Australian Coastal Flooding Trends and Forcing Factors

Abstract

Using the new Australian National Collection of Homogenized Observations of Relative Sea Level (ANCHORS) dataset, we assess trends in Australian relative sea levels over recent decades and subsequent coastal flooding impacts. We estimate a gauge average rate of mean sea level rise over the 1966–2019 period of 1.94 mm/yr with local variations around the Australian continent. Simultaneously, the frequency of coastal flooding impacts has increased at many major Australian cities including Sydney, Melbourne, Brisbane, Adelaide, and Perth. We find that this increase is not because storm surges are getting larger or more frequent, but because tides are reaching higher levels as they rise and fall about higher mean sea levels. This demonstrates that a major shift in the processes that lead to coastal flooding is underway, arising directly from global mean sea level rise, and is consistent with findings from the United States. This suggests that new perspectives on extreme sea levels are required, so research can be more impact-based and meet the needs of policymakers planning for these impacts. Considering extreme sea levels more broadly, we show that the seasonality of extreme sea levels is closely linked to the monthly variability in the heights of the highest tides. This framework provides a holistic assessment of coastal flood risk in Australia, based on established impact-based methodologies.