More meteorological events that drive compound coastal flooding are projected under climate change

A significant portion of the global population resides in low-lying coastal regions vulnerable to flooding from sea-level rise and extreme precipitation/river discharge. The simultaneous or near-simultaneous occurrence of these hazards—compound flooding—can lead to substantially greater impacts than individual events. Recent events in various locations worldwide have exemplified the devastating consequences of compound flooding. This type of flooding occurs when inland rainfall is intensified by high meteorological tides hindering drainage into the sea, or when meteorological tides are exacerbated by precipitation. While compound flooding has been studied at various scales for the present climate, understanding its future dynamics under climate change remains crucial, especially given the projected increase in coastal populations. Previous research has focused on mid-latitude events and assessed present-day compound flood hazard for specific regions, often relying on field observations with limited global coverage. Recent advancements in ocean modeling now provide continuous, sub-daily global sea-level time-series. This study leverages these advancements along with precipitation data to conduct a comprehensive global assessment of meteorological drivers of compound flooding and their projected changes under climate change. The analysis focuses on the probability of concurrent meteorological tide and precipitation extremes near the coast, providing insights into the large-scale rainfall-driven compound flooding potential in low-lying areas and estuaries of smaller rivers.

Existing literature on compound flooding encompasses local to global scales, focusing on present-day hazards. Studies of recent mid-latitude events have improved understanding of the mechanisms driving compound flooding. Global assessments of compound flood hazards have been conducted for regions like the United States, Australia, and Europe, often considering the co-occurrence of sea-level extremes and either precipitation or river discharge extremes. However, most earlier studies relied on field observations of sea level, lacking complete global coverage. Advances in ocean modeling have generated globally comprehensive, sub-daily sea-level time series. These improvements, combined with precipitation or river discharge estimates, have enabled more thorough global assessments of present-day compound flood hazard. For example, studies have highlighted the exacerbating effect of storm surges on 1-in-10-year flood levels in many global deltas. Previous work acknowledges the future influence of sea level rise (SLR) on compound flood hazards, but also recognizes that meteorological drivers like extreme precipitation and meteorological tides will be significantly altered by climate change. While some studies have examined these effects for specific regions, such as Europe's coasts, a comprehensive global assessment was lacking before this study.

This research combined outputs from global climate models (GCMs) and ocean models to evaluate the spatio-temporal dynamics of compound flooding's meteorological drivers along global coastlines. Daily meteorological tide time series were derived by superimposing storm surges and waves from ocean model simulations. These simulations utilized reanalysis data for the observed past and CMIP5 GCM climate projections for future climate change under a high emissions scenario (RCP8.5). To enhance the representation of tropical cyclone (TC)-driven meteorological tides, the reanalysis-based dataset incorporated dynamically downscaled atmospheric conditions and wave corrections based on satellite altimetry data. However, this refinement was not applied to CMIP5-based simulations, introducing some limitations to the projections in regions with significant TC activity. Daily precipitation time series from reanalysis data and GCMs were aggregated over 3-day windows. The aggregation of precipitation data did not aim to represent compound flood potential in large river estuaries but rather focus on local rainfall-driven compound floods and smaller and medium-sized rivers. Extremes of meteorological tide and aggregated precipitation were defined as events occurring once yearly on average in the present climate. The joint return period (inverse probability) of concurring extremes in present and future climates was assessed using bivariate distributions based on parametric copulas, which model pairs of high values. The study assessed present-day (1980–2014) probabilities of concurring meteorological extremes, analyzing seasonality and drivers. It then analyzed changes by the end of the century (2070–2099) compared to the recent past (1970–2004). Finally, it quantified the contribution of each meteorological driver (precipitation and meteorological tides) and their interdependence to the projected changes, also investigating the uncertainties.

The analysis revealed a strong correlation between ocean and inland meteorological extremes in most coastlines globally, indicating a significantly higher likelihood of co-occurrence than expected under independence. However, the concurrence probability shows substantial spatial variability, influenced by cyclonic atmospheric flows. Regions with high tropical (TC) or extratropical (ETC) cyclone activity, such as the United States, Europe, and parts of Australia, experience the highest concurrence probability. The Northern Hemisphere generally exhibits higher concurrence probabilities than the Southern Hemisphere. In the tropics, the highest frequency of concurring events tends to align with the TC season. At mid-latitudes, the concurrence frequency peaks in autumn-winter. Under a high emissions scenario (RCP8.5), the concurrence probability of meteorological drivers of compound flooding is projected to increase along approximately 60% of the global coastline by 2100. Globally, the median change in return period is -20%, meaning joint extreme events become 26% more probable. Latitudes above 40° North show the most significant increase, with an average frequency 2.6 times higher than the present. Conversely, the probability decreases in some regions, such as parts of northwestern Africa and western South Africa. The tropics and subtropics exhibit more uncertainty in the projections. Changes in extreme precipitation are the most substantial drivers of projected changes in compound meteorological extremes, increasing the concurrence frequency for 83% of global coastlines. Changes in meteorological tides have a weaker but still notable effect. Changes in the dependence between high meteorological tides and extreme precipitation show strong regional variations but have relatively small global average effects compared to natural variability. Regional level analysis showed precipitation changes were the dominant driver in most IPCC regions, except for South Europe and South Africa (where changes in meteorological tides dominated) and a few others where changes in dependence dominated. The uncertainty in the projections of future concurrence probability is largely driven by the uncertainty in the dependence between meteorological extremes. At the global scale, climate variability in the projected dependence accounts for approximately half of the overall uncertainty, with precipitation and meteorological tide uncertainties comprising the other half.

This study offers the first global-scale assessment of climate change's impact on the meteorological drivers of compound flooding. While the results do not directly quantify compound flood hazard but rather the probability of co-occurrence of meteorological extremes, this approach offers valuable insights into large-scale rainfall-driven compound flooding in low-lying coastal areas and smaller river estuaries. Sea-level rise (SLR) will significantly increase the probability of compound flooding by pushing mean sea levels upwards. This study reveals the substantial additional impact of changes in the joint probability of high meteorological tides and extreme precipitation, further contributing to compound flooding risks globally. The regional variations in projected changes highlight the need for local assessments in areas with robust and significant projected changes to refine compound flood hazard and impact assessments. This understanding is crucial for effective coastal community adaptation strategies. Considering the projected increase in frequency of concurrent extreme events, improved emergency response planning and more resilient protective structures are essential.

This research provides the first global-scale assessment of how climate change will affect the meteorological conditions that cause coastal compound flooding. Our findings show a significant projected increase in the probability of these events globally, particularly at higher northern latitudes. Changes in extreme precipitation are the main driver of this increase, while the dependence between precipitation and meteorological tides is a major source of uncertainty in the projections. These results emphasize the need for coastal communities to consider compound flooding risks in their adaptation plans, highlighting the importance of robust infrastructure and emergency response strategies.

The study acknowledges several limitations inherent to global-scale analyses, such as the use of relatively coarse-resolution input data in the CMIP5-based simulations, potentially underestimating the impact of tropical cyclones. The study also does not fully capture the physical interactions between waves, storm surges, astronomical tides, and sea-level rise. The focus on smaller and medium-sized rivers rather than large river systems also limits the generalizability of the findings to certain regions. Further high-resolution studies focusing on hotspot regions are needed to better refine the estimations and understand the local complexities of compound flooding.