Changes In Aridity Research Articles

Report on mandibular remains of Notiomastodon platensis (Mammalia, Proboscidea) and review of its fossil record in the paleoecological context of Valle del Cauca, Colombia

The Proboscidea were very prominent in South American ecosystems during the Pleistocene and part of the Holocene. Specifically, in Valle del Cauca (Colombia), fossils of these large mammals have been found, reflecting an abundant presence in the region. In this work, a mandibular fragment with a complete last molar (m3) is reported, found near the bed of the Cauca River, in the Juanchito municipality of Santiago de Cali. According to the morphological features of the specimen, it is proposed that the remains belong to the proboscidean Notiomastodon platensis. This study emphasizes the large geographical distribution of this proboscidean in South America, including the Valle del Cauca, and provides new information on its presence in Colombia. In a paleoecological context, this work supports a generalist condition based on the use of food resources associated with the tropical dry forest during the processes of glacial and interglacial fluctuation that this type of ecosystem experienced during the Pleistocene epoch and that led to various changes in habitat aridity, fragmentation, and heterogeneity. Keywords: Notiomastodon, Quaternary, paleoecology, megafauna, South America.

Linking soil fungi to bacterial community assembly in arid ecosystems.

Revealing the roles of biotic factors in driving community assembly, which is crucial for the understanding of biodiversity and ecosystem functions, is a fundamental but infrequently investigated subject in microbial ecology. Here, combining a cross-biome observational study with an experimental microcosm study, we provided evidence to reveal the major roles of biotic factors (i.e., soil fungi and cross-kingdom species associations) in determining soil bacterial biogeography and community assembly in complex terrestrial ecosystems of the arid regions of northwest China. The results showed that the soil fungal richness mediates the balance of assembly processes of bacterial communities, and stochastic assembly processes decreased with increasing fungal richness. Our results further suggest that the predicted increase in aridity conditions due to climate change will reduce bacterial α-diversity, particularly in desert soils and subsurface layer, and induce more negative species associations. Together, our study represents a significant advance in linking soil fungi to the mechanisms underlying bacterial biogeographic patterns and community assembly in arid ecosystems under climate aridity and land-use change scenarios.

Influence of Energy and Water Cycle Key Parameters on Drought in Mongolian Plateau during 1979–2020

Drought in the Mongolian Plateau (MP) has gradually intensified in recent decades. The energy and water cycles are key factors affecting drought. However, there are few quantitative studies on the mechanism of aridity change in this region. This study uses the ERA5, Moderate Resolution Imaging Spectroradiometer (MODIS) and Himawari 8 datasets and investigated the mechanism of drought change over the MP. The aridity index (the ratio of potential evaporation and total precipitation) is employed to detect drought changes. The results showed that the annual mean of aridity index increased by 0.73% per year (increased significantly since 1999) during the period 1979–2020. Moreover, the drought was most severe in the January to April of 2016–2020, mainly concentrated in the central and western parts of the MP. The potential evaporation increased (0.72% per year) and total precipitation decreased (0.16% per year) from 1979 to 2020. However, the surface temperature continued increasing from August to December in the period 2016–2020 (1.67% per year). This may result in an increase in potential evaporation and a decrease in volumetric soil water from August to December last year. The decrease of volumetric soil water resulted in the decrease of total cloud cover (0.25% per year) and total precipitation from January to April. The surface net radiation (increased by 0.42% per year) and the potential evaporation increased, which may aggravate the drought from January to April. The evaporation paradox is studied over the MP. The results show that the variation in evaporation is consistent with that of total precipitation, and the surface temperature will promote an increase in evaporation and potential evaporation. This study reveals that global warming, desertification and increased surface net radiation contribute to the increase in potential evaporation and reduced volumetric soil water, which together contribute to drought.

