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It is well known that global warming increases the atmospheric water vapor content, which results in substantial changes in the hydrological cycle. Using five observational data sets, the results show that an increasing trend of near‐surface water vapor pressure (AVP) over land and ocean was significant from 1975 to 1998, while such an increasing trend in AVP subsequently weakened from 1999 to 2019. This phenomenon is associated with decreased oceanic evaporation and land surface evapotranspiration in response to recent climate variations. One consequence of such a phenomenon is a large increase in near‐surface vapor pressure deficit (VPD), which in turn increases atmospheric demand for water vapor and thus aridity and drought over land. This result emphasizes the importance of water vapor change under global warming. Plain Language Summary As one of the key components of the atmosphere, water vapor plays a crucial role in regulating the processes in the climate system. It has long been established that water vapor generally increases with global rising mean air temperature as dictated by the Clausius‐Clapeyron theorem if relative humidity changes little. Here, we use five observational data sets to study the trend changes in near‐surface actual water vapor pressure (AVP) from 1975 to 2019. Our results show an increasing trend of global land and ocean AVP from 1975 to 1998, but the increasing trend has weakened since the late 1990s. This phenomenon in water vapor is associated with decreased oceanic evaporation and land surface evapotranspiration. This phenomenon further enhances the atmospheric vapor pressure deficit (VPD), which dominates the water and carbon cycling in the terrestrial ecosystem by regulating vegetation stomatal conductance. Key Points The increasing trend of near‐surface actual water vapor pressure (AVP) over land and ocean has weakened since the end of the 1990s The weakened water vapor pressure increase is associated with decreased oceanic evaporation and land surface evapotranspiration One consequence of the weakened water vapor pressure increase is a large increase in near‐surface vapor pressure deficit

Recent decades have been characterized by increasing temperatures worldwide, resulting in an exponential climb in vapor pressure deficit (VPD). VPD has been identified as an increasingly important driver of plant functioning in terrestrial biomes and has been established as a major contributor in recent drought-induced plant mortality independent of other drivers associated with climate change. Despite this, few studies have isolated the physiological response of plant functioning to high VPD, thus limiting our understanding and ability to predict future impacts on terrestrial ecosystems. An abundance of evidence suggests that stomatal conductance declines under high VPD and transpiration increases in most species up until a given VPD threshold, leading to a cascade of subsequent impacts including reduced photosynthesis and growth, and higher risks of carbon starvation and hydraulic failure. Incorporation of photosynthetic and hydraulic traits in ‘next-generation’ land-surface models has the greatest potential for improved prediction of VPD responses at the plant- and global-scale, and will yield more mechanistic simulations of plant responses to a changing climate. By providing a fully integrated framework and evaluation of the impacts of high VPD on plant function, improvements in forecasting and long-term projections of climate impacts can be made.

This updated version of the popular Handbook further explains all aspects of physical vapor deposition (PVD) process technology from the characterizing and preparing the substrate material, through deposition processing and film characterization, to post-deposition processing. The emphasis of the new edition remains on the aspects of the process flow that are critical to economical deposition of films that can meet the required performance specifications, with additional information to support the original material. The book covers subjects seldom treated in the literature: substrate characterization, adhesion, cleaning and the processing. The book also covers the widely discussed subjects of vacuum technology and the fundamentals of individual deposition processes. However, the author uniquely relates these topics to the practical issues that arise in PVD processing, such as contamination control and film growth effects, which are also rarely discussed in the literature. In bringing these subjects together in one book, the reader can understand the interrelationship between various aspects of the film deposition processing and the resulting film properties. The book is intended to be both an introduction for those who are new to the field and a valuable resource to those already in the field.

