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There is no consensus on whether climate change has yet affected the statistics of tropical cyclones, owing to their large natural variability and the limited period of consistent observations. In addition, projections of future tropical cyclone activity are uncertain, because they often rely on coarse-resolution climate models that parameterize convection and hence have difficulty in directly representing tropical cyclones. Here we used convection-permitting regional climate model simulations to investigate whether and how recent destructive tropical cyclones would change if these events had occurred in pre-industrial and in future climates. We found that, relative to pre-industrial conditions, climate change so far has enhanced the average and extreme rainfall of hurricanes Katrina, Irma and Maria, but did not change tropical cyclone wind-speed intensity. In addition, future anthropogenic warming would robustly increase the wind speed and rainfall of 11 of 13 intense tropical cyclones (of 15 events sampled globally). Additional regional climate model simulations suggest that convective parameterization introduces minimal uncertainty into the sign of projected changes in tropical cyclone intensity and rainfall, which allows us to have confidence in projections from global models with parameterized convection and resolution fine enough to include tropical cyclones. Climate model simulations reveal that recent destructive tropical cyclones would have been equally intense in terms of wind speed but would have produced less rainfall if these events had occurred in pre-industrial climates, and in future climates they would have greater wind speeds and rainfall.

Category 4 landfalling hurricane Harvey poured more than a metre of rainfall across the heavily populated Houston area, leading to unprecedented flooding and damage. Although studies have focused on the contribution of anthropogenic climate change to this extreme rainfall event 1 – 3 , limited attention has been paid to the potential effects of urbanization on the hydrometeorology associated with hurricane Harvey. Here we find that urbanization exacerbated not only the flood response but also the storm total rainfall. Using the Weather Research and Forecast model—a numerical model for simulating weather and climate at regional scales—and statistical models, we quantify the contribution of urbanization to rainfall and flooding. Overall, we find that the probability of such extreme flood events across the studied basins increased on average by about 21 times in the period 25–30 August 2017 because of urbanization. The effect of urbanization on storm-induced extreme precipitation and flooding should be more explicitly included in global climate models, and this study highlights its importance when assessing the future risk of such extreme events in highly urbanized coastal areas. Modelling the contribution of urbanization to the impacts associated with hurricane Harvey in August 2017 shows that urbanization worsens rainfall and flooding.

Analysis of global historical data in the Northern and Southern hemispheres reveals a statistically significant, poleward migration of 1° per decade in the average latitude at which tropical cyclones have achieved their lifetime-maximum intensity over the past 30 years. Tropical cyclones pushed towards the poles Attempts to monitor changes in tropical cyclone activity have been hampered by inconsistencies in global data sets, such as measures of frequency, storm duration and intensity. Jim Kossin and colleagues by-pass this long-standing problem by instead focusing on the latitude at which tropical cyclones reached their lifetime maximum intensity, a far more robust measurement. They find that during the past 30 years the position of peak intensity has migrated steadily poleward, at a rate of about 60 km per decade. This shift appears to be associated with changes in vertical wind shear and potential intensity, which the authors suggest may be associated with recent increases in the width of the tropical belt associated with global warming. Temporally inconsistent and potentially unreliable global historical data hinder the detection of trends in tropical cyclone activity 1 , 2 , 3 . This limits our confidence in evaluating proposed linkages between observed trends in tropical cyclones and in the environment 4 , 5 . Here we mitigate this difficulty by focusing on a metric that is comparatively insensitive to past data uncertainty, and identify a pronounced poleward migration in the average latitude at which tropical cyclones have achieved their lifetime-maximum intensity over the past 30 years. The poleward trends are evident in the global historical data in both the Northern and the Southern hemispheres, with rates of 53 and 62 kilometres per decade, respectively, and are statistically significant. When considered together, the trends in each hemisphere depict a global-average migration of tropical cyclone activity away from the tropics at a rate of about one degree of latitude per decade, which lies within the range of estimates of the observed expansion of the tropics over the same period 6 . The global migration remains evident and statistically significant under a formal data homogenization procedure 3 , and is unlikely to be a data artefact. The migration away from the tropics is apparently linked to marked changes in the mean meridional structure of environmental vertical wind shear and potential intensity, and can plausibly be linked to tropical expansion, which is thought to have anthropogenic contributions 6 .