Spatiotemporal changes in global aridity in terms of multiple aridity indices: An assessment based on the CRU data

Aridity is a complex environmental hazard, posing potential risks to more than 38% of the world's population in terms of poverty, reduction in crop and livestock production, water scarcity, and loss of biodiversity. Despite its complex nature and the subsequent adverse impacts, limited attention has been given to studying the aridity phenomenon, in terms of multiple aridity indices. The present study assessed spatiotemporal changes in global aridity over the period 1901–2019, using five aridity indices, i.e., De Martonne aridity index (AIDM), Emberger aridity index (AIE), Erinç aridity index (AIM), the Pinna combinative aridity index (AIP), and United Nations Environment Programme (UNEP) aridity index (AIUNEP). The study's findings revealed that the maximum intensity and expansion of global aridity have been observed in the recent decades (1991–2019), with major hotspots of aridity located in the equator and subtropical zones of the world. Except for the AIUNEP index, the rest of the indices displayed an obvious and consistent increase in aridity over Northern Hemisphere. The increase (decrease) in temperature (soil moisture) has intensified the process of potential evapotranspiration (PET) and aridity across the globe. Though the selected indices characterize the aridity patterns and their intensities with high spatial variability; therefore, these indices need to be applied at regional and local scales for more robust results, and better planning, adaptation, and mitigation measures to ensure sustainability in agriculture and water resources. A detailed assessment along with a numerical model experiment is also required for a conclusive quantification of the relative influence of the associated variables in the variability of aridity over different regions of the world.

Asymmetrical Trends of Burned Area Between Eastern and Western Siberia Regulated by Atmospheric Oscillation

AbstractWildfires in Siberian cause organic carbon released from Arctic ecosystems to the atmosphere, which further enhances greenhouse effects. However, the trends of Siberian wildfires and their mechanisms are not well understood. We show that the burned fraction trends were opposite in the eastern and western Siberia. The contrasting changes are explained by vapor pressure deficit and soil moisture inferring atmospheric and ground aridity conditions, respectively. The difference in aridity change in the eastern and western Siberia is regulated by increased positive phase of Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO), which leads to reduced moisture transport to the east but increased transport to the west from the North Atlantic and Arctic Ocean mainly during May–August. The growing trend of positive phase of AO and NAO will continuously lead to the asymmetrical trends of aridity condition, which could enhance wildfire in the eastern Siberia.

Human-caused long-term changes in global aridity

Widespread aridification of the land surface causes substantial environmental challenges and is generally well documented. However, the mechanisms underlying increased aridity remain relatively underexplored. Here, we investigated the anthropogenic and natural factors affecting long-term global aridity changes using multisource observation-based aridity index, factorial simulations from the Coupled Model Intercomparison Project phase 6 (CMIP6), and rigorous detection and attribution (D&A) methods. Our study found that anthropogenic forcings, mainly rising greenhouse gas emissions (GHGE) and aerosols, caused the increased aridification of the globe and each hemisphere with high statistical confidence for 1965–2014; the GHGE contributed to drying trends, whereas the aerosol emissions led to wetting tendencies; moreover, the bias-corrected CMIP6 future aridity index based on the scaling factors from optimal D&A demonstrated greater aridification than the original simulations. These findings highlight the dominant role of human effects on increasing aridification at broad spatial scales, implying future reductions in aridity will rely primarily on the GHGE mitigation.

Lithium isotopic composition of soil pore water: Responses to evapotranspiration

Abstract Lithium isotopes show great potential to trace Earth surface processes due to the large mass discrimination between 6Li and 7Li associated with clay uptake. However, factors controlling the Li isotopic composition (δ7Li) of river water, especially those with a water-bedrock δ7Li difference higher than that of the equilibrium fractionation associated with clay formation (ΔW-C), have not yet been fully resolved. Traditional interpretation involves the Rayleigh distillation, but it unrealistically separates the stage of clay formation from that of silicate dissolution using fractionation factors that are much lower than laboratories can constrain. We propose an in situ mechanism that simulates high δ7Li by evapotranspiration. A model with coupled mineral dissolution and clay precipitation shows that evaporative enrichment of pore-water Li progressively increases the incorporation of Li into clays with light δ7Li, resulting in higher δ7Li values in the residual water than ΔW-C. We also provide evidence from the Chinese Loess Plateau, where an evaporative effect readily explains the observed δ7Li. The influence of evapotranspiration on riverine δ7Li implies that changes in aridity may partly explain the variations of seawater δ7Li. The same principle may also apply to other stable isotopic systems whereby incorporation into secondary precipitates controls the isotopic fractionation.

