Terrestrial Aridity Research Articles

Quantification of human contribution to soil moisture-based terrestrial aridity

Current knowledge of the spatiotemporal patterns of changes in soil moisture-based terrestrial aridity has considerable uncertainty. Using Standardized Soil Moisture Index (SSI) calculated from multi-source merged data sets, we find widespread drying in the global midlatitudes, and wetting in the northern subtropics and in spring between 45°N–65°N, during 1971–2016. Formal detection and attribution analysis shows that human forcings, especially greenhouse gases, contribute significantly to the changes in 0–10 cm SSI during August–November, and 0–100 cm during September–April. We further develop and apply an emergent constraint method on the future SSI’s signal-to-noise (S/N) ratios and trends under the Shared Socioeconomic Pathway 5-8.5. The results show continued significant presence of human forcings and more rapid drying in 0–10 cm than 0–100 cm. Our findings highlight the predominant human contributions to spatiotemporally heterogenous terrestrial aridification, providing a basis for drought and flood risk management.

Humidification of Central Asia and equatorward shifts of westerly winds since the late Pliocene

The production, transport, and deposition of mineral dust exert major influences on climate change and Earth’s biogeochemical cycles. Furthermore, their imprint, as recorded in pelagic sediments, provides an avenue for determining past changes in terrestrial aridity and atmospheric circulation patterns in response to global climate change. Here, by examining geochemical and magnetic data obtained from a ferromanganese crust in the western Pacific Ocean, we investigate the eolian dust source-region conditions and dust transport mechanisms from the Asian interior to the Pacific Ocean since the Pliocene. We identify a gradual provenance change in the dust source regions, from a dominant Gobi Desert source during the early Pliocene to a mixed Gobi-Taklimakan Desert source during the late Pliocene and Pleistocene, alongside increasing chemical weathering in those source areas. Climate model simulations suggest that these changes were related to an equatorward shift of the westerly jet and humidification of Central Asia during the gradual transition from a warm Pliocene climate to the cool Pleistocene.

Historical and future shifts of a sharp zonal aridity gradient: A case study of the Hu Line in China

Ongoing climate change considerably affects terrestrial aridity from multiple perspectives. As a typical sharp zonal aridity gradient, China’s Hu Line effectively divides the country into the arid northwest and the humid southeast and has exerted profound impacts on human settlement, agricultural and socio-economic development. Previous studies of regions experiencing sharp zonal aridity gradients (e.g., the 100th meridian in the USA and the Sahel belt in Northern Africa) reported significant and widespread aridity changes in the context of anthropogenic climate change, yet how changes in aridity may geographically shift the Hu Line remains unknown. Here we assess the historical and future shifts of the Hu Line from agroecological, atmospheric and hydrologic perspectives. Our results show that the effective arid-humid divide from the agroecological perspective has moved northwest with a mean shift of 0.48° (over 1982–2016), whereas from the atmospheric and hydrologic perspectives its position remains effectively unchanged (very small northwest movements) over the past 35 years. For future periods (under a high emission scenario, SSP585), the effective arid-humid divides from all three perspectives move northwest towards the end of the 21st century, suggesting an overall decrease of agroecological, atmospheric and hydrologic aridity in the region. In addition, we find that vegetation response to elevated atmospheric CO2 concentration plays a key role in determining the predicted shifts of the Hu Line, which alleviates aridity increases under warming and effectively drives the arid-humid divides toward more arid northwest. Our finding of the northwest movement of the Hu Line under future climate change, in particular due to rising CO2, suggests a future potential for expansion of China’s agricultural and socioeconomy northwest and also sheds light on studying other regions experiencing sharp zonal aridity gradients.

