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Heatwaves represent a significant natural hazard in Australia, arguably more hazardous to human life than bushfires, tropical cyclones and floods. In the 2008/2009 summer, for example, many more lives were lost to heatwaves than to that summer’s bushfires which were among the worst in the history of the Australian nation. For many years, these other forms of natural disaster have received much greater public attention than heatwaves, although there are some signs of change. We propose a new index, called the excess heat factor (EHF) for use in Australian heatwave monitoring and forecasting. The index is based on a three-day-averaged daily mean temperature (DMT), and is intended to capture heatwave intensity as it applies to human health outcomes, although its usefulness is likely to be much broader and with potential for international applicability. The index is described and placed in a climatological context in order to derive heatwave severity. Heatwave severity, as characterised by the climatological distribution of heatwave intensity, has been used to normalise the climatological variation in heatwave intensity range across Australia. This methodology was used to introduce a pilot national heatwave forecasting service for Australia during the 2013/2014 summer. Some results on the performance of the service are presented.

The establishment of an effective policy response to rising heatwave impacts is most effective when the history of heatwaves, their current impacts and future risks, are mapped by a common metric. In response meteorological agencies aim to develop seamless climate, forecast, and warning heat impact services, spanning all temporal and spatial scales. The ability to diagnose heatwave severity using the Excess Heat Factor (EHF) has allowed the Australian Bureau of Meteorology (the Bureau) to publicly release 7-day heatwave severity maps since 2014. National meteorological agencies in the UK and the United States are evaluating global 7-day and multi-week EHF heatwave severity probability forecasts, whilst the Bureau contributes to a Copernicus project to supply the health sector with global EHF severity heatwave projection scenarios. In an evaluation of impact skill within global forecast systems, EHF intensity and severity is reviewed as a predictor of human health impact, and extended using climate observations and human health data for sites around the globe. Heatwave intensity, determined by short and long-term temperature anomalies at each locality, is normalized to permit spatial analysis and inter-site comparison. Dimensionless heatwave event moments of peak severity and accumulated severity are shown to correlate with noteworthy events around the globe, offering new insights into current and future heatwave variability and vulnerability. The EHF severity metric permits the comparison of international heatwave events and their impacts, and is readily implemented within international heatwave early warning systems.

The Russian heatwave in 2010 killed tens of thousands of people, and was by far the worst event in Europe since at least 1950, according to recent studies and a novel universal heatwave index capturing both the duration and magnitude of heatwaves. Here, by taking an improved version of this index, namely the heat wave magnitude index daily, we rank the top ten European heatwaves that occurred in the period 1950–2014, and show the spatial distribution of the magnitude of the most recent heatwave in summer 2015. We demonstrate that all these events had a strong impact reported in historical newspapers. We further reveal that the 1972 heatwave in Finland had a comparable spatial extent and magnitude as the European heatwave of 2003, considered the second strongest heatwave of the observational era. In the next two decades(2021–2040), regional climate projections suggest that Europe experiences an enhanced probability for heatwaves comparable to or greater than the magnitude, extent and duration of the Russian heatwave in 2010. We demonstrate that the probability of experiencing a major European heatwave in the coming decades is higher in RCP8.5 than RCP4.5 even though global mean temperature projections do not differ substantially. This calls for a proactive vulnerability assessment in Europe in support of formulating heatwave adaptation strategies to reduce the adverse impacts of heatwaves.

China has experienced varying degrees of increase or decrease in daytime and nighttime heatwaves, but studies have mostly been at the site or grid scale, and it remains unclear how daytime and nighttime heatwave events in China vary regionally when spatial scales are considered. Here, we redefine the different types of heatwave events in China from 1979 to 2018 as combined day and night heatwave events (CDNHWEs) and independent daytime (nighttime) heatwave events. Due to more pronounced spatiotemporal characteristics, CDNHWEs are the dominant heatwave events in China. Further analysis of the dominant heatwave events indicates that their increase in intensity is stronger at night than during the day, i.e. for CDNHWEs, the daytime-dominated events are gradually replaced by nighttime-dominated events. Compared to 1979–2003, there has been an increase in dominant heatwave events mainly in the south during the day and a nationwide increase at night since 2004. For CDNHWEs, daytime and nighttime processes are regulated by different mechanisms. During the daytime, fewer clouds enhance solar shortwave radiation, favouring daytime heatwaves; however, the increase in aerosols in northern China suppresses solar shortwave radiation. At night, the increase in humidity allows for increased longwave radiation, which favours the formation of nighttime heatwaves across the country. These findings further demonstrate the regional variability of heatwave hazards experienced in China and that targeted heatwave-mitigation measures should be developed based on regional characteristics.