Recent changes in global dryland temperature and precipitation

AbstractChanges in dryland temperature and precipitation are essential components of dryland climate change. This study applies the use of the monthly reanalysis data product of ERA5, which has a spatial resolution of 0.25°, to investigate the changes in the dryland climate between 1979 and 2018. The result revealed warming and drying of the drylands in recent decades, most notably in the current decade. The general global dryland temperature and precipitation increased and decreased significantly (0.01 significance level) at the rate of 0.032°C·year−1 and 0.074 mm·month−1·year−1, respectively, in the last 40 years. Although precipitation generally decreased over the drylands, summer precipitation increased over southern Africa and Australia as well as northern Africa and south Asia dryland areas. Variation in the warming rate over the dryland zones at different periods was also observed. The semiarid and dry subhumid warmed continually from the first decade (0.02°C·year−1 and 0.017°C·year−1) to the fourth decade (0.077°C·year−1 and 0.065°C·year−1, respectively), while the hyperarid warmed at a slower rate in the last two decades compared to the prior decades. More intense warming was also observed in the areas where precipitation decreased, suggesting the effect of reduced evaporative cooling. These changes have implications on the dryland environment, such as aridity changes, which could affect the sustainable development of the areas through the corresponding impact on the ecosystem.

Projected changes in air temperature, precipitation and aridity in Serbia in the 21st century

AbstractIn this study, the projected changes of the average monthly and annual air temperatures, precipitation amounts and De Martonne aridity index in Serbia in the 21st century are examined. For that purpose, the spatial distribution of air temperature and precipitation amount is statistically modelled using data of 64 climatologic stations and 421 precipitation stations for the period 1976–2005 using multiple linear regression. The De Martonne aridity index is calculated from the spatially distributed data. In order to obtain regional averages and variability, we divide Serbia into three subdomains: the western zone (the Dinarides), eastern mountainous (Carpathian‐Balkan) zone and the terrains with altitudes lower than 500 m a.s.l. (Lowland). As a second step, the members of the COordinated Regional Downscaling EXperiment (CORDEX) for Europe (EURO‐CORDEX) are evaluated in terms of near‐surface air temperature and precipitation for Serbia. By comparing respective values for the period 1976–2005 with future periods 2021–2050 and 2071–2100, we derive the climate change signal for near‐surface air temperature, precipitation and the De Martonne index for both scenarios RCP4.5 and RCP8.5. Hereby, the ensemble averages are calculated as weighted averages using the inverse standardized Euclidean distance by comparing observation and model results for near‐surface air temperature and precipitation as a weight. We find for the period 2071–2050 that most ensemble members predict a statistically significant increase in monthly and annual near‐surface air temperature in both scenarios in all three subdomains, and a statistically significant decrease in the yearly De Martonne index in the RCP8.5 scenario in the Carpathian‐Balkan and Dinarides. For the last decades of the 21st century, our findings indicate much higher temperatures and increased risks of drought for the RCP 8.5 scenario compared to both the observations from 1976 to 2005 and the RCP 4.5 scenario.

