Assessment of agricultural drought over Africa and its relation to El Niño-Southern Oscillation using remote sensing-based time series

Ranked amongst the most destructive natural disasters of the world, droughts may have severe impacts on ecosystems and society. Particularly in Africa, where water is a limiting factor and countries strongly rely on rain-fed agriculture, droughts have constantly led to widespread crop failure, food shortages and even humanitarian crises. In regions over eastern and southern Africa, such dry conditions have been attributed to the effect of El Niño-Southern Oscillation (ENSO). Given the recent El Niño episode of 2015/16 and the associated severe droughts that occurred in many parts of Africa, this interconnection has once again become an issue of importance. In this regard, remote sensing data and image analysis provide new opportunities for generating substantial information on the evolution of droughts at large spatial and temporal scales. This thesis focusses on monitoring agricultural droughts over Africa during 2000-2016 and their relation to ENSO by means of remote sensing time series. The used continental-scale approach is based on drought indices. In particular, TRMM-based Standardized Precipitation Index (SPI) and MODIS-derived Vegetation Condition Index (VCI) were used for analysing the spatio-temporal patterns of agricultural droughts. All in all, a comprehensive insight into the evolution of agricultural droughts in Africa was gained. The applicability of SPI and VCI as indices for continental-scale drought monitoring was proven. Observed discrepancies were linked with variabilities in sensitivity of vegetation to rainfall over Africa, which in turn merits further research. Moreover, the relation between droughts and ENSO was examined by applying a correlation analysis between time series of drought indices and Multivariate ENSO Index (MEI). This complex relationship could be described in its fundamentals. Based on revealed correlation patterns, droughts tend to occur during El Ni~no over large parts of southern Africa. In contrast, a divided pattern was observed in eastern Africa, where areas with bimodal annual rainfall cycles tend to be affected by droughts during La Niña and, in zones of unimodal rainfall regimes, droughts tend to arise during the onset of El Niño. However, no universal El Niño- or La Niña-related response pattern of droughts could be deduced. In this regard, multi-year atmospheric fluctuations and characteristics of ENSO variants were discussed as possible influencing factors. Regional impacts of the drought episodes during El Niño 2002/03 and La Niña 2010/11 were illuminated by comparing observed regional drought patterns with statistics on national crop production. Focus is laid on each southern and eastern Africa, where decreases in production numbers were observed for major drought-affected countries. In order to achieve improvements in quality and reliability of the output, the incorporation of more accurate cropland information, adaptions of the correlation analysis as well as an uncertainty assessment and a validation are proposed. Using remote sensing data as a toolset for drought monitoring, this thesis represents an attempt to contribute to a better understanding of spatio-temporal patterns of agricultural droughts in Africa and their dependencies. Such knowledge is essential as it forms the basis for implementing strategies of drought hazard mitigation in the affected regions

Global land use changes are four times greater than previously estimated

Die Quantifizierung der Dynamik von Landnutzungsänderungen ist entscheidend für die Bewältigung globaler gesellschaftlicher Herausforderungen wie Ernährungssicherheit, Klimawandel und Verlust der biologischen Vielfalt. Hier analysieren wir die Dynamik des globalen Landnutzungswandels mit einer noch nie dagewesenen räumlichen Auflösung, indem wir mehrere offene Datenströme (Fernerkundung, Rekonstruktionen und Statistiken) kombinieren, um das HIstoric Land Dynamics Assessment + (HILDA +) zu erstellen. Wir schätzen, dass der Landnutzungswandel in nur sechs Jahrzehnten (1960-2019) fast ein Drittel (32 %) der globalen Landfläche betroffen hat und damit etwa viermal so groß ist, wie bisher aus langfristigen Landänderungsanalysen geschätzt. Wir identifizieren auch geografisch divergierende Prozesse der Landnutzungsänderung, mit Aufforstung und Aufgabe von Ackerland im globalen Norden und Entwaldung und landwirtschaftlicher Expansion im Süden. Hier zeigen wir, dass die beobachteten Phasen des beschleunigten (~1960-2005) und verlangsamten (2006-2019) Landnutzungswandels durch die Auswirkungen des globalen Handels auf die landwirtschaftliche Produktion erklärt werden können

Three billion new trees in the EU’s biodiversity strategy: low ambition, but better environmental outcomes?

