Desertification in the Sahel Region: A Product of Climate Change or Human Activities? A Case of Desert Encroachment Monitoring in North-Eastern Nigeria Using Remote Sensing Techniques

In Nigeria, desertification has become one of the most pronounced ecological disasters, with the impacts mostly affecting eleven frontline States. This has been attributed to a range of both nat-ural and man-made factors. This study applied a remote sensing-based change detection and indicator analysis to explore land use/land cover changes and detect major conversions from ecologically active land covers to sand dunes. Results indicate that areas covered by sand dunes (a major indicator of desertification) have doubled over the 25 years under consideration (1990 to 2015). Although about 0.71 km2 of dunes have been converted to vegetation, indicative of the success of various international, national, local, and individual afforestation efforts, conversely about 10.1 km2 of vegetation were converted to sand dunes, implying around 14 times more de-forestation compared to afforestation. Juxtaposing the progression of sand dune with climate records of the study area and examining the relationship between indicators of climate change and desertification suggested a mismatch between both processes as increasing rainfall and lower temperatures observed in 1994, 2005, 2012, and 2014 did not translated into positive feedbacks for desertification in the study area. On average, our results reveal that sand dune is progressing at a mean annual rate of about 15.2 km2 in the study area. Based on this study’s land cover change, trend and conversion assessment, visual reconciliation of climate records with land cover data, statistical analysis, observations from ground-truthing, as well as previous literature, it can be inferred that desertification in Nigeria is less a function of climate change, but more a product of human activities driven by poverty, population growth and failed government policies. Further projections by this study also reveal a high probability of more farmlands being converted to sand dunes by the year 2030 and 2045 if current practices prevail.

Regional Precipitation Trends Since 1500 Ce, Reconstructed from Calcite Sublayers of a Varved Mediterranean Lake Record (Central Pyrenees)

The Mediterranean is one of the regions of the world where human-induced climate warming is expected to have large impacts on water and environmental resources. To predict shifts in the current climate system, more regional climate records, including seasonal-to-century scale variability spanning longer than the instrumental periods, are needed. To help fill this gap, we provide a reconstruction of autumn precipitation variations for the Central Pyrenees range since 1500 Common Era (CE) using the varved sediments of Lake Montcortès. To assess the suitability of the calcite sublayer width of the sediments of this lake as a proxy for precipitation anomalies, we performed an analysis and smoothing of the temporal structure of the width series, calibration of the new series with the available instrumental climate records, calculation of a transfer function and testing and comparison of the reconstructed series against available empirical data.The prediction model was statistically robust and showed that the climatic signal was captured in the calcite sublayers. The reconstruction provides the first estimations of regional autumn precipitation shifts in the Central Pyrenees at annual resolution, since 1500 CE. Pronounced interdecadal shifts in precipitation were noticeable; no increasing nor decreasing linear trends or periods of extreme precipitation events were identified. The reconstructed precipitation anomalies suggest a decrease in rainfall during the coldest phase within the coldest period of the Little Ice Age and also during the 20th century, probably associated to current Global Warming. Correlations between autumn precipitation and the North Atlantic Oscillation, Western Mediterranean Oscillation and Southern Oscillation indices were weak to moderate. A potential relationship with the Atlantic Multidecadal Oscillation pattern is suggested. The reconstructed autumn precipitation trends are coherent with other palaeohydrological reconstructions in similar Mediterranean settings, and consistent at a regional level.

June–July Temperature Reconstruction of Kashmir Valley from Tree Rings of Himalayan Pindrow Fir

The Himalaya is one of the major mountain ecosystems that is most likely to be impacted by climate change. The main drawback in understanding climate change in the remote Himalayan ecosystems is the lack of long-term instrumental climate records. Reconstructing past climates from tree-rings offers a useful proxy for adding data to the instrumental climate records. In this study, climatically sensitive tree-rings of Himalayan fir (Abies pindrow) were used for reconstruction of mean June–July temperatures of Kashmir valley. Total ring-width chronology was built from 60 tree-ring cores growing near the higher altitudinal limits of the species. The radial growth showed a strong positive response to growing season temperature. The strong response of site chronology to mean June–July temperatures was used for reconstruction purposes. Mean June–July temperatures of Kashmir valley were reconstructed since 1773 from residual site chronology. Though the reconstruction did not show any strong long-term trend, on a centennial-scale, 20th-century summers were the warmest with a mean annual summer temperature of 22.99 °C. Seven of the warmest years and five of the warmest decades were seen in the 20th century. The reconstruction for 1773–2012 showed 23 extreme hot summers above the hot threshold of a 23.47 °C mean temperature and 19 extreme cold years below the cold threshold of a 22.46 °C mean summer temperature. The cold years in the reconstruction did not coincide with known volcanic eruptions. This reconstruction will help in providing a better understanding of regional climate change.