Astronomically paced climate evolution during the Late Paleozoic icehouse-to-greenhouse transition

Late Paleozoic deglaciation is Earth's first icehouse-to-greenhouse transition with extensive vegetation, but the response of tropical climate to this transition has not yet been fully addressed. Here, cyclostratigraphic analysis was conducted on the magnetic susceptibility (MS) record of a deep marine carbonate succession in South China to construct the astrochronology of late Cisuralian (Early Permian), and decipher the evolutionary response to the climate transition in the MS and δ13Ccarb time series. The constructed astronomical time scale indicates an age of 272.83 ± 0.2 Ma for the Cisuralian–Guadalupian boundary in South China. Synchronizing global records demonstrates a complicated linkage between low and high latitudes, involving secular changes in atmospheric pCO2, ice volume, tropical climate and carbon cycling. In the final stage of the Early Permian icehouse (~290–288.2 Ma), the delivery of moisture from low to high latitudes was reduced in comparison with that in the icehouse apex. A more humid climate facilitated the development of tropical coal forests and more storage of 12C on land. In a full greenhouse condition (~281–272 Ma), the tropical responses to a rise in atmospheric pCO2 involved continental drying and ocean stagnation. On a shorter time scale, the nodes of 1.36 Myr obliquity cycles triggered ice-sheet expansion and enhanced tropical precipitation during an icehouse condition. With the gradual transition to an arid greenhouse, the insolation-climate relationship began to change at ~285.1 Ma, and the obliquity nodes became associated instead with terrestrial aridity and marine anoxia. These results bring into a focus pattern of shifting dynamics involving Earth's astronomical parameters and climate change for icehouse and greenhouse worlds in the Late Paleozoic Era.

Shellfish resilience to prehistoric human consumption in the southern Red Sea: Variability in Conomurex fasciatus across time and space

Intertidal environments have been the main source for mollusc gathering and consumption for at least the last 164,000 years. However, our knowledge of long-term trends is compromised by the fact that the majority of Pleistocene and early Holocene shorelines, and in turn their archaeological sites, are either currently submerged under water or have long been destroyed by sea-level change. Ecological information on the resilience of intertidal resources is crucial in assessing how attractive they were to past humans as a long-term source of food. Of particular interest is the southern Red Sea and its function as the southern gateway out of Africa into Arabia during a period of aridity. The role that marine food sources likely played in this dispersal is underplayed and largely ignored when interpreting periods of terrestrial aridity. Here we analyse the resilience of Conomurex fasciatus and report size measurements of over 15,000 specimens from the Holocene shell middens on the Farasan Islands, Saudi Arabia, as an ecological baseline for prehistoric shellfish exploitation to determine the long-term sustainability of shellfish harvesting in an arid environment. Changes in shell-size and relative abundance can indicate whether a species was subjected to changes in the intensity of human harvests and we use this dataset to reconstruct how the species was affected by a known intense exploitation period between 7360 and 4780 cal BP. Our results indicate no signs of resource depletion throughout the occupation period and add to the growing body of evidence that marine resources along arid shorelines are an important part of a mixed diet. Further, by measuring size changes occurring during early life stages of C. fasciatus we were able to reveal changes in size that were unaffected by human harvesting pressure and instead suggest patch-selection as the main control. These results have implications for the interpretation of shellfish harvesting during periods of terrestrial aridity and specifically the potential of shellfish as a reliable food source during Palaeolithic migrations out of Africa.

Cloud Cover over the Arabian Peninsula from Global Remote Sensing and Reanalysis Products