Recent evidence for warmer and drier growing seasons in climate sensitive regions of Central America from multiple global datasets

AbstractSmallholder livelihoods throughout Central America are built on rain‐fed agriculture and depend on seasonal variations in temperature and precipitation. Recent climatic shifts in this highly diverse region are not well understood due to sparse observations, and as the skill of global climate products have not been thoroughly evaluated. We examine the performance for several reanalysis and satellite‐based global climate data products (CHIRPS/CHIRTS, ERA5, MERRA‐2, PERSIANN‐CDR) as compared to the observation‐based GPCC precipitation dataset. These datasets are then used to evaluate the magnitude and spatial extent of hydroclimatic shifts and changes in aridity and drought over the last four decades. We focus on water‐limited regions that are important for rain‐fed agriculture and particularly vulnerable to further drying, and newly delineate those regions for Central America and Mexico by adapting prior definitions of the Central American Dry Corridor. Our results indicate that the CHIRPS dataset exhibits the greatest skill for the study area. A general warming of 0.2–0.8°C·decade−1 was found across the region, particularly for spring and winter, while widespread drying was indicated by several measures for the summer growing season. Changes in annual precipitation have been inconsistent, but show declines of 20–25% in eastern Honduras/Nicaragua and in several parts of Mexico. Some regions most vulnerable to drying have been subject to statistically significant trends towards summer drying, increases in drought and aridity driven by precipitation declines, and/or a lengthening of the winter dry season, highlighting areas where climate adaptation measures may be most urgent.

Climatic displacement exacerbates the negative impact of drought on plant performance and associated arthropod abundance.

Climate change is acting on species and modifying communities and ecosystems through changes not only with respect to mean abiotic conditions, but also through increases in the frequency and severity of extreme events. Changes in mean aridity associated with climate change can generate ecotype by environment mismatch (i.e., climatic displacement). At the same time, variability around these shifting means is predicted to increase, resulting in more extreme droughts. We characterized the effects of two axes of climate change, climatic displacement and drought, on the shrub Artemisia californica and its arthropods. We established common gardens of plants sourced along an aridity gradient (3.5-fold variation in mean annual precipitation) in an arid region of the species distribution, thus generating a gradient of climatic displacement (sustained increase in aridity) as predicted with climate change. We surveyed plants and arthropods over eight years where precipitation varied sixfold, including both extreme drought and relatively mesic conditions. These two axes of climate change interacted to influence plant performance, such that climatically displaced populations grew slowly regardless of drought and suffered substantial mortality during drought years. Conversely, local populations grew quickly, increased growth during wet years, and had low mortality regardless of drought. Effects on plant annual arthropod yield were negative and additive, with drought effects exceeding that of climatic displacement by 24%. However, for plant lifetime arthropod yield, incorporating effects on both plant growth and survival, climatic displacement exacerbated the negative effects of drought. Collectively these results demonstrate how climatic displacement (through increasing aridity stress) strengthens the negative effects of drought on plants and, indirectly, on arthropods, suggesting the possibility of climate-mediated trophic collapse.

Pervasive changes in stream intermittency across the United States

Non-perennial streams are widespread, critical to ecosystems and society, and the subject of ongoing policy debate. Prior large-scale research on stream intermittency has been based on long-term averages, generally using annually aggregated data to characterize a highly variable process. As a result, it is not well understood if, how, or why the hydrology of non-perennial streams is changing. Here, we investigate trends and drivers of three intermittency signatures that describe the duration, timing, and dry-down period of stream intermittency across the continental United States (CONUS). Half of gages exhibited a significant trend through time in at least one of the three intermittency signatures, and changes in no-flow duration were most pervasive (41% of gages). Changes in intermittency were substantial for many streams, and 7% of gages exhibited changes in annual no-flow duration exceeding 100 days during the study period. Distinct regional patterns of change were evident, with widespread drying in southern CONUS and wetting in northern CONUS. These patterns are correlated with changes in aridity, though drivers of spatiotemporal variability were diverse across the three intermittency signatures. While the no-flow timing and duration were strongly related to climate, dry-down period was most strongly related to watershed land use and physiography. Our results indicate that non-perennial conditions are increasing in prevalence over much of CONUS and binary classifications of ‘perennial’ and ‘non-perennial’ are not an accurate reflection of this change. Water management and policy should reflect the changing nature and diverse drivers of changing intermittency both today and in the future.

