Global warming may exacerbate soil moisture droughts. However, evaluations of future droughts are not conclusive because of the uncertainty in estimates of future warming. Here, we estimate the impacts of differential climate change at 1-3 K on the largest soil moisture droughts across Europe to understand the implications of the goal of the 2015 Paris climate change agreement to constrain global warming to below 1.5 degrees. The results show that under an increase of 3 K compared to 1.5 K, drought area will increase by 40% (± 24%) and will potentially affect 42% more people. Similarly, an event like the 2003 drought will become two times more frequent. Adapting to a temperature increase of 3 K implies adjusting to an increase

Abstract. Data assimilation (DA) holds considerable potential for improving hydrologic predictions as demonstrated in numerous research studies. However, advances in hydrologic DA research have not been adequately or timely implemented in operational forecast systems to improve the skill of forecasts for better informed real-world decision making. This is due in part to a lack of mechanisms to properly quantify the uncertainty in observations and forecast models in real-time forecasting situations and to conduct the merging of data and models in a way that is adequately efficient and transparent to operational forecasters.The need for effective DA of useful hydrologic data into the forecast process has become increasingly recognized in recent years. This motivated a hydrologic DA workshop in Delft, the Netherlands in November 2010, which focused on advancing DA in operational hydrologic forecasting and water resources management. As an outcome of the workshop, this paper reviews, in relevant detail, the current status of DA applications in both hydrologic research and operational practices, and discusses the existing or potential hurdles and challenges in transitioning hydrologic DA research into cost-effective operational forecasting tools, asPublished by Copernicus Publications on behalf of the European Geosciences Union. Y. Liu et al.: Advancing data assimilation in operational hydrologic forecastingwell as the potential pathways and newly emerging opportunities for overcoming these challenges. Several related aspects are discussed, including (1) theoretical or mathematical aspects in DA algorithms, (2) the estimation of different types of uncertainty, (3) new observations and their objective use in hydrologic DA, (4) the use of DA for real-time control of water resources systems, and (5) the development of community-based, generic DA tools for hydrologic applications. It is recommended that cost-effective transition of hydrologic DA from research to operations should be helped by developing community-based, generic modeling and DA tools or frameworks, and through fostering collaborative efforts among hydrologic modellers, DA developers, and operational forecasters.

Since the spring 2018, a large part of Europe has been in the midst of a record-setting drought. Using long-term observations, we demonstrate that the occurrence of the 2018-2019 (consecutive) summer drought is unprecedented in the last 250 years, and its combined impact on the growing season vegetation activities is stronger compared to the 2003 European drought. Using a suite of climate model simulation outputs, we underpin the role of anthropogenic warming on exacerbating the future risk of such a consecutive drought event. Under the highest Representative Concentration Pathway, (RCP 8.5), we notice a seven-fold increase in the occurrence of the consecutive droughts, with additional 40 (± 5) million ha of cultivated areas being affected by such droughts, during the second half of the twenty-first century. The occurrence is significantly reduced under low and medium scenarios (RCP 2.6 and RCP 4.5), suggesting that an effective mitigation strategy could aid in reducing the risk of future consecutive droughts. Human-induced climate change is evident and it poses a great concern to society, primarily due to its potential to intensify extreme events around the globe 1,2. In the past 2 decades, Europe experienced an increased frequency of droughts 3,4 with estimated loss of about EUR 100 billion 5. One such devastating event was the drought in summer 2003, which was an exceptionally warm and dry year across most of central and western Europe. Historical reconstructions since 1500 C.E. suggest that it was one of the hottest summers 6 , and the event was estimated to result in a 30% reduction in gross primary production compared to previous years between 1998-2002 3. Although, the 2003 drought event was rare and exceptional, even in a multi-centennial time window, its likelihood is expected to increase in the near future 7 , mainly due to the anthropogenic warming 8-11. In the summer of 2018, temperature anomaly broke the record again in several locations across Europe, but with distinct spatial patterns. While in summer 2003 the increase in temperature was more concentrated in central and southern Europe (Fig. 1a), summer 2018 was characterised by an anomalous increase in central and northeastern Europe (Fig. 1b). Unlike the 2003 event-where the temperature anomaly (Supplementary Fig. S1) and the ecosystem carbon and energy fluxes recovered early after the summer 12 , the extreme event of 2018 persisted to the subsequent year 2019 (Fig. 1c). For all these years, the impact was strongest in the Central European region, where the increase in temperature was accompanied by concurrent significant reduction of summer precipitation (Fig. 1d-f), which led to extreme drought conditions. The intensity and spatial extent of droughts significantly affects the plant and agricultural productivity 13,14 , underlying the severity of the drought impact in Central European region, where the focus on agriculture is strong 3,7,15-17. With the use of remote sensing data-sets 18 , we find that the concurrent increased temperature with de...