Abstract Clouds – their coverage, nature, and interactions with the energy budget of the planet – represent one of the primary sources of uncertainty in future climate prediction. Despite the crucial role of desert clouds in the distribution of water and energy budgets, their climatology is still largely incomplete. With arid regions projected to become dryer under global warming conditions, understanding the characteristics of their cloud cover can provide critical insights. In this work, cloud coverage was investigated over one of the Earth's most arid regions – the Arabian Peninsula. Four total cloud cover (TCC) products, namely the International Satellite Cloud Climatology Project H (ISCCP), the CM SAF Cloud, Albedo and Surface Radiation AVHRR 2 (CLARA) satellite datasets, the National Centers for Environmental Prediction – National Center for Atmospheric Research (NCEP–NCAR) Reanalysis (R-2) (NCEP) and the ECMWF Interim Re-Analysis (ERA) reanalyses, were used to construct a climatology of desert clouds over the peninsula between 1984 and 2009, accounting for the different products' uncertainties and limitations. Satellite retrievals and reanalysis fields were first validated against ground observations from the United Arab Emirates, for which homogeneity assessments were conducted. The validation was done using statistical indicators, including Normalized Root Mean Square Errors (nRMSE), relative biases (rBIAS), and correlation coefficients, on monthly and seasonal scales. The ISCCP dataset resulted in the highest correlations with the ground observations (overall 0.38) and the lowest nRMSE (overall 0.54), while CLARA had the lowest rBIAS (overall 0.02). All products showed discrepancies when compared to the ground observations, both annually and on a seasonal basis. When extended to the entire Arabian Peninsula, the satellite and reanalysis products showed decreasing spring and increasing summer (except for ISCCP) TCC evolution across the region. At inter-annual scales, the TCC over the peninsula showed a significant discontinuity in 1998. This shift could be linked to the forcing of the El Nino Southern Oscillation on water vapor transport over the region, as well as to documented artifacts in satellite retrievals and model outputs. These discrepancies are indicative of a need for detailed assessments to be made of the uncertainties existing in TCC data for the Arabian Peninsula. Plain Language Summary Clouds represent a primary source of uncertainty in future climate predictions. This is particularly pronounced in dryland clouds, given their sporadic and intermittent nature. With global warming predicted to enhance aridification trends in terrestrial arid zones, historical cloudiness trends over arid and hyper arid regions are very important – yet their climatology to date is still largely incomplete. The quality of observations and/or model outputs are an important consideration, making validation and intercomparison assessments imperative. This work investigated cloud cover over the Arabian Peninsula, one of the most arid regions on the Earth. Four cloud cover datasets were used, derived from satellite measurements and atmospheric reanalysis model outputs. The selected study period was between 1984 and 2009. The four datasets were initially compared with observations taken at ground stations in the United Arab Emirates. The four products were then studied over the entire Arabian Peninsula. Results showed that cloud cover displayed marked seasonal characteristics and large inter-annual temporal evolution. An abrupt change in regional cloud cover time series was found to occur in 1998. This could be attributed to the forcing of the El Nino Southern Oscillation, although significant documented uncertainties in satellite products over this time span call for deeper investigations of this causal relation.

Abiotic Reduction of Chlorate by Fe(II) Minerals: Implications for Occurrence and Transformation of Oxy-Chlorine Species on Earth and Mars

Recent investigations have reported a widespread occurrence of chlorate (ClO3–) and perchlorate (ClO4–) throughout the solar system, including terrestrial arid environments. ClO3– and ClO4– are deposited/accumulated at an approximate equal molar ratio, with some exceptions, such as the Antarctica Dry Valley soils (MDV) and perhaps Martian surface material, where ClO4– is the dominate ClOx– species. All known ClO4– production mechanisms produce molar ratios of ClO3–/ClO4– equal to or much greater than 1, suggesting that reduced ratios may be due to post-depositional mechanism(s). The objective of this study was to investigate potential iron-mediated abiotic reduction of ClO3–, similar to transformation mechanisms reported for nitrate (NO3–) by Fe(II) minerals. Three types of Fe(II)-containing minerals, wustite (FeO), siderite (FeCO3), and sulfate green rust (GRSO42–), were investigated in completely mixed batch reactors as potential ClO3– reductants at a range of pH (4–9) and iron mineral concentrations (1...