Bias‐corrections on aridity index simulations of climate models by observational constraints

AbstractAridity Index (AI) indicates the balance between water supply and water demand on the atmosphere–land interface. Despite continuous improvements, coupled climate models still have significant systematic errors in simulating AI in terms of temporal and spatial variabilities. One of the approaches to bias‐correct simulations is extracting the linear relationship between historical observations and model outputs by utilizing the empirical orthogonal function (EOF). In this study, the methodology of ensemble EOF‐based bias‐correction by observational constraints is developed based on previous bias‐correction approach, with the improvement on seeking the optimal combinations of the leading modes with sensitivity test and replacing the certain correction with the ensemble means of optimal members. In verification, the ensemble mean of Coupled Model Intercomparison Project phase 5 (CMIP5‐EM) is bias‐corrected towards the CPC/GLDAS observations, and the extracted leading modes present high correlations with internal climate variability. By cross‐validation and posteriori independent validation of hindcasts over the historical period (1948–2005), the ensemble EOF‐based bias‐correction could better present spatial patterns compared to the CMIP5‐EM after systematic bias‐correction, as indicated by the anomaly correlation and the root mean square error. The verifications also indicate that the temporal variability in aridity over different dryland regions is much closer to that in the observations and that the dryland subtype changes are improved significantly by bias‐corrections. Besides, another observational dataset of UDel/CRU is applied to assess the uncertainty on different datasets and the improvement on skill scores is robust. The above results verify that the ensemble EOF‐based bias‐corrections provide better reference for assessing and projecting global aridity changes by climate models.

Stochastic community assembly decreases soil fungal richness in arid ecosystems.

Uncovering the linkages between community assembly and species diversity is a fundamental issue in microbial ecology. In this study, a large-scale (transect intervals of 1257.6km) cross-biome soil survey was conducted, which ranged over agricultural fields, forests, wetlands, grasslands and desert, in the arid regions of northwest China. The aim was to investigate the biogeographic distribution, community assembly and species co-occurrence of soil fungi. The fungal communities in agricultural soils exhibited a steeper distance-decay slope and wider niche breadths, and were more strongly affected by stochastic assembly processes, than fungi in other natural habitats. A strong relationship was revealed between soil fungal richness and community assembly in arid ecosystems, with the influence of stochastic assembly processes decreasing with increasing fungal richness. Moreover, aridity was the most important environmental factor influencing fungal richness, β-diversity and species co-occurrence patterns. Specifically, the predicted increase in arid conditions will probably reduce fungal richness and network complexity. These findings represent a considerable advance in linking fungal richness to mechanisms underlying the biogeographic patterns and assembly processes of fungal communities in arid ecosystems. These results can thus be used to forecast species co-occurrence and diversities pattern of soil fungi under climate aridity and land-use change scenarios.

Long-term relative decline in evapotranspiration with increasing runoff on fractional land surfaces

Abstract. Evapotranspiration (ET) accompanied by water and heat transport in the hydrological cycle is a key component in regulating surface aridity. Existing studies documenting changes in surface aridity have typically estimated ET using semi-empirical equations or parameterizations of land surface processes, which are based on the assumption that the parameters in the equation are stationary. However, plant physiological effects and its responses to a changing environment are dynamically modifying ET, thereby challenging this assumption and limiting the estimation of long-term ET. In this study, the latent heat flux (ET in energy units) and sensible heat flux were retrieved for recent decades on a global scale using a machine learning approach and driven by ground observations from flux towers and weather stations. This study resulted in several findings; for example, the evaporative fraction (EF) – the ratio of latent heat flux to available surface energy – exhibited a relatively decreasing trend on fractional land surfaces. In particular, the decrease in EF was accompanied by an increase in long-term runoff as assessed by precipitation (P) minus ET, accounting for 27.06 % of the global land areas. The signs are indicative of reduced surface conductance, which further emphasizes that surface vegetation has major impacts in regulating water and energy cycles, as well as aridity variability.

