Climate-related extreme events pose a significant burden on humanity, having already caused hundreds of thousands of deaths and trillions of dollars in damage in the first two decades of this century. These events, such as floods, droughts, Heat waves, fires and tropical cyclones are likely to become more frequent in the future. However, the combined occurrence of (simultaneous) different climate threats and impacts may have even more serious consequences for society.

The study, published in Earth's Future in 2025, provides a global mapping of future changes in the combined occurrence of six categories of climate-related hazards or impacts: river floods, droughts, heat waves, wildfires, tropical cyclone-induced winds, and crop failures. It draws on a large dataset from the ISIMIP (Inter-Sectoral Impact Model Intercomparison Project), which uses outputs from global climate and impact models.

The results of the study confirm, in line with existing literature a sharp increase in the incidence of many individual threats and impacts, especially heat waves and fires. Fires are currently one of the most common threats, but heat waves, which are currently relatively rare, will increase dramatically in the future and become the dominant threat globally, although fires will also remain common with a geographically widespread increase in incidence. Other threats, such as river floods and droughts (especially around the Mediterranean), also show a large increase in incidence. Crop failures will increase significantly in Africa, South Asia and the Americas.

The key finding of the study is that Under the medium-high emissions scenario (RCP6.0), many regions of the world move from experiencing a given threat or impact category in isolation to commonly experiencing combined threats or impacts. In the historical period (1956-2005) the ratio of clustered to isolated events was low. However, in the future (2050-2099) the RCP6.0 scenario projects a clear shift towards a world dominated by clustered threats/impacts, particularly in the tropics and lower mid-latitudes.

Not only the occurrence, but also the recurrence of associated threats or impacts in the same region changes significantly in the futureWhile currently, cluster events rarely recur in consecutive years, in the future, periods lasting one or more decades will become common when a given location experiences specific cluster events at least once a year.

The most common future threat/impact pairs will be those involving heat waves. Pair heat waves and fires stands out for the scope and magnitude of the changes, which cover almost the entire world. The occurrence of pairs will also increase significantly drought and heat waves, heat waves and crop failures a river floods and heat wavesThe increase in the incidence of associated events is driven mainly by the rapid increase in the occurrence of individual threats/impacts, especially heat waves.

The study also highlights significant nonlinearity of the increase in associated events depending on the level of global warming. Increasing levels of warming lead to a much greater percentage increase in associated events.

Findings indicate qualitative shift in regime from a world dominated by isolated climate threats and impacts to a world where clustered events become the norm. This has significant implications for adaptation planning. Single-threat approaches may misestimate risks and vulnerabilities. Effective adaptation strategies must explicitly take into account the combined nature of future threats and impactsThe increasing simultaneous impact of larger geographic areas will also strain regional resource-sharing mechanisms and global trade networks. The study therefore underlines the importance of a holistic view of risk that considers multiple interacting threats and impacts. Spring

Glossary of key terms

• Climate extremes: They refer to extreme values of one or more climate variables (e.g. extreme precipitation, extreme temperatures).
• Dangers: Elements that trigger impacts. These are events or trends that pose a threat to people, property or the environment (e.g. floods, droughts, heat waves, forest fires, tropical cyclones). They are sometimes distinguished in the study from “derived hazards”, which are partly or entirely driven by (extreme) climate conditions.
• Influences: Specific social, economic or environmental consequences resulting from hazards (e.g. loss of life, financial losses, environmental damage, crop failure).
• Compound occurrence: The co-occurrence of two or more hazards or impacts at the same location. In the study, this is understood as occurring at least once during a given year.
• Recurrence: It quantifies whether individual or compound hazards/impacts recur over time by counting the stretches of consecutive years with occurrence at a given location.
• Propensity: A metric that quantifies the relative importance of the occurrence of a single hazard/impact compared to the average occurrence of all six analyzed categories at a given location.
• Co-occurrence ratio: A metric that shows where and when composite occurrences dominate the occurrence of individual, isolated hazards/impacts at a given location.
• Mode shift: A qualitative change in the nature or pattern of occurrence of climate hazards and impacts, for example a shift from a predominance of isolated events to a predominance of compound events.
• Medium-high emission scenario: A scenario of future greenhouse gas emissions that is considered medium-high. The study often refers to the RCP6.0 scenario.
• ISIMIP (Inter-Sectoral Impact Model Intercomparison Project): A project that coordinates and compares impact models from different sectors (e.g. water, agriculture, ecosystems) that are driven by outputs from global climate models (GCMs).
• GCM (Global Climate Model): Numerical models that simulate processes in the Earth's climate system and are used to project future climate conditions.
• Impact models: Models that simulate the impacts of climate conditions (often from GCMs) on specific systems such as rivers, crops, or forest fires.
• Addiction: In the statistical sense, as used in the study, it refers to the degree to which the occurrences of two or more hazards/effects are statistically related. It does not necessarily imply a physical or causal relationship.
• RCP (Representative Concentration Pathway): Greenhouse gas concentration scenarios that are used as input to climate models to project future climate change. RCP2.6 is a low-emission scenario, while RCP6.0 and RCP8.5 are higher-emission scenarios.