The climatological characteristics of water vapor transport over the Tibetan Plateau (TP) were investigated in this study by using the ERA-interim and JRA55 monthly reanalysis dataset. The trends of water vapor budget and water vapor sources during the past 40 years were also revealed. The analyses show that the TP is a water vapor convergence area, where the convergence was enhanced from 1979 to 2018. In addition, the convergence is much stronger in JJA, with a linear trend that is twice the annual average trend. The climatological water vapor sources over the TP were identified mainly at the southern and western boundaries, with the vapor sources at the southern boundaries originating from the Arabian Sea and Bay of Bengal and the vapor sources at the western boundary being transported by mid-latitude westerlies. The TP is a moisture sink at a climatological mean, with an annual average net water vapor flux of 11.86 × 106kg ∙ s−1. Water vapor transport is much stronger in JJA than in other times of the year, and the net water vapor flux is 29.60 × 106kg ∙ s−1. The net water vapor flux in the TP increased with a linear trend of 0.12×106kg ∙ s−1 ∙ year−1 (α = 0.01), while the increase in the flux was more significant in JJA than in other times of the year with a linear trend of 0.30 ×106kg ∙ s−1 ∙ year−1 (α = 0.01). Detailed features in the water vapor flux and transport changes across the TP’s four boundaries were explored by simulating backward trajectories with a Lagrangian trajectory model (hybrid single-particle Lagrangian integrated trajectory model, HYSPLIT). In the study period, the water vapor contribution rate of western channel is increased. However, the Southern channel’s water vapor contribution decreased.

Water vapor transport in the dry soil layer (DSL) plays a critical role in water and energy exchange between soil and atmosphere in semi‐arid and arid regions. However, monitoring water vapor transport in extremely dry soils remains challenging. This study directly measured changes in water vapor content and temperature within sand pores during evaporation using fiber Bragg grating relative humidity sensing technology. Results indicated that soil temperature reached a minimum when relative humidity dropped from 100%, confirming that the sensors successfully captured the evaporating front and its dynamic migration within the soil. Water vapor fluxes exhibited a mono‐convex temporal pattern, peaking at the evaporating front. Additionally, deeper evaporating fronts migrated more slowly in sands with varying initial water content. Furthermore, the relative humidity distribution within the DSL was found to be depth‐dependent and could be described by a nonlinear function of depth. These findings suggest that our method offers a novel approach for investigating the mechanisms of water vapor transport in dry soils in semi‐arid and arid regions. Plain Language Summary In semi‐arid and arid regions, shallow soil typically has very low water content, with the evaporating front remaining within the soil most of the year. At this subsurface evaporating front, liquid water converts to vapor, which moves through the dry soil layer (DSL) into the atmosphere. Direct measurement of water vapor transport within DSLs has been challenging with current technology. In this study, optical fiber sensing technology directly measured changes in relative humidity, temperature, water vapor density, and water vapor flux during evaporation in four sand samples with very low water content. As relative humidity decreased from 100%, the soil temperature reached its minimum, confirming the sensor accurately captured the evaporating front. The water vapor flux exhibited a characteristic pattern, rising from zero, peaking at the evaporating front, and then declining back to zero. Moreover, in sand with varying initial moisture content, a deeper evaporating front was associated with a slower downward migration rate. The relative humidity distribution within the DSL was depth‐dependent and could be characterized by a nonlinear function of depth. This study introduces a novel approach to investigating the mechanisms of water vapor transport in dry soils in semi‐arid and arid regions. Key Points The drop in soil relative humidity from 100% during evaporation precisely aligns with the moment of minimum soil temperature Sensors effectively captured the evaporating front and tracked its dynamic migration within the soil Water vapor fluxes exhibited a mono‐convex temporal pattern, peaking at the evaporating front

Compound extremes such as cooccurring soil drought (low soil moisture) and atmospheric aridity (high vapor pressure deficit) can be disastrous for natural and societal systems. Soil drought and atmospheric aridity are 2 main physiological stressors driving widespread vegetation mortality and reduced terrestrial carbon uptake. Here, we empirically demonstrate that strong negative coupling between soil moisture and vapor pressure deficit occurs globally, indicating high probability of cooccurring soil drought and atmospheric aridity. Using the Global Land Atmosphere Coupling Experiment (GLACE)-CMIP5 experiment, we further show that concurrent soil drought and atmospheric aridity are greatly exacerbated by land–atmosphere feedbacks. The feedback of soil drought on the atmosphere is largely responsible for enabling atmospheric aridity extremes. In addition, the soil moisture–precipitation feedback acts to amplify precipitation and soil moisture deficits in most regions. CMIP5 models further show that the frequency of concurrent soil drought and atmospheric aridity enhanced by land–atmosphere feedbacks is projected to increase in the 21st century. Importantly, land–atmosphere feedbacks will greatly increase the intensity of both soil drought and atmospheric aridity beyond that expected from changes in mean climate alone.