Tree Diversity, Site Index, and Carbon Storage Decrease With Aridity in Douglas-Fir Forests in Western Canada

Forests are important for biodiversity, timber production and carbon accumulation, but these ecosystem services may be impacted by climate change. Field data collected from individual forest types occurring across a climatic gradient can contribute to forecasting these consequences. We examined how changes in temperature, precipitation and aridity affect ecosystem services in 23 mature Douglas-fir (Pseudotsuga menziesii) forests in nine climatic regions across a 900 km gradient in British Columbia, Canada. Using Canadian National Forest Inventory methodology, we assessed richness and diversity of plant functional groups, site index, and above- and below-ground carbon stocks. As aridity increased, ecosystem-level tree species richness declined on average from four to one species, Douglas-fir site index declined from 30 to 15 m, and ecosystem carbon storage decreased from 565 to 222 Mg ha–1. Tree species richness was positively and herb species richness negatively correlated with carbon storage. Carbon storage by ecosystem compartment was largest in aboveground live tree biomass, declining in the following order: mineral soils > coarse woody debris and dead standing trees > forest floor > small and fine woody debris > understory plants. Mineral soil carbon at depths of 0-15 cm, 15-35 cm, and 35-55 cm increased with increasing mean annual precipitation and decreasing aridity. Our results indicate that as aridity increases and precipitation decreases, tree species richness, site index and carbon storage in existing Douglas-fir forests declines. However, assisted or natural migration of Douglas-fir into more humid regions could be associated with more diverse, productive, carbon-rich forests. This study informs carbon stock vulnerability and provides empirical data essential for carbon stock forecasts.

Time-varying responses of dryland aridity to external forcings over the last 21 ka

Identifying potential external forcings on dryland aridity at various timescales and clarifying underlying mechanisms can advance our knowledge of dryland climate behaviors. To address this issue, we examine the past 21 ka using a set of transient simulations driven by realistic climatic forcings, as well as multiple types of proxies. The simulations suggest the evolution of dryland aridity and dominant forcings vary over time and region. On average, global drylands feature two-phase aridification at the orbital scale: the first phase occurs before the Holocene and is dominated by ice sheet melting and rising greenhouse gas (GHG) concentrations, followed by the second phase driven by the summer insolation decrease over northern high latitudes after the early Holocene; different from the wetness during the orbital-scale glacial period, millennium-scale cold events forced by meltwater discharges correspond to desiccation, punctuating the long-term trend during the last deglaciation. In addition, the average aridity changes for drylands are larger than those for global landmasses under all forcings except GHG concentrations. Spatially, the dominant external forcings and aridity responses to the same forcing vary regionally, mainly related to the latitude and hemisphere. Further diagnosis indicates the manners to affect aridity, namely through altering precipitation or evaporative capacity, vary by forcing. The aforementioned evolution of the simulated aridity matches well with the proxies in most dryland zones. This study provides a scenario of time-varying responses of dryland aridity to external forcings over the last 21 ka and may shed light on the future changes.

Landscapes, climate and choice: Examining patterns in animal provisioning across the Near East c. 13,000-0 BCE

Understanding the organisation of food production is vital for understanding ancient societies. Multiple factors may influence decision making, including the local environmental capacity of a given area and individual and cultural preferences. This study compares zooarchaeological data from sites across the length and breadth of the Holocene Near East with modelled patterns of land use. The goal is to determine how far variation in the capacities of local landscapes impacted the choices made in animal production. Our approach allows us to investigate trends through time as well as between different regions of the Near East. The spatial and temporal scales employed also mean we can investigate the relationship between food production and climate trends. We find substantial patterning in the choices made in animal production, reflecting complex and regionally diverse production approaches. We demonstrate a prioritisation of individual and societal preferences to produce specific animals which is rarely impacted by either short or long term changes in aridity. We also find that the emergence of urban sites has a major impact on provisioning structures, and argue that the resulting organisational forms may have resulted in urban sustainability at the expense or rural sites.

