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Delta progradation in Greenland driven by increasing glacial mass loss

Abstract

Climate changes are pronounced in Arctic regions and increase the vulnerability of the Arctic coastal zone1. For example, increases in melting of the Greenland Ice Sheet and reductions in sea ice and permafrost distribution are likely to alter coastal morphodynamics. The deltas of Greenland are largely unaffected by human activity, but increased freshwater runoff and sediment fluxes may increase the size of the deltas, whereas increased wave activity in ice-free periods could reduce their size, with the net impact being unclear until now. Here we show that southwestern Greenland deltas were largely stable from the 1940s to 1980s, but prograded (that is, sediment deposition extended the delta into the sea) in a warming Arctic from the 1980s to 2010s. Our results are based on the areal changes of 121 deltas since the 1940s, assessed using newly discovered aerial photographs and remotely sensed imagery. We find that delta progradation was driven by high freshwater runoff from the Greenland Ice Sheet coinciding with periods of open water. Progradation was controlled by the local initial environmental conditions (that is, accumulated air temperatures above 0 °C per year, freshwater runoff and sea ice in the 1980s) rather than by local changes in these conditions from the 1980s to 2010s at each delta. This is in contrast to a dominantly eroding trend of Arctic sedimentary coasts along the coastal plains of Alaska2, Siberia3 and western Canada4, and to the spatially variable patterns of erosion and accretion along the large deltas of the main rivers in the Arctic5,6,7. Our results improve the understanding of Arctic coastal evolution in a changing climate, and reveal the impacts on coastal areas of increasing ice mass loss and the associated freshwater runoff and lengthening of open-water periods.

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Figure 1: Progradation in two types of deltas.
Figure 2: Delta changes in the period between 1940s–1980s and 1980s–2010s.
Figure 3: Structural equation model representing connections between delta changes in a progradation period and factors influencing coastal evolution.

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Acknowledgements

This study would not have been possible without the aid of the Danish Geodata Agency (GST), who gave us access to the historical aerial photographs. The modern satellite imagery was obtained through the freely available online database Google Earth. M.B., A.K., A.W.-N. and B.E. acknowledge financial support from the Danish National Research Foundation (CENPERM DNRF100). L.L.I. was funded by the Carlsberg Foundation (grant 0604-02230B). A.A.B. acknowledges support from the Danish Council for Independent Research, grant DFF-610800469, and from the Inge Lehmann Scholarship from the Royal Danish Academy of Science and Letters. I.O. received support from the US National Science Foundation (NSF) Office of Polar Programs (grant ARC-0909349) and INSTAAR. S.A.K. was funded by the Danish Council for Independent Research (grant DFF-4181-00126). K.R.B. acknowledges support from the Annenberg Public Policy Center and NSF SI2-SSI Award 1450409. J.A. acknowledges the ClimateBasis programme of the Greenland Ecosystem Monitoring system (www.g-e-m.dk) and Asiaq Greenland Survey. K.L. was supported by Asiaq Greenland Survey.

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Contributions

M.B. and L.L.I. designed the study, and, together with A.A.B., B.E. and A.K., framed the research questions. M.B. and A.A.B collected the data used in the photographic data analysis, and J.A. and K.L. assembled parts of this data. I.O. and K.R.B. analysed sea-ice data, A.W.-N. and J.E.B. analysed the data for the regional climate model, S.A.K. analysed isostasy data, and M.B. and L.L.I. analysed the data and wrote the manuscript with contributions and inputs from all authors.

Corresponding author

Correspondence to Mette Bendixen.

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Reviewer Information Nature thanks M. Fritz, W. Pollard and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Examples of the identification of the delta extent.

The land–water boundary is drawn where the high-water line (a) can be identified. Presence of snowcover (b) or icebergs (c) aids the identification process. Mouth bars (d) are included as part of the delta extent. All imagery provided by Google Earth.

Extended Data Figure 2 Meta-model showing hypothesized causal links from which a structural equation model was constructed.

Justification and argumentation of each path is given in the main text and Methods section. Solid boxes indicate factors from which two options for each variable were constructed, one representing initial values in the 1980s and one representing changes from 1980s to the 2010s. Dashed boxes indicate variables that were expected to have a confounding effect on delta changes, potentially obscuring our target hypotheses.

Extended Data Figure 3 Total effect of TDD on delta changes derived from the SEM.

The top-right insert indicates the direction of the pathways from TDD to delta changes in the structural equation model presented in Fig. 3. Green bars represent unique pathways via runoff; yellow bars represent unique pathways via sea-ice extent.

Extended Data Figure 4 Spatial distribution of runoff and thawing degree days (TDD).

a, Mean runoff; b, yearly change in runoff 1981–2014; c, mean TDD; d, yearly change in TDD from 1981–2014.

Extended Data Figure 5 Spatial distribution of open-water days.

a, Mean number of open-water days; b, yearly change in open-water days from 1981 to 2014.

Extended Data Figure 6 Total runoff from the studied delta catchments as a function of the ice coverage in the catchments.

The dotted line indicates mean trend from 0% to 100% ice coverage. There is a significant linear increase in the log total runoff in the 1980s when the percentage of ice bodies in the catchment increases: slope estimate β = 0.017 [0.006; 0.028], (mean [95% confidence limits]), P < 0.01.

Extended Data Table 1 Model variables
Extended Data Table 2 Standardized partial effect sizes (and standard errors) and proposed interpretations
Extended Data Table 3 Individual paths not included in the final structural equation
Extended Data Table 4 AICc values of local submodels derived from the initial meta model

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Bendixen, M., Lønsmann Iversen, L., Anker Bjørk, A. et al. Delta progradation in Greenland driven by increasing glacial mass loss. Nature 550, 101–104 (2017). https://doi.org/10.1038/nature23873

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