Marine heatwaves (MHWs) are extreme events characterized by abnormally high sea surface temperatures, and they have significant impacts on marine ecosystems and human society. The rapid and accurate forecasting of MHWs is crucial for preventing and responding to the impacts they can lead to. However, the research on relevant forecasting methods is limited, and a dedicated forecasting system specifically tailored for the South China Sea (SCS) region has yet to be reported. This study proposes a novel forecasting system utilizing U-Net and ConvLSTM models to predict MHWs in the SCS. Specifically, the U-Net model is used to forecast the intensity of MHWs, while the ConvLSTM model is employed to predict the probability of their occurrence. The indication of an MHW relies on both the intensity forecasted by the U-Net model exceeding threshold T and the occurrence probability predicted by the ConvLSTM model surpassing threshold P. Incorporating sensitivity analysis, optimal thresholds for T are determined as 0.9 °C, 0.8 °C, 1.0 °C, and 1.0 °C for 1-, 3-, 5-, and 7-day forecast lead times, respectively. Similarly, optimal thresholds for P are identified as 0.29, 0.30, 0.20, and 0.28. Employing these thresholds yields the highest forecast accuracy rates of 0.92, 0.89, 0.88, and 0.87 for the corresponding forecast lead times. This innovative approach gives better predictions of MHWs in the SCS, providing invaluable reference information for marine management authorities to make well-informed decisions and issue timely MHW warnings.

Considerable attention has been directed at understanding the consequences and impacts of long-term anthropogenic climate change. Discrete, climatically extreme events such as cyclones, floods, and heatwaves can also significantly affect regional environments and species, including humans. Climate change is expected to intensify these events and thus exacerbate their effects. Climatic extremes also occur in the ocean, and recent decades have seen many high-impact marine heatwaves (MHWs)—anomalously warm water events that may last many months and extend over thousands of square kilometers. A range of biological, economic, and political impacts have been associated with the more intense MHWs, and measuring the severity of these phenomena is becoming more important. Progress in understanding and public awareness will be facilitated by consistent description of these events. Here, we propose a detailed categorization scheme for MHWs that builds on a recently published classification, combining elements from schemes that describe atmospheric heatwaves and hurricanes. Category I, II, III, and IV MHWs are defined based on the degree to which temperatures exceed the local climatology and illustrated for 10 MHWs. While there is a long-term increase in the occurrence frequency of all MHW categories, the largest trend is a 24% increase in the area of the ocean where strong (Category II) MHWs occur. Use of this scheme can help explain why biological impacts associated with different MHWs can vary widely and provides a consistent way to compare events. We also propose a simple naming convention based on geography and year that would further enhance scientific and public awareness of these marine events.

Compound drought‐heatwave (CDHW) events threaten ecosystem productivity and are often characterized by low soil moisture (SM) and high vapor pressure deficit (VPD). However, the relative roles of SM and VPD in constraining forest productivity during CDHWs remain controversial. In the summer of 2022, China experienced a record‐breaking CDHW event (DH2022). Here, we applied satellite remote‐sensing data and meteorological data, and machine‐learning techniques to quantify the individual contributions of SM and VPD to forest productivity variations and investigate their interactions during the development of DH2022. The results reveal that SM, rather than VPD, dominates the forest productivity decline during DH2022. We identified a possible critical tipping point of SM below which forest productivity would quickly decline with the decreasing SM. Furthermore, we illuminated the evolution of SM, VPD, evapotranspiration, forest productivity, and their interactions throughout DH2022. Our findings broaden the understanding of forest response to extreme CDHWs at the ecosystem scale. Plain Language Summary Low soil moisture (SM) and high vapor pressure deficit (VPD) are widely recognized as the dominant drivers of forest productivity decline during compound drought‐heatwave (CDHW) events. In the summer of 2022, a record‐breaking CDHW (DH2022) struck China. In this study, we decoupled the respective impacts of SM and VPD in determining forest productivity decline during DH2022. We found that during DH2022, SM, rather than VPD, is the dominant driver of forest productivity decline, and once SM decreases below a certain threshold, forest productivity would decline sharply. We illuminated the evolution of SM, VPD, evapotranspiration, forest productivity, and their interactions throughout DH2022. Our findings promote the understanding of forest response to extreme CDHWs at the ecosystem scale and thus potentially improve terrestrial ecosystem models' ability to evaluate and predict the impacts of CDHWs. Key Points Soil moisture (SM), rather than vapor pressure deficit, dominates the forest productivity decline in the 2022 China compound drought‐heatwave event Forest productivity would decline sharply once SM drops below a certain threshold during extreme compound drought‐heatwave events Evolution of the 2022 China compound drought‐heatwave event and its impacts on forests were illuminated

