Land clearance and hydrological change in the Sahel: SW Niger

In the West African semiarid belt of the Sahel, for the second half of the XXth century, lasting droughts (1970s–1980s) and one of the World's highest population growths have resulted in major land cover and hydrological changes that can be quantified using aerial photographs. This paper aims to provide one of the longest combined observations of land cover and hydrological changes for semiarid areas using a time series of normalised mosaics of aerial photographs dating back from 1950, field inquiries, and updated groundwater data. The 500 km² study area in southwest Niger was chosen (i) for its rural environment representative of the rain-fed agriculture belt of the Sahel and (ii) to encompass the main hydrological study sites investigated in this region over the past two decades (Hapex-Sahel and AMMA experiments, 1990–2000s). Results have significant implications for future freshwater availability and food security in the Sahel.Between 1950 and 1992,  80% of the study area has been cleared, firstly to open new areas for agriculture and secondly for firewood supply (59% of the plateaux, 42% of the valley bottoms, and 87% of the hillslopes). Intermediate aerial photograph surveys (1960, 1975) attest an accelerated loss in the woody savannah that could not be recovered on the short term. A strong, indirect impact of land clearance is observed on the water resources. Land clearance has resulted in a modification of the soil properties and infiltration capacity and has led to an increase in Hortonian runoff collected in numerous gullies and ponds. Between 1950 and 1992, aerial photographs show a  2.5 fold increase of the drainage density with the development of large drainage systems and new ponds. Groundwater data also indicate a continuous rise in the water table, mostly noticeable since the 1980s with a mean groundwater level rise of  4 m for the 1963–2005 period (+ 15% in aquifer reserves). The relatively short  30 year time-lag between the onset of land clearance and the beginning of the water table rise is linked to the process of indirect groundwater recharge and is timed with the connectivity of the drainage network and the formation of new ponds. Finally, the sustained increase in surface runoff and groundwater recharge during the past four decades indicates that the indirect impact of land clearance on the terrestrial water balance has been stronger than that of the long-lasting Sahelian drought. As the rate of land clearance increased for the past century in semiarid Africa, its main hydrological effects may not yet be fully perceptible.     

Estimation of surface albedo increase during the eighties Sahel drought from Meteosat observations

The devastating drought in the Sahel during the 70s and the 80s is among the most undisputed and largest recent climate event recognized by the research community. This dramatic climate event has generated numerous sensitivity analyses on land-atmosphere feedback mechanisms with contradicting conclusions on surface albedo response to precipitation changes. Recent improvements in the calibration and quantitative exploitation of archived Meteosat data for the retrieval of surface albedo have permitted to compare surface albedo of 1884, the driest year of the 80s, with year 2003 which had similar precipitation rate than conditions prevailing prior to the 80s drought. This analysis reveals detailed information on the geographical extension and magnitude of the surface albedo increase during from the 80s drought. A mean zonal increase in broadband surface albedo of about 0.06 between 1984 and 2003 has been estimated from the analysis of Meteosat observations. Regions particularly affected by the 1980s drought are essentially located into a narrow band of about 2° width along 16°N running from 18°W up to 20°E. Within this geographical area, surface albedo changes are not homogeneous and largest differences might locally exceed 0.15 whereas other places remained almost unaffected. The variety of previously published results might be explained by these important spatial variations observed around 16°N.     

A hydrologic and archeologic study of climate change in Al Ain,United Arab Emirates

Aridity trends established for Al Ain, United Arab Emirates, for the past 4500 years correlate with the trends of increased well depths and declining groundwater levels. Depth of wells found at archeologic sites at Hili near Al Ain were correlated to groundwater levels. Trends of declining groundwater levels were related to trends of increasing aridity (climate change). The increasing aridity had a pronounced affect on man's development in Al Ain area as well. For example, nonirrigation farming could not be successfully sustained at the end of the Bronze Age. This thwarted the economic development until the falaj (a water conveyance structure) was introduced in the Iron Age.The aridity trends in Al Ain correspond to contemporaneous aridity trends noted in Mesopotamia and the Dead Sea area, as well as the Middle East, Mediterranean, and northern Africa, in general. Other global climatic changes that are contemporaneous with climate change at Al Ain have been noted. The increased aridity (desertification) trends at Al Ain are contemporaneous with increased atmospheric CO₂ trends as reported by Indermuhle et al. [Nature (398) 121].     