Variations in δ13C values of sedimentary organic matter since late Miocene time in the Indus Fan (IODP Site 1457) of the eastern Arabian Sea

AbstractA 1108.6 m long core was recovered at Site U1457 located on the Indus Fan in the Laxmi Basin of the eastern Arabian Sea during IODP Expedition 355. Shipboard examinations defined five lithologic units (I to V) of the lower Paleocene to Holocene sedimentary sequence. In this study, δ13C values of sedimentary organic matter (SOM) confirm the differentiation of the lithologic units and further divide units III and IV into two subunits (1 and 2). Based on the underlying assumption that the SOM is decided primarily by a mixture of marine and terrestrial origins, δ13CSOM values at Site U1457 provide information on the terrestrial catchment conditions since late Miocene time. Low δ13CSOM values from late Miocene to late Pleistocene times are similar (c. −22.0 ‰) for the most part, reflecting a consistent contribution of terrestrial organic matter from the catchment areas characterized by dominant C3 land plants. Significantly lower δ13CSOM values (c. −24.0 ‰) in Unit III-2 (∼8 to ∼7 Ma) might be due to a greater input of C3 terrestrial organic matter. The increase in δ13CSOM values at ∼7 Ma and the appearance of high δ13CSOM values (c. −18.0 ‰) within Unit III-1 (∼7 to ∼2 Ma) indicate that C4 biomass overwhelmed the terrestrial catchment environment as a result of enhanced terrestrial aridity in the Himalayan foreland. The three-end-member simple mixing model, estimating the relative contributions of SOM from terrestrial C3 and C4 plants and marine phytoplankton, supports our interpretation of the distribution of C3 and C4 land plants in the terrestrial catchment environment.

Is Himalayan-Tibetan Plateau “drying”? Historical estimations and future trends of surface soil moisture

The Himalayan-Tibetan Plateau (HTP), often known as the “Third Pole” and the “Asian Water Tower”, is the source of water resources for many Asian rivers and in turn for hundreds of millions of people living downstream. The HTP has direct impacts on the establishment and maintenance of Asian monsoon, and therefore on the climate of its surrounding areas. Besides, soil moisture plays a critical role in the hydrological cycle and is a critical link between land surface and atmosphere. Hence, soil moisture was greatly emphasized by Global Climate Observing System Programme as an Essential Climate Variable. However, little is known about soil moisture changes on the HTP from a long-term perspective. By comparing remotely sensed and modelled soil moisture datasets against in-situ observations from 100 observation stations, here we find that Noah performed better than other soil moisture datasets. In past years, soil moisture first decreased and then increased obviously. In most regions on HTP, precipitation changes can be taken as the major cause behind soil moisture variations. In future, there is persistently decreasing soil moisture trend since ~2010 with a decreasing rate of −0.044 kg/m2/10a, −0.031 kg/m2/10a and −0.0p 88 kg/m2/10a under RCP2.6, RCP4.5 and RCP8.5 scenarios, respectively, in CMIP5 (Coupled Model Intercomparision Project Phase 5). Specifically, a sudden decrease of soil moisture with a rate of −0.372 kg/m2/10a can be expected after ~2080 under RCP8.5 scenario. Amplifying terrestrial aridity due to increasing precipitation but more significant increasing potential evapotranspiration potentially results in drying HTP. Potential water deficiency for Asian rivers due to drying HTP should arouse considerable concerns.

Disconnection Between Trends of Atmospheric Drying and Continental Runoff

AbstractThe ratio of potential evapotranspiration (E0) over precipitation (P), known as the aridity index (AI), has been commonly used to stratify global aridity zones and widely adopted to assess changes in aridity globally. Anthropogenic climate change, in particular atmospheric warming, is projected to increase AI, which has in most cases been interpreted as increasing terrestrial aridity. In this study we demonstrate, for both past and future conditions, that such an interpretation requires reconsideration. Using catchment observations over the past 30 years and climate model projections for the 21st century, we show that increased AI does not ubiquitously lead to a decreased water availability over land, using surface runoff (Q) as an indicator. This is primarily caused by a higher Q sensitivity to changes in P (SP) and a lower Q sensitivity to changes in E0 (SE0), with the ratio of SP over SE0 being higher than the relative changes of E0 compared to P (i.e., |P × SP| > |E0 × SE0|). Assessment of Coupled Model Intercomparison Project Phase 5 model outputs indicates that both Q and AI change‐induced Q changes are increasing over the majority of the globe for the 21st century despite increasing AI (a drying atmosphere). Our findings demonstrate a disconnection between the atmospheric drying trends and surface runoff trends and call for caution when interpreting retrospective and future changes in terrestrial aridity based on AI and related measures.