Intense tropical cyclones (TCs), which often peak in autumn 1 , 2 , have destructive impacts on life and property 3 , 4 – 5 , making it crucial to determine whether any changes in intense TCs are likely to occur. Here, we identify a significant seasonal advance of intense TCs since the 1980s in most tropical oceans, with earlier-shifting rates of 3.7 and 3.2 days per decade for the Northern and Southern Hemispheres, respectively. This seasonal advance of intense TCs is closely related to the seasonal advance of rapid intensification events, favoured by the observed earlier onset of favourable oceanic conditions. Using simulations from multiple global climate models, large ensembles and individual forcing experiments, the earlier onset of favourable oceanic conditions is detectable and primarily driven by greenhouse gas forcing. The seasonal advance of intense TCs will increase the likelihood of intersecting with other extreme rainfall events, which usually peak in summer 6 , 7 , thereby leading to disproportionate impacts. We identify a seasonal advance of intense tropical cyclones that is closely related to the seasonal advance of rapid intensification events, favoured by the observed earlier onset of favourable oceanic conditions.

About a third of the sediment delivery of the Mekong River is shown to be associated with rainfall generated by tropical cyclones, suggesting that future delta stability will be strongly moderated by changes to tropical cyclone intensity, frequency and track. Cyclones shaping tropical mega-deltas The delivery of sediment to deltas is crucial for their survival, especially when faced with rising sea levels. Human activities, such as dam building and land-cover alterations, can affect sediment supply, but Stephen Darby et al . show that, for the Mekong River, about a third of the sediment delivered is associated with rainfall generated by tropical cyclones. More than half of the decline in suspended sediment supply to the delta between 1981 and 2005 arose from shifts in tropical-cyclone climatology, suggesting that future delta stability will also be strongly moderated by additional changes to tropical-cyclone intensity and track. The world’s rivers deliver 19 billion tonnes of sediment to the coastal zone annually 1 , with a considerable fraction being sequestered in large deltas, home to over 500 million people. Most (more than 70 per cent) large deltas are under threat from a combination of rising sea levels, ground surface subsidence and anthropogenic sediment trapping 2 , 3 , and a sustainable supply of fluvial sediment is therefore critical to prevent deltas being ‘drowned’ by rising relative sea levels 2 , 3 , 4 . Here we combine suspended sediment load data from the Mekong River with hydrological model simulations to isolate the role of tropical cyclones in transmitting suspended sediment to one of the world’s great deltas. We demonstrate that spatial variations in the Mekong’s suspended sediment load are correlated ( r  = 0.765, P  < 0.1) with observed variations in tropical-cyclone climatology, and that a substantial portion (32 per cent) of the suspended sediment load reaching the delta is delivered by runoff generated by rainfall associated with tropical cyclones. Furthermore, we estimate that the suspended load to the delta has declined by 52.6 ± 10.2 megatonnes over recent years (1981–2005), of which 33.0 ± 7.1 megatonnes is due to a shift in tropical-cyclone climatology. Consequently, tropical cyclones have a key role in controlling the magnitude of, and variability in, transmission of suspended sediment to the coast. It is likely that anthropogenic sediment trapping in upstream reservoirs is a dominant factor in explaining past 5 , 6 , 7 , and anticipating future 8 , 9 , declines in suspended sediment loads reaching the world’s major deltas. However, our study shows that changes in tropical-cyclone climatology affect trends in fluvial suspended sediment loads and thus are also key to fully assessing the risk posed to vulnerable coastal systems.

Tropical cyclones have far-reaching impacts on livelihoods and population health that often persist years after the event 1 – 4 . Characterizing the demographic and socioeconomic profile and the vulnerabilities of exposed populations is essential to assess health and other risks associated with future tropical cyclone events 5 . Estimates of exposure to tropical cyclones are often regional rather than global 6 and do not consider population vulnerabilities 7 . Here we combine spatially resolved annual demographic estimates with tropical cyclone wind fields estimates to construct a global profile of the populations exposed to tropical cyclones between 2002 and 2019. We find that approximately 560 million people are exposed yearly and that the number of people exposed has increased across all cyclone intensities over the study period. The age distribution of those exposed has shifted away from children (less than 5 years old) and towards older people (more than 60 years old) in recent years compared with the early 2000s. Populations exposed to tropical cyclones are more socioeconomically deprived than those unexposed within the same country, and this relationship is more pronounced for people exposed to higher-intensity storms. By characterizing the patterns and vulnerabilities of exposed populations, our results can help identify mitigation strategies and assess the global burden and future risks of tropical cyclones. A global profile of tropical cyclone population exposure for the period 2002–2019 shows a steady increase, with approximately 560 million people exposed yearly and a disproportionate exposure among those with lower socioeconomic status.

From Atlantic hurricanes to the Indian monsoons, storms are getting worse and becoming more erratic. From Atlantic hurricanes to the Indian monsoons, storms are getting worse and becoming more erratic.