The regional hydrologic impacts of global climate change: the role of climate models

The requirement to increase understanding of the complex interaction between society and the environment is well documented. Dramatic evidence of the vulnerability of anthropogenic systems to short-term weather fluctuations abounds. Taking an historical perspective provides an equally impressive picture of the potential upheaval caused by longer term climate changes. However, the past (and present) may not provide an adequate analoque for the future. The greenhouse theory of climate change suggests that the changes in climate regime to be expected from enhanced atmospheric CO₂ will be of similar magnitude to the glacial-interglacial mean temperature difference, but will occur in a fraction of the time. Consequently, considerable emphasis is being placed on the role of physical climate models in determining projections of future global and regional temperature and precipitation patterns. The latter climate changes will have important implications for the distribution (in time and space) of water, a principal natural resource and basic requirement for a variety of human activities. Consequently, climate models are being applied to the question of determining the regional hydrologic response to global climate change. The latter objective is a prerequisite to assessing the likely impacts on the water resources sector. This paper reviews current progress in achieving this aim and outlines some future research directions.     

A review on East Asian dust storm climate, modelling and monitoring

In arid and semi-arid area of Asia, dust storms occur frequently. Asian dust storms have a major impact on the air quality of the densely populated areas of China, Korea and Japan, and are important to the global dust cycle. In extreme cases, they result in the loss of human lives and disruptions of social and economic activities. In recent years, systematic research on Asian dust storms has been carried out. Much progress has been made in the development of integrated dust storm monitoring and modeling systems by making use of advanced numerical models, satellite remote sensing and GIS data. In this paper, we summarize the recent achievements in Asian dust storm research with an emphasis on dust climatology, modeling and satellite monitoring. The concept of integrated dust storm monitoring and modeling system is described and a summary of the developments in key research areas is given, including new dust models and techniques in satellite remote sensing and system integration. We then discuss the current research frontiers and the challenges for future studies.     

Sediment flux sensitivity to climate change: A case study in the Longchuanjiang catchment of the upper Yangtze River, China

Climate change may affect the sediment generation and transportation processes and the consequent sediment flux in a river. The sensitivity of suspended sediment flux to climate change in the Longchuanjiang catchment is investigated with Artificial Neural Networks (ANNs). ANNs were calibrated and validated using sediment flux data from 1960 to 1990 during which the influence from human activities was relatively stable. The established ANN is used to predict the responses of sediment flux to 25 hypothetical climate scenarios, which were generated by adjusting the baseline temperature up to − 1, 1, 2 and 3 °C and by scaling the baseline precipitation by +/− 10% and +/− 20%. The results indicated when temperature remains unchanged, an increase in rainfall will lead to a rise in sediment flux; when rainfall level remains unchanged, an increase in temperature is likely to result in a decrease in sediment flux. Same percentage of changes in rainfall and temperature are likely to trigger higher responses in wetter months than in drier months. However, it is the combination of the change in temperature and rainfall that determines the change of sediment flux in a river. Higher sediment flux is expected to appear under wetter and warmer climate, when higher transport capacity is accompanied by higher erosion rate.     

Edge adapted wavelets,solar magnetic activity,and climate change

The continuous wavelet transform is adapted to account for signal truncation through renormalization and by modifying the shape of the analyzing window. Comparison is made of the instant, integrated, and mean wavelet power with previous algorithms. The edge adapted and renormalized admissible wavelet transforms are used to estimate the level of solar magnetic activity from the sunspot record. The solar activity is compared to Oerlemans’ temperature reconstruction and to the Central England Temperature record. A correlation is seen for years between 1610 and 1990, followed by a strong deviation as the recently observed temperature increases.     