Simulating the Effects of Anthropogenic Aerosols on Terrestrial Aridity Using an Aerosol–Climate Coupled Model

Abstract The comprehensive effects of anthropogenic aerosols (sulfate, black carbon, and organic carbon) on terrestrial aridity were simulated using an aerosol–climate coupled model system. The results showed that the increase in total anthropogenic aerosols in the atmosphere from 1850 to 2010 had caused global land annual mean precipitation to decrease by about 0.19 (0.18, 0.21) mm day−1, where the uncertainty range of the change (minimum, maximum) is given in parentheses following the mean change, and reference evapotranspiration ET0 (representing evapotranspiration ability) to decrease by about 0.33 (0.31, 0.35) mm day−1. The increase in anthropogenic aerosols in the atmosphere from 1850 to 2010 had caused land annual mean terrestrial aridity to decrease by about 3.0% (2.7%, 3.6%). The areal extent of global total arid and semiarid areas had reduced due to the increase in total anthropogenic aerosols in the atmosphere from preindustrial times. However, it was found that the increase in anthropogenic aerosols in the atmosphere had enhanced the terrestrial aridity and thus resulted in an expansion of arid and semiarid areas over East and South Asia. The projected decrease in anthropogenic aerosols in the atmosphere from 2010 to 2100 will increase global land annual mean precipitation by about 0.15 (0.13, 0.16) mm day−1 and ET0 by about 0.26 (0.25, 0.28) mm day−1, thereby producing a net increase in terrestrial aridity of about 2.8% (2.1%, 3.6%) and an expansion of global total arid and semiarid areas.

Competing influences of anthropogenic warming, ENSO, and plant physiology on future terrestrial aridity.

The 2011-2016 Californian drought illustrates that drought-prone areas do not always experience relief once a favorable phase of El Niño-Southern Oscillation (ENSO) returns. In the 21st century, such an expectation is unrealistic in regions where global warming induces an increase in terrestrial aridity larger than the aridity changes driven by ENSO variability. This premise is also flawed in areas where precipitation supply cannot offset the global warming-induced increased evaporative demand. Here, atmosphere-only experiments are analyzed to identify land regions in which aridity is currently sensitive to ENSO, and where projected future changes in mean aridity exceed the range caused by ENSO variability. Insights into the drivers of these aridity changes are obtained in simulations with incremental addition of three different factors to current climate: ocean warming, vegetation response to elevated CO2 levels, and intensified CO2 radiative forcing. The effect of ocean warming overwhelms the range of ENSO-driven temperature variability worldwide, increasing potential evapotranspiration (PET) in most ENSO-sensitive regions. Additionally, ~39% of the regions currently sensitive to ENSO receive less precipitation in the future, independent of the ENSO phase. Aridity increases consequently in 67-72% of the ENSO-sensitive area. When both radiative and physiological effects are considered, the area affected by aridity rises to 75-79% when using PET-derived measures of aridity, but declines to 41% when total soil moisture aridity indicator is employed. This reduction mainly occurs because plant stomatal resistance increases under enhanced CO2 concentrations, which results in improved plant water use efficiency, and hence reduced evapotranspiration and soil desiccation. Imposing CO2-invariant stomatal resistance may overestimate future drying in PET-derived indices.

