Research ArticleClimate-induced trends in global riverine water discharge and suspended sediment dynamics in the 21st century
Introduction
Human influence on the climate through anthropogenic greenhouse gas (GHG) emissions is leading to warming of the global climate system (IPCC, 2014). Climate warming has caused substantial changes in the hydrological cycle, altering the quantity and quality of available water resources in many regions worldwide (Bates et al., 2008). This has placed increased attention on the future of global rivers, especially how changes in climate will induce behavioral changes in fluvial systems (Bates et al., 2008; Syvitski et al., 2003; Walling, 2009). A comprehensive understanding of the response of fluvial systems to future changes in climate warrants detailed analysis of future riverine water discharge and sediment fluxes (Shrestha et al., 2016). Sediment transport by rivers plays an essential role in the functioning and connectivity of the earth's natural systems, by directly influencing ecohydrological, biogeochemical and geomorphological processes (Vörösmarty et al., 2003; Walling and Fang, 2003). It serves as an important sensitive indicator of changes in the Earth's processes (Fryirs, 2013; Walling, 2009), and is essential for studying nutrient cycles, contaminant pathways, biodiversity and habitat conditions in riverine, coastal and marine ecosystems (Mukundan et al., 2013; Syvitski and Milliman, 2007; Walling, 2009). Sediments are responsible for structuring landscape features such as deltas (Darby et al., 2015; Dunn et al., 2019) and controlling channel geometry and morphology (Pelletier, 2012; Vercruysse et al., 2017). In addition to the key role in natural planetary functions, sediment dynamics has important engineering and socio-economic implications on, e.g., dam sustainability, flood hazards, and water quality (Vercruysse et al., 2017). Although there is extensive literature with regard to estimation of sediment fluxes (e.g. Pelletier, 2012; Syvitski and Milliman, 2007; Syvitski et al., 2003; Walling and Fang, 2003), simulating global riverine sediment fluxes still remains challenging owing to the multiscale nature (Cohen et al., 2014; Pelletier, 2012; Vercruysse et al., 2017) and the non-linear relationship of the processes involved (Coulthard et al., 2012; Fryirs, 2013).
A major factor affecting changes in sediment transport and river discharge is climate (Aerts et al., 2006; Haddeland et al., 2014; Syvitski, 2003a; Syvitski, 2003b). Future changes in climate, particularly rises in temperature driven by increased GHG emissions, are projected to considerably alter 21st century precipitation intensity and distribution (IPCC, 2014; Lu et al., 2013; Oki and Kanae, 2006; Pendergrass et al., 2017). Research has shown that moderate changes in average climate conditions (i.e. changes of 1–2 °C, 10–20% precipitation) can lead to substantial changes in rivers including sediment yield (Knox, 1993; see Syvitski, 2003b). Not only average climate conditions, but also projected increases in extreme events due to climate change can have profound and complex impacts on hydrological responses of a catchment (Fryirs, 2013).
Human interferences on hydrological systems e.g., damming, soil erosion and conservation measures also have substantial influences on rivers (Walling, 2009; Wang et al., 2011; Syvitski et al., 2005). The increasing impacts of both human activities and climate change necessitate the need to identify and quantify the impacts from individual drivers on fluvial water and sediment discharges (Yang et al., 2015). Isolating the effects of changing climate as one of the primary drivers of changes in fluvial systems can facilitate more informed decision making with regard to human activities affecting hydrological systems. However it is difficult, in most cases, to disentangle the signal of climate from other human impacts (Lu et al., 2013; Walling, 2009).
A number of studies have been carried out to explore the recent trends in discharge and suspended sediment loads in global rivers at a range of scales (e.g. Cohen et al., 2014; Syvitski, 2002; Syvitski et al., 2003; Walling and Fang, 2003; Wang et al., 2011). Basin scale studies provide evidence of marked changes in the sediment loads and water discharge in recent years (Dai et al., 2009; López and Torregroza, 2017; Walling, 2009). In many instances, these changes are predicted based on the interactions between climate change and human impacts (Dai et al., 2009; Syvitski and Milliman, 2007; Syvitski et al., 2003; Walling and Fang, 2003; Wang et al., 2011). Although there is a wealth of literature related to the effects of GHG-induced global warming on future water discharge of rivers at a global scale (e.g. Milly et al., 2005; Nakaegawa et al., 2013; Nijssen et al., 2001; Sperna Weiland et al., 2012), assessments of sediment flux in response to climate change are mostly at the river catchment scale (Coulthard et al., 2012; Darby et al., 2015; Rodríguez-Blanco et al., 2016; Zhu et al., 2008). More recent studies such as Dunn et al. (2019) and Nienhuis et al. (2020) looked at changes in sediment delivery to river deltas worldwide and the different drivers responsible for these changes.
This paper is focused on providing a comprehensive and spatially explicit analysis of the natural sensitivity of global riverine water discharge and suspended sediment fluxes to future climate change trajectories. In order to achieve this objective, the study was conducted under conditions that mimic a pristine world without anthropogenic activities. This gives the opportunity to identify the direction and relative strength of the unmixed signal of GHG-induced climate change in the 21st century on global riverine fluxes, for different climate change scenarios. Existing anthropogenic activities (e.g. dams, land management practices) may hinder this signal and counter-balance the changes predicted based only on climate change.
Section snippets
Model description
Global riverine water discharge and suspended sediment fluxes were simulated using the spatially and temporally explicit global riverine sediment flux model WBMsed v2.0 (Cohen et al., 2014). WBMsed is an extension of the WBMplus global hydrology model (Wisser et al., 2010; see Cohen et al., 2013). A comprehensive description of the model infrastructure and input parameters can be found in Cohen et al. (2013 and 2014). WBMsed employs the BQART model (Syvitski and Milliman, 2007) as its governing
Model validation
Cohen et al., 2013, Cohen et al., 2014 evaluated the WBMsed model predictions of long-term averaged suspended sediment flux and water discharge (using observed climate inputs) and found a correlation of R2 = 0.66 to observed sediment flux and R2 = 0.70 to water discharge for 95 global sites. A stronger correlation was found to observed sediment flux for 11 USGS sites (R2 = 0.94). In this study, the model's forecasting capability using GCM forcings was assessed based on the ensemble hindcast
Discussion
Due to the different structures and parameters used in GCMs, projected future changes in temperature and precipitation have large spatial and temporal uncertainties even for the same radiative forcing levels (Cai et al., 2009; Knutti and Sedláček, 2013). Therefore, studies that investigate climate change responses of fluvial systems show varying degrees and directions of changes over the 21st century (Arnell, 2003; Haddeland et al., 2014; Hagemann et al., 2013; Schewe et al., 2013; van Vliet et
Conclusion
In order to isolate the signal of projected future climate change on global riverine water discharge and suspended sediment fluxes in the 21st century under pristine conditions, a numerical model (WBMsed) was forced with precipitation and temperature projections from five GCMs each driven by four RCPs. The results, based on an ensemble of model outputs, revealed that natural global river discharge and sediment fluxes are highly sensitive to anthropogenic climate change in the 21st century.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We express our gratitude to Dr. Hamid Moradkhani and Dr. Sarah Praskievicz for their constructive comments, and the anonymous reviewers for their valuable suggestions that helped us to improve the quality of this paper. This research was partly funded by the University Corporation for Atmospheric Research (UCAR), United States grant number SUBAWD000837.
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