In this study, we investigate changes in temperature and precipitation extremes over West and Central Africa (hereafter, WAF domain) as a function of global mean temperature with a focus on the implications of global warming of 1.5 • C and 2 • C according the Paris Agreement. We applied a scaling approach to capture changes in climate extremes with increase in global mean temperature in several subregions within the WAF domain: Western Sahel, Central Sahel, Eastern Sahel, Guinea Coast and Central Africa including Congo Basin.While there are several uncertainties and large ensemble spread in the projections of temperature and precipitation indices, most models show high-impact changes in climate extremes at subregional scale. At these smaller scales, temperature increases within the WAF domain are projected to be higher than the global mean temperature increase (at 1.5 • C and at 2 • C) and heat waves are expected to be more frequent and of longer duration. The most intense warming is observed over the drier regions of the Sahel, in the central Sahel and particularly in the eastern Sahel, where the precipitation and the soil moisture anomalies have the highest probability of projected increase at a global warming of 1.5 • C. Over the wetter regions of the Guinea Coast and Central Africa, models project a weak change in total precipitation and a decrease of the length of wet spells, while these two regions have the highest increase of heavy rainfall in the WAF domain at a global warming of 1.5 • C. Western Sahel is projected by 80% of the models to experience the strongest drying with a significant increase in the length of dry spells and a decrease in the standardized precipitation evapotranspiration index. This study suggests that the 'dry gets drier, wet gets wetter' paradigm is not valid within the WAF domain.
Rivers receive large amounts of nitrogen (N) from their watershed and are the final sites of nutrient processing before delivery to coastal waters. Transformations of dissolved inorganic N (DIN) to gaseous N within rivers can impact both coastal eutrophication and greenhouse gas emissions. Vegetated shallows of rivers are sites of active metabolism and may act as hot spots for N transformation, but little is known about the variability of denitrification within shallows or the role of vegetation structure in controlling this variability. We measured in situ N loss and accumulation of N2 and N2O in vegetated shallows of the tidal Hudson River and used regression models to determine the role of plant species in different monospecific beds in ecosystem N loss. N2 production was highly variable between vegetated shallows and was associated with species‐driven differences in dissolved oxygen (DO) dynamics during the ebb tide. N2 production was extremely high (37–71 mmol N·m−2·d−1) in beds with invasive floating‐leaved plants (Trapa natans) but was insignificant in submersed native vegetation (Vallisneria americana). In Trapa sites, N2 production was strongly related to metabolism. Change in DO concentrations in the surrounding water due to atmospheric venting by the plants during ebb tide, combined with changes in water temperatures, explained 87% of the variation of the observed N2 production. Despite these high denitrification losses, beds acted as N2O sinks where N2O concentrations became undersaturated during ebb tide. An estimate of summertime N2 production in Trapa beds, based on continuously measured oxygen and temperature by moored sondes, suggests that these beds are a major seasonal hot spot for N removal. Large Trapa beds represent only 2.7% of the total area of the tidal Hudson, but they remove between 70% and 100% of the total N retained in this river reach during summer months. Although they are active for only three months of the year, Trapa shallows contribute to as much as 25% of the annual N removal. Trapa activity represents an important ecosystem service, modulated by its impacts on DO as a function of Trapa's growth form trait and modulated by the physical properties of the environment.
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