Large volcanic eruptions as a natural hazard: The impact of the 536/540 CE double event on the atmospheric circulation, surface climate, vegetation and society in Scandinavia

<p>Large volcanic eruptions that reach the stratosphere cool the surface climate and impact the atmospheric circulation, feeding back on the local climate. The mid-6<sup>th</sup> century is an outstanding period in climate history that featured an extreme cold period, including one of the coldest decades in the past 2000 years. It was triggered by the 536/540 CE volcanic double event, creating the strongest decadal volcanic forcing in the last two millennia. During this period societal changes are recorded around the world, like the Great Migration period and the outbreak of the Justinian Plague. However, not a lot is known about the causal relationships between global cooling and societal change. Less is known also, about the impact of the large-scale atmospheric circulation on the regional climate, vegetation and society in Scandinavia after this volcanic double event. Here we aim to improve this understanding by combining global climate and regional growing-degree-day (GGD) modeling with climate proxies and archaeological records from Southeastern Norway.</p><p>We use PMIP4 past2k runs and the MPI-ESM ensemble simulation of the 6<sup>th/7th </sup> century (520-680 CE), to analyze the atmospheric circulation, surface climate and vegetation changes as a response to the volcanic double event of 536/540 CE, over Scandinavia, specifically Southeastern Norway. Thereby we focus on the response of the major circulation patterns that influence the climate over Northern Europe: the positive and negative North Atlantic Oscillation, the Scandinavian blocking and the Atlantic ridge. The results of the GDD model, driven with the MPI-ESM model input, are compared to local pollen and climate records and archaeological data (e.g. grave density and settlement records) to shed more light on the local climate, vegetation and society impact. This comparison allows us to better understand how a natural hazard influenced local areas and climate records in Southeastern Norway. This study is part of the VIKINGS project, which focuses on the impact of volcanic eruptions on climate, environment and society in Norway/ Scandinavia.</p>

The development and relevance of a consistent flagging strategy for multi-sensor satellite soil moisture climate records

<p>The quality of soil moisture retrievals from passive microwave satellite sensors is limited during certain conditions, e.g. snow coverage, radio-frequency interference and dense vegetation. Therefore, masking the retrievals in these conditions by data flagging algorithms is vital for the production of reliable satellite-based products. However, these products utilise different flagging methods. A clear overview and comparison of these methods and their impact on the data are lacking. For long-term soil moisture records such as the ESA CCI soil moisture products, the impact of any flagging inconsistency from combining multiple sensor datasets was not assessed.</p><p>Recently, Van der Vliet et al. (2020) provided a review of the data flagging system that is used within multi-sensor ESA CCI soil moisture products as well as the flagging systems of two other soil moisture datasets from sensors that are also used for the ESA CCI soil moisture products: The level 3 Soil Moisture and Ocean Salinity (SMOS) and the Soil Moisture Active/Passive (SMAP). Substantial differences were detected between the SMOS and SMAP soil moisture flagging systems in terms of the number and type of conditions considered, critical flags, and data source dependencies. The impact on the data availability of the different flagging systems was shown to differ globally and especially for northern high latitudes, mountainous regions, and equatorial latitudes (up to 37%, 33%, and 32% respectively) with large seasonal variability. These results highlighted the relevance of a consistent and well-performing flagging approach that is applicable to all individual products used in long-term soil moisture data records.</p><p>Consequently, Van der Vliet et al. (2020) designed a consistent and model-independent flagging strategy to improve soil moisture climate records. For the snow cover, ice, and frozen conditions, which were found to have the highest impact on data availability, a uniform satellite driven flagging strategy was designed and evaluated against two ground observation networks. Compared to the individual flagging approaches adopted by the SMOS and SMAP soil moisture datasets, the new flagging approach was demonstrated to be a robust flagging alternative, with a similar performance, but with the applicability to the full ESA CCI historical record without the use of modelled approximations. </p><p>A part of the designed flagging decision tree demonstrated to form a good base for the filtering of bare grounds and heavy precipitation events as well. A future extension of the flagging strategy is expected to mask these conditions, as well as other conditions such as radio frequency interference and dense vegetation.</p>

Drought, Not War, Felled Some Ancient Asian Civilizations

Radiocarbon dating, luminescent sand grains, and climate records point to drought as the reason for the civilizations’ demise.