Extreme hazards - rare, high-impact events - pose a serious and underestimated threat to humanity. The extremes of the broad ensemble of natural and anthropogenic hazards can lead to global disasters and catastrophes.

Because they are rare and modern society lacks experience with them, they tend to be ignored in disaster risk management. While the probabilities of most natural hazards do not change much over time, the sensitivity of the built environment and the vulnerability of the embedded socio-economic fabric have increased rapidly. Exposure to geohazards has increased dramatically in recent decades and continues to do so.

In particular, growing urban environments - including megacities - are in harm's way. Because of the increasing complexity of modern society even moderate hazards can cause regional and global disasters.

Natural hazards that occur frequently on our dynamic planet are increasingly causing loss of human life and damage to goods and infrastructures at the local, regional and global scale, depending on their intensity.

The Science Position Paper 'Extreme Geohazards: Reducing the Disaster Risk and Increasing Resilience' analyses the potential effects of low-probability high-impact events, which might cause global disasters and even bring our already stressed global society beyond the limits of sustainability.

The paper, a joint initiative by the European Science Foundation (ESF), the Group on Earth Observations (GEO) and the Geohazard Community of Practice (GHCP), following a high-level ESF-COST Conference on the subject, addresses several types of geohazards, but puts special emphasis on the impending risk of catastrophic effects on populations and infrastructures should our growing and increasingly interconnected modern society be exposed to a very large volcanic eruption.

The paper highlights the urgency of establishing an effective dialogue with a large community of stakeholders in order to develop robust risk management, disaster risk reduction, resilience, and sustainability plans in the coming years and decades.

It also underlines the need to develop the methodology to assess the potentially global impacts that a major hazard would have on our modern society, which would provide guidance to reduce vulnerability where possible and increase general resilience in the face of surprise events. It concludes that preparedness requires a global monitoring system that could provide sufficiently early warnings, should such a major hazardous event develop.