Changes in terrestrial aridity for the period 850–2080 from the Community Earth System Model

AbstractThis study examines changes in terrestrial aridity due to both natural and anthropogenic forcing for the period 850–2080 by analyzing the Community Earth System Model (CESM) Last Millennium Ensemble simulations for 850–2005 and the CESM Large Ensemble simulations for 1920–2080. We compare terrestrial aridity in the Medieval Warm Period (MWP) (950–1250) with that in the Little Ice Age (LIA) (1550–1850), present day (PD) (1950–2005) with the last millennium (LM) (850–1850), and the future (F8.5) (2050–2080) with the LM, to place anthropogenic changes in the context of changes due to natural forcings. The aridity index defined as the ratio of annual precipitation to potential evapotranspiration, averaged over land, becomes smaller (i.e., a drier terrestrial climate) by 0.34% for MWP versus LIA (MWP‐LIA), 1.4% for PD versus LM (PD‐LM), and 7.8% for F8.5 versus LM (F8.5‐LM). The change of terrestrial‐mean aridity in PD‐LM and F8.5‐LM due to anthropogenic forcing is thus 4 and 20 times of that from MWP‐LIA due to natural forcing, respectively. It is shown that a drier climate in PD than LM is largely due to a decrease of precipitation while a drier climate in F8.5 than LM, and MWP than LIA, is mainly caused by an increase of temperature. The terrestrial‐mean aridity change in PD‐LM is, however, largely driven by greenhouse gas increases as in F8.5‐LM. This is because anthropogenic aerosols have a small effect on terrestrial‐mean aridity but at the same time they totally alter the attributions of aridity changes to meteorological variables by causing large negative anomalies in surface air temperature, available energy, and precipitation. Different from MWP‐LIA and F8.5‐LM, there are large spatial inhomogeneities in P/PET changes for PD‐LM in both magnitudes and signs, caused by anthropogenic aerosols, greenhouse gases, and land surface changes. The changes of terrestrial‐mean P and P − E (precipitation minus evaporation) for 850–2080 are also examined. The relative changes in P (P − E) are 0.4% (0.6%) for MWP‐LIA, −2.6% (−3.8%) for PD‐LM, and 4.7% (11.8%) for F8.5‐LM. The signs of changes in P − E and P are the same.

Simulated responses of terrestrial aridity to black carbon and sulfate aerosols

AbstractAridity index (AI), defined as the ratio of precipitation to potential evapotranspiration (PET), is a measure of the dryness of terrestrial climate. Global climate models generally project future decreases of AI (drying) associated with global warming scenarios driven by increasing greenhouse gas and declining aerosols. Given their different effects in the climate system, scattering and absorbing aerosols may affect AI differently. Here we explore the terrestrial aridity responses to anthropogenic black carbon (BC) and sulfate (SO4) aerosols with Community Earth System Model simulations. Positive BC radiative forcing decreases precipitation averaged over global land at a rate of 0.9%/°C of global mean surface temperature increase (moderate drying), while BC radiative forcing increases PET by 1.0%/°C (also drying). BC leads to a global decrease of 1.9%/°C in AI (drying). SO4 forcing is negative and causes precipitation a decrease at a rate of 6.7%/°C cooling (strong drying). PET also decreases in response to SO4 aerosol cooling by 6.3%/°C cooling (contributing to moistening). Thus, SO4 cooling leads to a small decrease in AI (drying) by 0.4%/°C cooling. Despite the opposite effects on global mean temperature, BC and SO4 both contribute to the twentieth century drying (AI decrease). Sensitivity test indicates that surface temperature and surface available energy changes dominate BC‐ and SO4‐induced PET changes.

