Attributing the 2017 Bangladesh floods from meteorological and hydrological perspectives

Abstract

Abstract. In August 2017 Bangladesh faced one of its worst river flooding events in recent history. This paper presents for the first time an attribution of this precipitation-induced flooding from a combined meteorological and hydrological perspective. Experiments were conducted with three observational data sets and two climate models to estimate changes in extreme 10-day precipitation event frequency over the Brahmaputra basin. The precipitation fields were then used as meteorological input for four different hydrological models to estimate the corresponding changes in river discharge, allowing for comparison between approaches and for the robustness of the attribution results to be assessed. In all three observational precipitation data sets the climate change trends for extreme precipitation similar to observed in August 2017 are not significant, however in two out of three series, the sign of this insignificant trend is positive. One climate model shows a significant positive influence of anthropogenic climate change, whereas the other simulates a cancellation between the increase due to greenhouse gases and a decrease due to sulphate aerosols. Considering discharge rather than precipitation, the hydrological models show that attribution of the change in discharge towards higher values is somewhat less uncertain than for precipitation, but the 95 % confidence interval still encompasses no change in risk. For the future, all models project an increase in probability of extreme events at 2 °C global heating since pre-industrial times, becoming more than 1.7 times more likely for high 10-day precipitation, and about a factor 1.5 more likely for discharge. Our best estimate on the trend in flooding events similar to the Brahmaputra event of August 2017 is derived by synthesizing the observational and model results: We find the change in risk to be greater than one and of similar order of magnitude (between 1 and 2) for both the meteorological and hydrological approach. This study shows that, for precipitation-induced flooding events, investigating changes in precipitation is useful, either as an alternative when hydrological models are not available, or as an additional measure to confirm qualitative conclusions. Besides, it highlights the importance of using multiple models in attribution studies, particularly where the climate change signal is not strong relative to natural variability or is confounded by other factors such as aerosols.

Added Value of Large Ensemble Simulations for Assessing Extreme River Discharge in a 2 °C Warmer World

“…Projected changes include changes in climatological precipitation patterns, precipitation types, evaporation amounts, soil moisture availability, and discharge levels (e.g., Berghuijs et al, 2014;Held & Soden, 2006;Intergovernmental Panel on Climate Change, 2013;Milly et al, 2005;Wanders & Wada, 2015a). For instance, the recent drought in South Africa (2015 to present, Baudoin et al, 2017) and the 2017 floods in Bangladesh (Philip et al, 2018) had severe negative consequences on people's livelihoods, agricultural production, and the national economy. These extreme events can have severe negative impacts on societies and ecosystems.…”

“…These extreme events can have severe negative impacts on societies and ecosystems. For instance, the recent drought in South Africa (2015 to present, Baudoin et al, 2017) and the 2017 floods in Bangladesh (Philip et al, 2018) had severe negative consequences on people's livelihoods, agricultural production, and the national economy. Scientific understanding of the probability of occurrence, severity, and characteristics of extreme hydrological events is therefore of societal importance.…”

Development of ecosystem‐based flood mitigation approach – investigations by experiments and numerical simulation

“…Despite optimal tsunami mitigation measures, the violent flooding caused by the Great East Japan tsunami destroyed not only coastal embankments, but also affected coastal forests, tsunami gates, inland buildings and human lives (Suppasri et al, 2013). Similar trends were observed in South Asian countries like India, Bangladesh and Indonesia where intense rainfall has affected millions of people and caused significant damages to infrastructures (Marfai and Hizbaron, 2011;Houze et al, 2017;Philip et al, 2019). In Pakistan alone, around 12,330 people lost their lives, 197,273 villages were damaged or destroyed and an area of more than 616,598 km 2 was flooded during the last 71 years (Flood Forecasting and Warning Centre, 2017).…”

“…Their study included the VOF method along with various turbulence models to estimate water surface profiles. In addition, VOF multiphase modelling was employed for open channel flows (Patel and Gill, 2006). Debnath et al .…”

Climate change attribution and the economic costs of extreme weather events: a study on damages from extreme rainfall and drought

“…Furthermore, cascading impacts-impacts in which anthropogenic climate change effects are mediated by other systems-present additional challenges for climate change attribution frameworks (Otto et al 2017;Challinor et al 2018;Lawrence et al 2019). One future development that is directly relevant is the integration of climate and hydrological work within an attribution setting (Hidalgo et al 2009;Kay et al 2018;Philip et al 2019). Utilising an integrated hydrological and climate infrastructure to calculate FARs would allow better quantification of flood risk, as well as improvements in our understanding of current and future climate risks.…”