The EU Biodiversity strategy aims to plant 3 billion trees by 2030, in order to improve ecosystem restoration and biodiversity. Here, we compute the land area that would be required to support this number of newly planted trees by taking account of different tree species and planting regimes across the EU member states. We find that 3 billion trees would require a total land area of between 0.81 and 1.37 Mha (avg. 1.02 Mha). The historic forest expansion in the EU since 2010 was 2.44 Mha, meaning that despite 3 billion trees sounding like a large number this target is considerably lower than historic afforestation rates within the EU, i.e. only 40% of the past trend. Abandoned agricultural land is often proposed as providing capacity for afforestation. We estimate agricultural abandoned land areas from the HIstoric Land Dynamics Assessment+ database using two time thresholds (abandonment since 2009 or 2014) to identify potential areas for tree planting. The area of agricultural abandoned land was 2.6 Mha (potentially accommodating 7.2 billion trees) since 2009 and 0.2 Mha (potentially accommodating 741 million trees) since 2014. Our study highlights that sufficient space could be available to meet the 3 billion tree planting target from abandoned land. However, large-scale afforestation beyond abandoned land could have displacement effects elsewhere in the world because of the embodied deforestation in the import of agricultural crops and livestock. This would negate the expected benefits of EU afforestation. Hence, the EU\u27s relatively low ambition on tree planting may actually be better in terms of avoiding such displacement effects. We suggest that tree planting targets should be set at a level that considers physical ecosystem dynamics as well as socio-economic conditions

TerraSAR-X and Wetlands: A Review

Since its launch in 2007, TerraSAR-X observations have been widely used in a broad range of scientific applications. Particularly in wetland research, TerraSAR-X\u27s shortwave X-band synthetic aperture radar (SAR) possesses unique capabilities, such as high spatial and temporal resolution, for delineating and characterizing the inherent spatially and temporally complex and heterogeneous structure of wetland ecosystems and their dynamics. As transitional areas, wetlands comprise characteristics of both terrestrial and aquatic features, forming a large diversity of wetland types. This study reviews all published articles incorporating TerraSAR-X information into wetland research to provide a comprehensive study of how this sensor has been used with regard to polarization, and the function of the data, time-series analyses, or the assessment of specific wetland ecosystem types. What is evident throughout this literature review is the synergistic fusion of multi-frequency and multi-polarization SAR sensors, sometimes optical sensors, in almost all investigated studies to attain improved wetland classification results. Due to the short revisiting time of the TerraSAR-X sensor, it is possible to compute dense SAR time-series, allowing for a more precise observation of the seasonality in dynamic wetland areas as demonstrated in many of the reviewed studies

Spatiotemporal pattern of global forest change over the past 60 years and the forest transition theory

Forest ecosystems play an indispensable role in addressing various pressing sustainability and social-ecological challenges such as climate change and biodiversity loss. However, global forest loss has been, and still is today, an important issue. Here, based on spatially explicit data, we show that over the past 60 years (1960–2019), the global forest area has declined by 81.7 million ha (i.e. 10% more than the size of the entire Borneo island), with forest loss (437.3 million ha) outweighing forest gain (355.6 million ha). With this forest decline and the population increase (4.68 billion) over the period, the global forest per capita has decreased by over 60%, from 1.4 ha in 1960 to 0.5 ha in 2019. The spatiotemporal pattern of forest change supports the forest transition theory, with forest losses occurring primarily in the lower income countries in the tropics and forest gains in the higher income countries in the extratropics. Furthermore, economic growth has a stronger association with net forest gain than with net forest loss. Our results highlight the need to strengthen the support given to lower income countries, especially in the tropics, to help improve their capacity to minimize or end their forest losses. To help address the displacement of forest losses to the lower income countries in the tropics, higher income nations need to reduce their dependence on imported tropical forest products