Aridity Changes and Related Climatic Drivers in the Drylands of China during 1960–2019

AbstractDrylands cover about one-half of the land surface in China and are highly sensitive to climate change. Understanding climate change and its impact drivers on dryland is essential for supporting dryland planning and sustainable development. Using meteorological observations for 1960–2019, the aridity changes in drylands of China were evaluated using aridity index (AI), and the impact of various climatic factors [i.e., precipitation P; sunshine duration (SSD); relative humidity (RH); maximum temperature (Tmax); minimum temperature (Tmin); wind speed (WS)] on the aridity changes was decomposed and quantified. Results of trend analysis based on Sen’s slope estimator and Mann–Kendall test indicated that the aridity trends were very weak when averaged over the whole drylands in China during 1960–2019 but exhibited a significant wetting trend in hyperarid and arid regions of drylands. The AI was most sensitive to changes in water factors (i.e., P and RH), followed by SSD, Tmax, and WS, but the sensitivity of AI to Tmin was very small and negligible. Interestingly, the dominant climatic driver to AI change varied in the four dryland subtypes. The significantly increased P dominated the increase in AI in the hyperarid and arid regions. The significantly reduced WS and the significantly increased Tmax contributed more to AI changes than the P in the semiarid and dry subhumid regions of drylands. Previous studies emphasized the impact of precipitation and temperature on the global or regional dry–wet changes; however, the findings of this study suggest that, beyond precipitation and temperature, the impact of wind speed on aridity changes of drylands in China should be given equal attention.

New characteristics about the climate humidification trendin Northwest China

<p indent="0mm">At the beginning of 21st century, a study suggested that Northwest China’s climate is becoming warmer and more humid. However, there were doubts about this conclusion because the humidification changes on a small scale only last for a short period, and also suspicions about the development, duration, and impact of the warming-wetting trend in this region. This paper sets out several factors, such as the area, intensity of humidification, and comprehensiveness of the wetness index, to characterize the wetting degree. Furthermore, by using measured climate data, CRU data, climate scenario predictions, and ecological and hydrological data, we systematically analyze temporal and spatial evolution of the warming-wetting trend in Northwest China. Studies have shown that Northwest China has experienced two warming periods during the last century, i.e., the 1940s, and the period from the 1960s to the present (the current warming period). The latter is more substantial. The precipitation in the west and east part of Northwest China has a seesaw change process in many periods. However, humidification has simultaneously occurred in both regions since the beginning of this century. The current precipitation in the west part of Northwest China has far exceeded any period over the past <sc>100 a.</sc> In contrast, the east part’s recent rainfall is still currently lower than that of the 1950s and 1960s. Since 1961, Northwest China’s west region has experienced consistent humidification, which has been more pronounced since 1987. Simultaneously, the east part was drying from 1961 to 1997, and has become humid since 1997. The temperature, precipitation, and aridity index changes in the west part show the same patterns. In most periods, the west part was either cold-dry or warm-wet. In comparison, such consistency of the temperature, precipitation, and aridity changes does not exist in the east region. Before this century, warm-wet climates rarely occurred in the east region, having become more frequent. This is probably due to the different dominant temperatures and precipitation processes in the west and east regions. From a spatial point of view, the temperature increases in the whole of Northwest China, and the precipitation increases in the west region and decreases in the east region. The area with increasing rainfall has become larger during the past three climatic periods, and currently covers the whole of Northwest China. The precipitation, aridity index, and comprehensive humidity index in Northwest China have a non-linear increasing trend. The expanding humidity trend has been significant since the beginning of this century. The current warming and humidification have not changed the basic climate pattern in Northwest China. However, if the current trend continues, they would change the climate pattern in the future. Such climate change may result from the simultaneous enhancement of the westerly circulation and the East Asian summer monsoon circulations in this century. The climate warming and humidification in Northwest China significantly impacted extreme weather events, water resources, ecological environment, and agriculture during this century. This research will provide a basis for decision-making on issues such as implementing the Western development strategy in the new era, the ecological protection and high-quality development of the Yellow River Basin, environmental construction planning, and water resources management policies in Northwest China.