The effect of human activity on radiative forcing of climate change: a review of recent developments

Human activity has perturbed the Earth's energy balance by altering the properties of the atmosphere and the surface. This perturbation is of a size that would be expected to lead to significant changes in climate. In recent years, an increasing number of possible human-related climate change mechanisms have begun to be quantified. This paper reviews developments in radiative forcing that have occurred since the second assessment report of the Intergovernmental Panel on Climate Change (IPCC), and proposes modifications to the values of global-mean radiative forcings since pre-industrial times given by IPCC. The forcing mechanisms which are considered here include those due to changes in concentrations of well-mixed greenhouse gases, tropospheric and stratospheric ozone, aerosols composed of sulphate, soot, organics and mineral dust (including their direct and indirect effects), and surface albedo. For many of these mechanisms, the size, spatial pattern and, for some, even the sign of their effect remain uncertain. Studies which have attributed observed climate change to human activity have considered only a subset of these mechanisms; their conclusions may not prove to be robust when a broader set is included.     

Borehole temperatures, climate change and the pre-observational surface air temperature mean: allowance for hydraulic conditions

Joint analysis of surface air temperature series recorded at weather stations together with the inversion of the temperature-depth profiles logged in the near-by boreholes enables an estimate of the conditions existing prior to the beginning of the meteorological observation, the so-called pre-observational mean (POM) temperature.Such analysis is based on the presumption of pure diffusive conditions in the underground. However, in real cases a certain subsurface fluid movement cannot be excluded and the measured temperature logs may contain an advective component. The paper addresses the correction for the hydraulic conditions, which may have perturbed the climate signal penetrating from the surface into the underground. The method accounts for vertical conductive and vertical advective heat transport in a 1-D horizontally layered stratum and provides a simultaneous evaluation of the POM-temperature together with the estimate of the Darcy fluid velocity. The correction strategy is illustrated on a synthetic example and its use is demonstrated on temperature logs measured in four closely spaced boreholes drilled near Tachlovice (located about 15 km SW of Prague, Czech Republic). The results revealed that in a case of moderately advectively affected subsurface conditions (fluid velocities about 10⁻⁹ m/s), the difference between POM-values assessed for a pure conductive approach and for combined vertical conductive/advective approach may amount up to 0.3–0.5 K, the value comparable with the amount usually ascribed to the 20th century climate warming.     

The economic impact of climate change on Kenyan crop agriculture: A Ricardian approach

This paper measures the economic impact of climate on crops in Kenya. We use cross-sectional data on climate, hydrological, soil and household level data for a sample of 816 households. We estimate a seasonal Ricardian model to assess the impact of climate on net crop revenue per acre. The results show that climate affects crop productivity. There is a non-linear relationship between temperature and revenue on one hand and between precipitation and revenue on the other. Estimated marginal impacts suggest that global warming is harmful for crop productivity. Predictions from global circulation models confirm that global warming will have a substantial impact on net crop revenue in Kenya. The results also show that the temperature component of global warming is much more important than precipitation. Findings call for monitoring of climate change and dissemination of information to farmers to encourage adaptations to climate change. Improved management and conservation of available water resources, water harvesting and recycling of wastewater could generate water for irrigation purposes especially in the arid and semi-arid areas.     