Grassland Carbon Sequestration Ability in China: A New Perspective from Terrestrial Aridity Zones

Current climate change (e.g., temperature and precipitation variations) profoundly influences terrestrial vegetation growth and production, ecosystem respiration, and nutrient circulation. Grasslands are sensitive to climate change, and the carbon sequestration ability is closely related to water availability. However, how the terrestrial water budget influences regional carbon sequestration by the grassland ecosystem is still unclear. In this study, we modified a terrestrial biogeochemical model to investigate net ecosystem productivity (NEP) of Chinese grasslands under different aridity index (AI) levels from 1982 to 2008. The results showed that Chinese grasslands acted as a carbon sink of 33.7 TgC. yr−1, with a clear decrease in the spatial distribution from the humid end (near-forest) to the arid end (near-desert). During these 27 years, gross primary productivity (GPP) and net primary productivity (NPP) significantly increased with regional warming over the entire range of the AI, but no significant tendency was found for NEP. Meanwhile, only NPP in the arid zone (AR) and the semiarid zone (SAR) were significantly correlated with mean annual precipitation (MAP), and no significant correlation was found between heterotrophic respiration (Rh) and MAP; NPP and Rh were both positively correlated with mean annual temperature (MAT) in all AI zones except for NPP in AR; no significant correlation between NEP and MAP or MAT was found. These results revealed that the grasslands with different AI levels keep different response patterns to temperature and precipitation variations. On the basis of these results, we predicted that the gap of carbon sequestration ability between humid and arid grassland will expand. The total carbon sink in Chinese grasslands will continue to fluctuate, but there is a danger that it might shrink in the future because of a combination of climatic and human factors, although CO2 fertilization and N deposition might partly mitigate this reduction.

Terrestrial Aridity and Its Response to Greenhouse Warming across CMIP5 Climate Models

Abstract The aridity of a terrestrial climate is often quantified using the dimensionless ratio of annual precipitation (P) to annual potential evapotranspiration (PET). In this study, the climatological patterns and greenhouse warming responses of terrestrial P, Penman–Monteith PET, and are compared among 16 modern global climate models. The large-scale climatological values and implied biome types often disagree widely among models, with large systematic differences from observational estimates. In addition, the PET climatologies often differ by several tens of percent when computed using monthly versus 3-hourly inputs. With greenhouse warming, land P does not systematically increase or decrease, except at high latitudes. Therefore, because of moderate, ubiquitous PET increases, decreases (drying) are much more widespread than increases (wetting) in the tropics, subtropics, and midlatitudes in most models, confirming and expanding on earlier findings. The PET increases are also somewhat sensitive to the time resolution of the inputs, although not as systematically as for the PET climatologies. The changes in the balance between P and PET are also quantified using an alternative aridity index, the ratio , which has a one-to-one but nonlinear correspondence with . It is argued that the magnitudes of changes are more uniformly relevant than the magnitudes of changes, which tend to be much higher in wetter regions. The ratio and its changes are also found to be excellent statistical predictors of the land surface evaporative fraction and its changes.

Responses of terrestrial aridity to global warming

The dryness of terrestrial climate can be measured by the ratio of annual precipitation (P) to potential evapotranspiration (PET), where the latter represents the evaporative demand of the atmosphere, which depends on the surface air temperature, relative humidity, wind speed, and available energy. This study examines how the terrestrial mean aridity responds to global warming in terms of P/PET using the Coupled Model Intercomparison Project phase 5 transient CO2 increase to 2 × CO2 simulations. We show that the (percentage) increase (rate) in P averaged over land is ~1.7%/°C ocean mean surface air temperature increase, while the increase in PET is 5.3%/°C, leading to a decrease in P/PET (i.e., a drier terrestrial climate) by ~3.4%/°C. Noting a similar rate of percentage increase in P over land to that in evaporation (E) over ocean, we propose a framework for examining the change in P/PET, in which we compare the change in PET over land and E over ocean, both expressed using the Penman–Monteith formula. We show that a drier terrestrial climate is caused by (i) enhanced land warming relative to the ocean, (ii) a decrease in relative humidity over land but an increase over ocean, (iii) part of increase in net downward surface radiation going into the deep ocean, and (iv) different responses of PET over land and E over ocean for given changes in atmospheric conditions (largely associated with changes in temperatures). The relative contributions to the change in terrestrial mean aridity from these four factors are about 35%, 35%, 15%, and 15%, respectively. The slight slowdown of the surface wind over both land and ocean has little impact on the terrestrial mean aridity.