Fourteen propositions for resilience, fourteen years later

In 2006, Walker et al. published an article titled, “A Handful of Heuristics and Some Propositions for Understanding Resilience in Social-ecological Systems.” The article was incorporated into the Ecology and Society special feature, Exploring Resilience in Social-Ecological Systems. Walker et al. identified five heuristics and posed 14 propositions for understanding resilience in social-ecological systems. At the time, the authors hoped the paper would promote experimentation, critique, and application of these ideas in resilience and social-ecological systems research. To determine the extent to which these propositions have achieved the authors’ hopes, we reviewed the scientific literature on socialecological systems since the article was published. Using Scopus, we identified 627 articles that cited the Walker et al. article. We then identified and assessed the articles relative to each proposition. In addition, we conducted a more general Scopus review for articles that did not cite the Walker et al. article specifically but incorporated a proposition’s concepts. Overall, articles often cite Walker et al. as a reference for a definition of a heuristic or ecological resilience generally and not to reference a specific proposition. Nonetheless, every proposition was at least mentioned in the literature and used to advance resilience scholarship on social-ecological systems. Eleven propositions were tested by multiple articles through application of case studies or other research, and 7 of the 11 propositions were substantially discussed and advanced. Finally, three propositions were heavily critiqued either as concepts in resilience literature or in their application

Changes in land use and management led to a decline in Eastern Europe’s terrestrial carbon sink

Land-based mitigation is essential in reducing net carbon emissions. Yet, the attribution of carbon fluxes remains highly uncertain, in particular for the forest-rich region of Eastern Europe (incl. Western Russia). Here we integrate various data sources to show that Eastern Europe accounted for an above-ground biomass carbon sink of ~0.41 gigatons of carbon per year over the period 2010–2019, that is 78% of the entire European carbon sink. We find that this carbon sink is declining, mainly driven by changes in land use and land management, but also by increasing natural disturbances. Based on a random forest model, we show that land use and management changes are main drivers of the declining carbon sink in Eastern Europe, although soil moisture variability is also important. Specifically, the saturation effect of tree regrowth in abandoned agricultural areas, combined with increasing wood harvest removals, particularly in European Russia, contributed to the decrease in the Eastern European carbon sink

On the use of Earth Observation to support estimates of national greenhouse gas emissions and sinks for the Global stocktake process: lessons learned from ESA-CCI RECCAP2

The Global Stocktake (GST), implemented by the Paris Agreement, requires rapid developments in the capabilities to quantify annual greenhouse gas (GHG) emissions and removals consistently from the global to the national scale and improvements to national GHG inventories. In particular, new capabilities are needed for accurate attribution of sources and sinks and their trends to natural and anthropogenic processes. On the one hand, this is still a major challenge as national GHG inventories follow globally harmonized methodologies based on the guidelines established by the Intergovernmental Panel on Climate Change, but these can be implemented differently for individual countries. Moreover, in many countries the capability to systematically produce detailed and annually updated GHG inventories is still lacking. On the other hand, spatially-explicit datasets quantifying sources and sinks of carbon dioxide, methane and nitrous oxide emissions from Earth Observations (EO) are still limited by many sources of uncertainty. While national GHG inventories follow diverse methodologies depending on the availability of activity data in the different countries, the proposed comparison with EO-based estimates can help improve our understanding of the comparability of the estimates published by the different countries. Indeed, EO networks and satellite platforms have seen a massive expansion in the past decade, now covering a wide range of essential climate variables and offering high potential to improve the quantification of global and regional GHG budgets and advance process understanding. Yet, there is no EO data that quantifies greenhouse gas fluxes directly, rather there are observations of variables or proxies that can be transformed into fluxes using models. Here, we report results and lessons from the ESA-CCI RECCAP2 project, whose goal was to engage with National Inventory Agencies to improve understanding about the methods used by each community to estimate sources and sinks of GHGs and to evaluate the potential for satellite and in-situ EO to improve national GHG estimates. Based on this dialogue and recent studies, we discuss the potential of EO approaches to provide estimates of GHG budgets that can be compared with those of national GHG inventories. We outline a roadmap for implementation of an EO carbon-monitoring program that can contribute to the Paris Agreement