Multi-model Bayesian assessment of climate change in the northern annular mode

A typical question in climate change analysis is whether a certain observed climate characteristic, like a pronounced anomaly or an interdecadal trend, is an indicator of anthropogenic climate change or still in the range of natural variability. Many climatic features are described by one-dimensional index time series, like for instance the global mean temperature or circulation indices. Here, we present a Bayesian classification approach applied to the time series of the northern annular mode (NAM), which is the leading mode of Northern Hemisphere climate variability. After a pronounced negative phase during the 1950s and 1960s, the observed NAM index reveals a distinct positive trend, which is also simulated by various climate model simulations under enhanced greenhouse conditions. The objective of this study is to decide whether the observed temporal evolution of the NAM may be an indicator of global warming. Given a set of prior probabilities for disturbed and undisturbed climate scenarios, the Bayesian decision theorem decides whether the observed NAM trend is classified in a control climate, a greenhouse-gas plus sulphate aerosol climate or a purely greenhouse-gas induced climate as derived from multi-model ensemble simulations.The three climate scenarios are well separated from each other in terms of the 30-year NAM trends. The multi-model ensembles contain a weak but statistically significant climate change signal in the form of an intensification of the NAM. The Bayesian classification suggests that the greenhouse-gas scenario is the most probable explanation for the observed NAM trend since 1960, even if a high prior probability is assigned to the control climate. However, there are still large uncertainties in this classification result because some periods at the end of the 19th century and during the “warm” 1920s are also classified in an anthropogenic climate, although natural forcings are likely responsible for this early NAM intensification. This demonstrates a basic shortcoming of the Bayesian decision theorem when it is based on one-dimensional index time series like the NAM index.     

Uncertainties in climate change prediction: El Niño-Southern Oscillation and monsoons

The sensitivity of climate phenomena in the low latitudes to enhanced greenhouse conditions is a scientific issue of high relevance to billions of people in the poorest countries of the globe. So far, most studies dealt with individual model results. In the present analysis, we refer to 79 coupled ocean–atmosphere simulations from 12 different climate models under 6 different IPCC scenarios. The basic question is as to what extent various state-of-the-art climate models agree in predicting changes in the main features of El Niño-Southern Oscillation (ENSO) and the monsoon climates in South Asia and West Africa. The individual model runs are compared with observational data in order to judge whether the spatio-temporal characteristics of ENSO are well reproduced. The model experiments can be grouped into multi-model ensembles. Thus, climate change signals in the classical index time series, in the principal components and in the time series of interannual variability can be evaluated against the background of internal variability and model uncertainty.There are large differences between the individual model predictions until the end of the 21st century, especially in terms of monsoon rainfall and the Southern Oscillation index (SOI). The majority of the models tends to project La Niña-like anomalies in the SOI and an intensification of the summer monsoon precipitation in India and West Africa. However, the response barely exceeds the level of natural variability and the systematic intermodel variations are larger than the impact of different IPCC scenarios. Nonetheless, there is one prominent climate change signal, which stands out from model variations and internal noise: All forced model experiments agree in predicting a substantial warming in the eastern tropical Pacific. This oceanic heating does not necessarily lead to a modification of ENSO towards more frequent El Niño and/or La Niña events. It simply represents a change in the background state of ENSO. Indeed, we did not find convincing multi-model evidence for a modification of the wavelet spectra in terms of ENSO or the monsoons. Some models suggest an intensification of the annual cycle but this signal is fairly model-dependent. Thus, large model uncertainty still exists with respect to the future behaviour of climate in the low latitudes. This has to be taken into account when addressing climate change signals in individual model experiments and ensembles.     

Antarctic dry valleys: Microclimate zonation, variable geomorphic processes, and implications for assessing climate change on Mars