Long-term time series of surface ocean-atmosphere heat fluxes show that the mid-latitude North Atlantic ocean may influence atmospheric variability on multidecadal timescales. Long-term climate effects of the oceans In 1964 the Norwegian–American meteorologist Jacob Bjerknes conjectured that interactions between the atmosphere and the ocean have opposing effects, depending on the timescale of the interaction: at interannual scales, the atmosphere controls the sea surface temperature, but at multidecadal scales, the ocean is the main driver of atmospheric variability. The former is now accepted and modelling work has been used to support the latter, but until now no firm observational evidence has been offered in support of oceanic control of the atmosphere. Sergey Gulev et al . have now assembled the available observational evidence and demonstrate that, at least in the North Atlantic, the ocean does drive multidecadal variability in surface heat fluxes. Nearly 50 years ago Bjerknes 1 suggested that the character of large-scale air–sea interaction over the mid-latitude North Atlantic Ocean differs with timescales: the atmosphere was thought to drive directly most short-term—interannual—sea surface temperature (SST) variability, and the ocean to contribute significantly to long-term—multidecadal—SST and potentially atmospheric variability. Although the conjecture for short timescales is well accepted, understanding Atlantic multidecadal variability (AMV) of SST 2 , 3 remains a challenge as a result of limited ocean observations. AMV is nonetheless of major socio-economic importance because it is linked to important climate phenomena such as Atlantic hurricane activity and Sahel rainfall, and it hinders the detection of anthropogenic signals in the North Atlantic sector 4 , 5 , 6 . Direct evidence of the oceanic influence of AMV can only be provided by surface heat fluxes, the language of ocean–atmosphere communication. Here we provide observational evidence that in the mid-latitude North Atlantic and on timescales longer than 10 years, surface turbulent heat fluxes are indeed driven by the ocean and may force the atmosphere, whereas on shorter timescales the converse is true, thereby confirming the Bjerknes conjecture. This result, although strongest in boreal winter, is found in all seasons. Our findings suggest that the predictability of mid-latitude North Atlantic air–sea interaction could extend beyond the ocean to the climate of surrounding continents.

The co-occurrence of the 2020 Atlantic hurricane season and the ongoing coronavirus disease 2019 (COVID-19) pandemic creates complex dilemmas for protecting populations from these intersecting threats. Climate change is likely contributing to stronger, wetter, slower-moving, and more dangerous hurricanes. Climate-driven hazards underscore the imperative for timely warning, evacuation, and sheltering of storm-threatened populations – proven life-saving protective measures that gather evacuees together inside durable, enclosed spaces when a hurricane approaches. Meanwhile, the rapid acquisition of scientific knowledge regarding how COVID-19 spreads has guided mass anti-contagion strategies, including lockdowns, sheltering at home, physical distancing, donning personal protective equipment, conscientious handwashing, and hygiene practices. These life-saving strategies, credited with preventing millions of COVID-19 cases, separate and move people apart. Enforcement coupled with fear of contracting COVID-19 have motivated high levels of adherence to these stringent regulations. How will populations react when warned to shelter from an oncoming Atlantic hurricane while COVID-19 is actively circulating in the community? Emergency managers, health care providers, and public health preparedness professionals must create viable solutions to confront these potential scenarios: elevated rates of hurricane-related injury and mortality among persons who refuse to evacuate due to fear of COVID-19, and the resurgence of COVID-19 cases among hurricane evacuees who shelter together.

Tropical storms, such as cyclones, hurricanes and typhoons, present major threats to coastal communities. Around two million people worldwide have died and millions have been injured over the past two centuries as a result of tropical storms. Bangladesh is especially vulnerable to tropical cyclones, with around 718 000 deaths from them in the past 50 years. However, cyclone-related mortality in Bangladesh has declined by more than 100-fold over the past 40 years, from 500 000 deaths in 1970 to 4234 in 2007. The main factors responsible for these reduced fatalities and injuries are improved defensive measures, including early warning systems, cyclone shelters, evacuation plans, coastal embankments, reforestation schemes and increased awareness and communication. Although warning systems have been improved, evacuation before a cyclone remains a challenge, with major problems caused by illiteracy, lack of awareness and poor communication. Despite the potential risks of climate change and tropical storms, little empirical knowledge exists on how to develop effective strategies to reduce or mitigate the effects of cyclones. This paper summarizes the most recent data and outlines the strategy adopted in Bangladesh. It offers guidance on how similar strategies can be adopted by other countries vulnerable to tropical storms. Further research is needed to enable countries to limit the risks that cyclones present to public health.