Water addition regulates the metabolic activity of ammonia oxidizers responding to environmental perturbations in dry subhumid ecosystems.

Terrestrial arid and semi-arid ecosystems (drylands) constitute about 41% of the Earth's land surface and are predicted to experience increasing fluctuations in water and nitrogen availability. Mounting evidence has confirmed the significant importance of ammonia-oxidizing archaea (AOA) and bacteria (AOB) in nitrification, plant nitrogen availability and atmospheric N2 O emissions, but their responses to environmental perturbations in drylands remain largely unknown. Here we evaluate how the factorial combinations of irrigation and fertilization in forests and land-use change from grassland to forest affects the dynamics of AOA and AOB following a 6-year dryland field study. Potential nitrification rates and AOA and AOB abundances were significantly higher in the irrigated plots, accompanied by considerable changes in community compositions, but their responses to fertilization alone were not significant. DNA-stable isotope probing results showed increased (13) CO2 incorporation into the amoA gene of AOA, but not of AOB, in plots receiving water addition, coupled with significantly higher net mineralization and nitrification rates. High-throughput microarray analysis revealed that active AOA assemblages belonging to Nitrosopumilus and Nitrosotalea were increasingly labelled by (13) CO2 following irrigation. However, no obvious effects of land-use changes on nitrification rates or metabolic activity of AOA and AOB could be observed under dry conditions. We provide evidence that water addition had more important roles than nitrogen fertilization in influencing the autotrophic nitrification in dryland ecosystems, and AOA are increasingly involved in ammonia oxidation when dry soils become wetted.

A stable isotope aridity index for terrestrial environments

We use the oxygen isotopic composition of tooth enamel from multiple mammalian taxa across eastern Africa to present a proxy for aridity. Here we report tooth enamel delta(18)O values of 14 species from 18 locations and classify them according to their isotopic sensitivity to environmental aridity. The species are placed into two groups, evaporation sensitive (ES) and evaporation insensitive (EI). Tooth enamel delta(18)O values of ES animals increase with aridity, whereas the tooth enamel delta(18)O values of EI animals track local meteoric water delta(18)O values, demonstrating that bioapatite delta(18)O values of animals with different behaviors and physiologies record different aspects of the same environment. The enrichment between tooth enamel delta(18)O values of ES and EI animals records the degree of (18)O enrichment between evaporated water (ingested water or body water) and source water, which increases with environmental aridity. Recognition of the ES-EI distinction creates the opportunity to use the (18)O composition of bioapatite as an index of terrestrial aridity.

Ferric sulfates on Mars

Evidence is presented for the possible formation and existence of ferric sulfato complexes and hydroxo ferric sulfate minerals in the permafrost on Mars. Acidic groundwater, derived from atmospheric oxidation of volcanogenic H2S to H2SO4 aerosols, promoted chemical weathering of fayalitic olivines, iron‐rich pyroxenes, plagioclase feldspar, and pyrrhotite‐pentlandite mineral assemblages in crustal ultramafic and basic igneous rocks. The acidic groundwater entered into electrochemical reactions with the iron sulfides, yielding dissolved FeSO4+, Fe(SO4)2−, and FeOH2+ complex ions, and the precipitation of basic ferric sulfate minerals such as those belonging to the roemerite, copiapite, botryogen, and jarosite‐alunite groups. These phases are stabilized at low temperatures and pH in Martian permafrost. The occurrence of jarosites in terrestrial arid regions suggests that they could also survive on the surface of Mars. Melting of the permafrost and raising of the pH may have initiated the hydrolysis of dissolved ferric sulfato complex ions and led to the precipitation of FeOOH, which reacted with precipitated silica to form phyllosilicates. Alternatively, degradation of the hydrolysate FeOOH to Fe2O3 during sublimation of permafrost exposed on Mars' surface may account for the presence of eolian maghemite suspected to be the magnetic mineral observed on the Viking Landers.