The Antarctic Dry Valleys (ADV) are generally classified as a hyper-arid, cold-polar desert. The region has long been considered an important terrestrial analog for Mars because of its generally cold and dry climate and because it contains a suite of landforms at macro-, meso-, and microscales that closely resemble those occurring on the martian surface. The extreme hyperaridity of both Mars and the ADV has focused attention on the importance of salts and brines on soil development, phase transitions from liquid water to water ice, and ultimately, on process geomorphology and landscape evolution at a range of scales on both planets. The ADV can be subdivided into three microclimate zones: a coastal thaw zone, an inland mixed zone, and a stable upland zone; zones are defined on the basis of summertime measurements of atmospheric temperature, soil moisture, and relative humidity. Subtle variations in these climate parameters result in considerable differences in the distribution and morphology of: (1) macroscale features (e.g., slopes and gullies); (2) mesoscale features (e.g., polygons, including ice-wedge, sand-wedge, and sublimation-type polygons, as well as viscous-flow features, including solifluction lobes, gelifluction lobes, and debris-covered glaciers); and (3) microscale features (e.g., rock-weathering processes/features, including salt weathering, wind erosion, and surface pitting). Equilibrium landforms are those features that formed in balance with environmental conditions within fixed microclimate zones. Some equilibrium landforms, such as sublimation polygons, indicate the presence of extensive near-surface ice; identification of similar landforms on Mars may also provide a basis for detecting the location of shallow ice. Landforms that today appear in disequilibrium with local microclimate conditions in the ADV signify past and/or ongoing shifts in climate zonation; understanding these shifts is assisting in the documentation of the climate record for the ADV. A similar type of landform analysis can be applied to the surface of Mars where analogous microclimates and equilibrium landforms occur (1) in a variety of local environments, (2) in different latitudinal bands, and (3) in units of different ages. Documenting the nature and evolution of the ADV microclimate zones and their associated geomorphic processes is helping to provide a quantitative framework for assessing the evolution of climate on Mars.     

Future desertification and climate change: The need for land-surface system evaluation improvement

In already drought-stressed areas and places with the potential for desertification as a result of greenhouse-induced change, high quality model-derived climate projections are essential for sustainable management. Today's challenge is how to select from the plethora of models and proposed new analyses the tools most likely to be valid for areas already water-stressed and those threatened by future surface moisture reduction. Here, the land-surface skills of models involved in the IPCC Fourth Assessment Report and new techniques of isotopic enrichment of components of evapotranspiration are analyzed. Both are found to have shortcomings. Surprisingly poor reporting of fundamental components of the land-surface system in standard model output was the largest challenge for widely accepted models. We show that very few of a large group (20) of today's climate models report land-surface water and energy budgets correctly in a well-controlled international experiment and that most fail basic conservation tests. Our analysis of a smaller (5) experiment suggests that isotopic techniques employed in arid zone irrigation management may not transition to evaluation and model improvement. Land-surface conditions important for policy are found to be poorly reported which raises questions about equal weighting given by international assessments to all models: good and bad.     

The link between abrupt climate change and basin stratigraphy: a numerical approach

To use basin stratigraphy for studying past climate change, it is important to understand the influence of evolving boundary conditions (river discharge and sediment flux, initial bathymetry, sea level, subsidence) and the complex interplay of the redistribution processes (plumes, turbidity currents, debris flows). To provide understanding of this complexity, we have employed source to sink numerical models to evaluate which process dominates the observed variability in a sedimentary record of two coastal Pacific basins, Knight Inlet in British Columbia and the Eel Margin of northern California.During the last glacial period, the Eel River supplied comparatively more sediment with a less variable flux to the ocean, while today the river is dominated by episodic events. Model results show this change in the variability of sediment flux to be as important to the deposit character as is the change in the volume of sediment supply. Due to the complex interaction of flooding events and ocean storm events, the more episodic flood deposits of recent times are less well preserved than the flood deposits associated with an ice-age climate.In Knight Inlet, the evolving boundary conditions (rapidly prograding coastline, secondary transport by gravity flows from sediment failures) are a strong influence on the sedimentary record. The delta and gravity flow deposits punctuate the sedimentary record formed by hemipelagic sedimentation from river plumes. Missing time intervals due to sediment failures can take away the advantage of the otherwise amplified lithologic record of discharge events, given the enclosed nature of the fjord basin.     

