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Droughts are often long-lasting phenomena, without a distinct start or end and with impacts cascading across sectors and systems, creating long-term legacies. Nevertheless, our current perceptions and management of droughts and their impacts are often event-based, which can limit the effective assessment of drought risks and reduction of drought impacts. Here, we advocate for changing this perspective and viewing drought as a hydrological–ecological–social continuum. We take a systems theory perspective and focus on how “memory” causes feedback and interactions between parts of the interconnected systems at different timescales. We first discuss the characteristics of the drought continuum with a focus on the hydrological, ecological, and social systems separately, and then we study the system of systems. Our analysis is based on a review of the literature and a study of five cases: Chile, the Colorado River basin in the USA, northeast Brazil, Kenya, and the Rhine River basin in northwest Europe. We find that the memories of past dry and wet periods, carried by both bio-physical (e.g. groundwater, vegetation) and social systems (e.g. people, governance), influence how future drought risk manifests. We identify four archetypes of drought dynamics: impact and recovery, slow resilience building, gradual collapse, and high resilience–big shock. The interactions between the hydrological, ecological, and social systems result in systems shifting between these types, which plays out differently in the five case studies. We call for more research on drought preconditions and recovery in different systems, on dynamics cascading between systems and triggering system changes, and on dynamic vulnerability and maladaptation. Additionally, we advocate for more continuous monitoring of drought hazards and impacts, modelling tools that better incorporate memories and adaptation responses, and management strategies that increase societal and institutional memory. This will help us to better deal with the complex hydrological–ecological–social drought continuum and identify effective pathways to adaptation and mitigation. 

Abstract. Human-induced atmospheric composition changes cause a radiative imbalance atthe top of the atmosphere which is driving global warming. This Earth energy imbalance (EEI) is the most critical number defining the prospects for continued global warming and climate change. Understanding the heat gain ofthe Earth system – and particularly how much and where the heat isdistributed – is fundamental to understanding how this affects warmingocean, atmosphere and land; rising surface temperature; sea level; and lossof grounded and floating ice, which are fundamental concerns for society.This study is a Global Climate Observing System (GCOS) concertedinternational effort to update the Earth heat inventory and presents anupdated assessment of ocean warming estimates as well as new and updated estimatesof heat gain in the atmosphere, cryosphere and land over the period1960–2018. The study obtains a consistent long-term Earth system heat gainover the period 1971–2018, with a total heat gain of 358±37 ZJ,which is equivalent to a global heating rate of 0.47±0.1 W m−2.Over the period 1971–2018 (2010–2018), the majority of heat gain is reportedfor the global ocean with 89 % (90 %), with 52 % for both periods inthe upper 700 m depth, 28 % (30 %) for the 700–2000 m depth layer and 9 % (8 %) below 2000 m depth. Heat gain over land amounts to 6 %(5 %) over these periods, 4 % (3 %) is available for the melting ofgrounded and floating ice, and 1 % (2 %) is available for atmospheric warming. Ourresults also show that EEI is not only continuing, but also increasing: the EEIamounts to 0.87±0.12 W m−2 during 2010–2018. Stabilization ofclimate, the goal of the universally agreed United Nations Framework Convention on ClimateChange (UNFCCC) in 1992 and the ParisAgreement in 2015, requires that EEI be reduced to approximately zero toachieve Earth's system quasi-equilibrium. The amount of CO2 in theatmosphere would need to be reduced from 410 to 353 ppm to increase heatradiation to space by 0.87 W m−2, bringing Earth back towards energybalance. This simple number, EEI, is the most fundamental metric that thescientific community and public must be aware of as the measure of how wellthe world is doing in the task of bringing climate change under control, andwe call for an implementation of the EEI into the global stocktake based onbest available science. Continued quantification and reduced uncertaintiesin the Earth heat inventory can be best achieved through the maintenance ofthe current global climate observing system, its extension into areas ofgaps in the sampling, and the establishment of an international framework forconcerted multidisciplinary research of the Earth heat inventory aspresented in this study. This Earth heat inventory is published at the German Climate Computing Centre (DKRZ, https://www.dkrz.de/, last access: 7 August 2020) under the DOIhttps://doi.org/10.26050/WDCC/GCOS_EHI_EXP_v2(von Schuckmann et al., 2020).