explaining 68% of the observed trend; the IQR of all 180 residual trends is 0.30 to 0.36 hPa/decade, explaining 62 to 75% of the observed VPD trend.Open in a separate windowFig. 4.(A) Warm season mean VPD′ time series over the WUS from observations (black line), analogues (blue line; shading represents IQR for VPD′ from the 180 analogue schemes described in Methods), and residuals (observations minus analogue, red line, shading represents IQR). (B) PDF of the residual VPD anomalies for the periods 1979 to 2000 and 2001 to 2020, respectively, and box plots (see Fig. 3B for explanation). (C) Analogue VPD′ trend (1979 to 2020) in each state. The value shown by bold black font within each state shows the VPD trend of that state. The value shown by bold black font in the Lower Left corner is the VPD trend averaged over the entire WUS. One, two, or three asterisk(s) next to these trend numbers denotes trend significance at P < 0.1, 0.05, and 0.01, respectively. Numbers inside brackets are IQR of the trends calculated from 180 individual analogue schemes. (D) Same as C but for residual VPD′ trend (observations minus analogue). (E) Percentage of the analogue VPD trend relative to the observed VPD trend (IQR in brackets). Montana, Wyoming, and Washington have nonsignificant observed VPD trends at the P < 0.05 level (SI Appendix, Table S4), and the corresponding regions are therefore hatched in C–E.Fig. 4B shows the PDF of residual VPD′. The PDF curve is basically symmetric about zero during the first two decades of our analysis period (1979 to 2000), suggesting a dominant influence of random variability of the atmospheric circulation on VPD. During the recent two decades (2001 to 2020), the mean residual VPD′ shifted to the positive side by 1.00 hPa (0.54 σ) relative to the period of 1979 to 2000. This is primarily due to a shift of +1.37 hPa (0.53 σ) in the mean observed VPD′; the shift in the mean of the analogue VPD′ (+0.38 hPa or 0.14 σ) is less than a third of that observed (SI Appendix, Fig. S4).Fig. 4 C and D show the analogue and residual VPD′ trends averaged over each state in the WUS. Similar to the result for the entire WUS, most states (especially those with significant observed VPD′ trends; Fig. 1 and SI Appendix, Table S4) have an analogue trend that is considerably smaller than the residual trend. The trend ratio (analogue to observed) in Fig. 4E suggests that for the eight states with significantly positive trends for the observed VPD′, the circulation contribution ranged from 24% (Idaho) to 39% (Utah), leaving 76 to 61%, respectively, of the residual trend unexplained. Overall, these results indicate the analogue VPD′ trend associated with circulation changes can only explain about one-third of the observed VPD trend across most of the WUS.This residual VPD′ mainly represents the thermodynamically contributed VPD′ after removing the dynamically controlled analogue VPD′. It is contributed by both thermodynamic feedbacks to the natural circulation changes, such as land surface feedbacks, and warming due to anthropogenic forcing. The relatively small residual VPD′ values prior to 2000 are presumably dominated by the thermodynamic feedbacks, whereas the systematic increase of the residual VPD′ afterward are likely contributed by anthropogenic forcings and the associated thermodynamic feedbacks.In addition to atmospheric circulation changes, could reduced cloudiness and vegetation cover contribute to the increases of VPD? Such changes would enhance solar radiation and evaporative demand, resulting in warmer and drier conditions, and thus higher in VPD (21, 45). We note, however, that an increase in downward surface solar radiation is mostly confined in the coastal states (California, Oregon, and Washington) and to northern Idaho and southwestern Arizona, whereas the decreases of Normalized Difference Vegetation Index (NDVI) are confined to Southern California and southwestern Arizona (SI Appendix, Fig. S5 C and D). These changes cannot explain a widespread increase of VPD′ and residual VPD′ across the entire WUS (SI Appendix, Fig. S5 A and B). Only southwestern Arizona exhibits both an increase in downward solar radiation and reduced NDVI; in this particular region, therefore, trends in both factors could contribute to the strong increase in observed VPD′.The previous discussion and figures focused solely on the observations. We attempted to partition observed VPD trends into a component associated with circulation changes and a residual component likely to be dominated by the response of VPD to external forcing. In the following, we consider VPD trends in the CMIP6 models. Fig. 5A compares the VPD′ trend between models and observations over the same 1979 to 2020 period. Simulations with combined natural and anthropogenic forcings (which comprise historical simulations up to 2014 and the Shared Socioeconomic Pathway 5 - Representative Concentration Pathway 8.5 [SSP5-8.5] scenario integrations thereafter) show a significant (P < 0.01) warm season mean VPD trend of 0.48 ± 0.05 hPa/decade (Fig. 5 A and C), which is very similar to the observed VPD trend over the WUS.Open in a separate windowFig. 5.(A) Warm season mean VPD′ time series averaged over the WUS region and the trends during 1979 to 2020 calculated with climate models and observations (daily gridMET and monthly PRISM). The orange and blue line represents observed and residual VPD′ from gridMET, respectively; the yellow line represents observation from PRISM (a longer term monthly observational dataset covering 1895 to present that gridMET is based on); the black and cyan solid lines represent CMIP6-ALL and CMIP6-NAT simulations, and the thin gray and cyan lines are for all ensemble members from CMIP6-ALL and CMIP6-NAT, respectively. For the purposes of visual display, the VPD′ lines for ALL, NAT, and PRISM are forced to have the same mean value during 1979 to 2010 as gridMET. The VPD trends, 95% CI, and IQR (only for residual VPD′) labeled in the Upper Left corner are calculated for the 1979 to 2020 period. (B) PDF of VPD trend for PRISM observations and CMIP6-NAT; the vertical lines, dots, and whiskers for the box plots are defined as in Fig. 3B; VPD trend is calculated for every consecutive 42-y period within the periods listed above. (C) Same as B but for CMIP6-ALL. When calculating the ensemble-mean VPD trends and their PDFs in the CMIP6 simulations, VPD trend is first calculated for each ensemble member of each model, and weights are given to all the members in a way that all members from the same model are equally weighted and all models are also equally weighted.In contrast, historical runs with only natural solar and volcanic external forcings show a very small mean VPD trend of 0.06 ± 0.05 hPa/decade (P < 0.05), with a middle 99% range from −0.37 to 0.46 hPa/decade (Fig. 5B). The observed VPD trend exceeds over 99% of the trend values that can be explained by natural climate forcings (solar and volcanic) and internal variability. Our natural variability estimates are based on a large number (∼15,000) of 42-y trend samples from 14 different climate models. The observed VPD trend is very similar to the mean of the modeled trends in the CMIP6 simulations with combined natural and anthropogenic external forcings. Over 1979 to 2020, the PDF of model VPD trends under “all forcings” spans the range from 0 to 1 hPa/decade (Fig. 5C). This range arises from natural climate variability, from model differences in historical external forcing, and from model differences in the response to forcing.The difference between the multimodel ensemble with combined anthropogenic and natural forcings and the multimodel ensemble with natural forcing only—which we refer to as “ALL” and “NAT” hereafter—is widely used for estimating the anthropogenically forced component of climate change (28, 46–49). Here, differencing the means of the ALL and NAT multimodel ensembles yields an anthropogenically forced VPD trend of 0.42 hPa/decade, equivalent to 88% of the observed VPD trend over the period of 1979 to 2020. This is roughly 30% larger than our observationally derived residual VPD′ trend of 0.33 ± 0.16 hPa/decade.