As global warming gets worse, the extreme heat exposure time is expected to increase. Considering that the heatwave damages increased by the accumulation of heat stress, it is important to understand the heatwave onset and number of heatwave days (HWDs). Here, we show that the end of East Asian summer monsoon activity (i.e., Changma retreat date, CRD) could be an indicator in determining the onset of the heatwave, and the barotropic structure induced by the circumglobal and Pacific–Japan teleconnections is a key factor in lengthening the number of HWDs in Korea. The onset of the heatwave is delayed when CRD belated than the climatology late over Korea due to sufficient moisture transportation between the edge of western North Pacific subtropical high and cold polar air mass in July. The number of HWDs from July through August over Korea shows a positive linear relationship with the synoptic stagnation index because the upper-layer anticyclone associated with stagnation is formed around Korea by zonal wave activity. Barotropic anticyclone stabilizes the atmosphere and increases the number of clear sky days, which possibly leads to hot days. Fewer HWDs are observed when there is baroclinic instability due to strong upper-level jet stream and synoptic-scale weather systems move smoothly. By identifying the meteorological mechanism of heatwave occurrence and favorable conditions for sustained hot days over Korea, our results are eventually able to contribute to reducing damages caused by heatwaves.

Extreme environmental events are often catalysts for adaptive changes. Like many such events, the 2012 marine heatwave in the Gulf of Maine revealed unexpected connections within the ecosystem and between the natural and human components of the system. The strongest economic impacts were to Maine’s valuable lobster fishery. In 2012, early and intense landings led to a backlog in the supply chain and a drop in price. This experience prompted the lobster industry to implement changes throughout the supply chain to avoid the severe drop in price should another warm year with early landings occur. Here we describe a second heatwave in the Gulf of Maine that occurred in 2016. Despite a similar pattern in monthly landings, dockside prices for lobster were higher than expected. The contrast with 2012 suggests that the adaptations in the supply chain were successful. The 2012 and 2016 heatwaves are part of a broader pattern of temperature changes in this region. Warmer autumns allow sea turtles to remain in the Gulf of Maine longer, increasing their risk of being stunned or killed by cold water, and warm years followed by cold years typically lead to a decline in lobster landings. While there is evidence for adaptation to rising temperatures in the Gulf of Maine, the adaptation brought a significant shock to the system. Forecasts may help foster adaptation while avoiding shocks, but this will require a substantial shift in the mindsets of both forecast producers and consumers.

Harmful heatwave events that cause serious damage to human society and natural environment have occurred with increasing frequencies worldwide. In this paper, we classify summer heatwave events in the Northern Hemisphere through a new method considering more comprehensive event characteristics of the intensity, duration, affected area and humidity amplification. Apart from low-severity events in which all event characteristics are at normal levels, there are also five main different high-severity event types in nature, including high-intensity, high-humidity-amplification, long-duration, large-area and long-duration-large-area events. Different types of high-severity events usually occur at different locations in the Northern Hemisphere. The proportion of high-severity events, which is calculated as the ratio of the high-severity event number to the total heatwave number, increased significantly after 1989, and these increases probably came mostly from large-area and long-duration-large-area events. Further model analyses using the Detection and Attribution Model Intercomparison Project and Scenario Model Intercomparison Project of CMIP6 show that greenhouse gas-related external forcing is the leading cause of the significant increases in these two types of heatwave events. Anthropogenic activities are suggested to cause more rapidly increasing occurrences of extreme heatwaves with large affected areas and heatwaves with both long durations and large affected areas.