Global warming intensifies atmospheric water vapor transport between ocean and land, which increases the likelihood of extreme precipitation and floods. However, accurate estimations of water vapor exchange between ocean and land are difficult due to the lack of available data and effective methods. This study developed a novel eight-directionvector decomposition algorithm for calculating water vapor flux between ocean and land based on the ERA5 dataset, and the results showed that global water vapor exchange between ocean and land had significantly increased in the past 40 years, except for Antarctica. During 1980–2018, the average annual net water vapor inflow from ocean to land (Qnet) was 44.68 × 1015 kg yr−1, and Qnet increased at a rate of 1.48 × 1015 kg yr−1 decade−1. The intensified atmospheric water vapor exchange between ocean and land was directly caused by the increase of atmospheric water vapor content, which largely depended on the rising air temperature, and it was found that water vapor flux between ocean and land increased by over 8% K−1 with the increasing air temperature at the global average. This study also identified El Niño–Southern Oscillation (ENSO) as an important contributor to the global ocean–land water vapor exchange anomalies. A strong El Niño event (MEI = 1) can result in a 1.36 × 1015 kg yr−1 (3.03%) decrease in Qnet, and a strong La Niña event (MEI=-1) can increase Qnet by 1.38 × 1015 kg yr−1 (3.09%). The eight-direction-vector decomposition algorithm was effective in ocean–land water vapor flux estimations at different spatial and temporal scales, which could provide great insights into the mechanisms of extreme precipitation events.

Tree species growing along the forest–grassland ecotone are near the moisture limit of their range. Small increases in temperature can increase vapor pressure deficit (VPD) which may increase tree water use and potentially hasten mortality during severe drought. We tested a 40% increase in VPD due to an increase in growing temperature from 30 to 33°C (constant dewpoint 21°C) on seedlings of 10 tree species common to the forest–grassland ecotone in the southern Great Plains, USA. Measurement at 33 vs 30°C during reciprocal leaf gas exchange measurements, that is, measurement of all seedlings at both growing temperatures, increased transpiration for seedlings grown at 30°C by 40% and 20% for seedlings grown at 33°C. Higher initial transpiration of seedlings in the 33°C growing temperature treatment resulted in more negative xylem water potentials and fewer days until transpiration decreased after watering was withheld. The seedlings grown at 33°C died 13% (average 2 d) sooner than seedlings grown at 30°C during terminal drought. If temperature and severity of droughts increase in the future, the forest–grassland ecotone could shift because low seedling survival rate may not sufficiently support forest regeneration and migration.

Vapour pressure deficit (VPD), the difference between the saturation and actual air vapour pressures, indicates the level of atmospheric drought and evaporative pressure on plants. VPD increases during climate change due to changes in air temperature and relative humidity. Rising VPD induces stomatal closure to counteract the VPD-mediated evaporative water loss from plants. There are important gaps in our understanding of the molecular VPD-sensing and signalling mechanisms in stomatal guard cells. Here, we discuss recent advances, research directions and open questions with respect to the three components that participate in VPD-induced stomatal closure in Arabidopsis, including: (1) abscisic acid (ABA)-dependent and (2) ABA-independent regulation of the protein kinase OPEN STOMATA 1 (OST1), and (3) the passive hydraulic stomatal response. In the ABA-dependent component, two models are proposed: ABA may be rapidly synthesised or its basal levels may be involved in the stomatal VPD response. Further studies on stomatal VPD signalling should clarify: (1) whether OST1 activation above basal activity is needed for VPD responses, (2) which components are involved in ABA-independent regulation of OST1, (3) the role of other potential OST1 targets in VPD signalling, and (4) to which extent OST1 contributes to stomatal VPD sensitivity in other plant species.