Impact cratering as a cause of climate change,surface alteration,and resurfacing during the early history of Mars

Ancient valley networks (VNs) and related open- and closed-basin lakes are testimony to the presence of flowing liquid water on the surface of Mars in the Late Noachian and Early Hesperian. Uncertain, however, has been the mechanism responsible for causing the necessary rainfall and runoff and/or snowfall and subsequent melting. Impact cratering has been proposed (e.g., Segura et al. 2002) as a process for temporarily raising temperatures and inducing conditions that would produce rainfall, snowmelt, runoff, and formation of the VNs and associated lacustrine features. We refer to the collective effects of this process as the ICASE model (impact cratering atmospheric/surface effects). In this contribution, we assess the proposed impact cratering mechanism in order to understand its climatic implications for early Mars: we outline the step-by-step events in the cratering process and explore the predictions for atmospheric and surface geological consequences. For large and basin-scale impacts, rainfall should be globally and homogeneously distributed and characterized by very high temperatures. Rainfall rates are predicted to be high, ~2 m yr⁻¹, similar to rates in tropical rainforests on Earth, and runoff rates are correspondingly very high. These predicted characteristics do not seem to be consistent with the observed VNs, which are mainly equatorial and not homogeneous in their distribution. Prior to the Late Noachian, however, we predict that basin-scale impact effects are very likely to contribute significantly to degradation of crater rims and regional smoothing of terrain, implying vast resurfacing and resetting of crater ages following large crater and basin-scale impacts. Furthermore, the high temperatures of impact-induced rainwater and snowmelt and the pervasive penetration of heat into the regolith substrate are predicted to have a significant influence on the mineralogical alteration of the crust and its resulting physical properties. We conclude by describing a case example (Isidis basin) and describe how the ICASE model provides an alternative scenario for the interpretation of the layered phyllosilicates in the Nili Fossae and NE Sytris regions. We outline specific conclusions and recommendations designed to improve the ICASE model and to promote further understanding of its implications for the geological, mineralogical, and climate history of early Mars.     

The effect of climate change on carbon in Canadian peatlands

Peatlands, which are dominant features of the Canadian landscape, cover approximately 1.136 million km², or 12% of the land area. Most of the peatlands (97%) occur in the Boreal Wetland Region (64%) and Subarctic Wetland Region (33%). Because of the large area they cover and their high organic carbon content, these peatlands contain approximately 147 Gt soil carbon, which is about 56% of the organic carbon stored in all Canadian soils.A model for estimating peatland sensitivity to climate warming was used to determine both the sensitivity ratings of various peatland areas and the associated organic carbon masses. Calculations show that approximately 60% of the total area of Canadian peatlands and 51% of the organic carbon mass in all Canadian peatlands is expected to be severely to extremely severely affected by climate change.The increase in average annual air temperature of 3–5 °C over land and 5–7 °C over the oceans predicted for northern Canada by the end of this century would result in the degradation of frozen peatlands in the Subarctic and northern Boreal wetland regions and severe drying in the southern Boreal Wetland Region. In addition, flooding of coastal peatlands is expected because of the predicted rise in sea levels. As a result of these changes, a large part of the carbon in the peatlands expected to be severely and extremely severely affected by climate change could be released into the atmosphere as carbon dioxide (CO₂) and methane (CH₄), which will further increase climate warming.     

Comparing modelled responses of two high-permeability,unconfined aquifers to predicted climate change

The responses of two small, regional-scale aquifers to predicted climate change are compared. The aquifers are unconfined, heterogeneous, highly permeable, and representative of glaciofluvial environments in southern British Columbia, Canada and Washington State, USA. In one case, river–aquifer interactions dominate the hydraulic response. The climate change data set is that predicted by Canadian Global Climate Model 1 (CGCM1), for consecutive 30-yr intervals from present to 2069. Downscaling of GCM predictions and stochastic weather generation were done for each geographic location separately. Both studies employed identical methodologies and software for downscaling global climate model data, modelling weather for input to recharge models, determining the spatio-temporal distribution of recharge, and modelling groundwater flow using MODFLOW. Results suggest observable, but small, changes in groundwater levels, forced by changes in recharge. At the site in which river–aquifer interactions occur, water levels within the floodplain respond significantly and more directly to shifts in the river hydrograph under scenarios of climate change.