A weather forecast can tell you that heavy rain is coming, but that is not necessarily the information people need most. What people often really need to know is: Will my home flood? Will roads close? Will crops fail? Should I leave now, or wait?
The gap between simply forecasting what the weather will be and actually knowing what the weather will do lies in understanding how hazards can turn into actual impacts. This is at the centre of transitioning towards “impact-based early warning”, addressed in this recent UNU-EHS guide. The guide for practitioners on impact-based early warning looks at how early warning systems can move beyond hazard forecasts and become more useful for real-world decision-making by including information about risks and impacts, especially in places facing recurring floods and droughts.
What is impact-based early warning?
Traditional early warning systems are mostly weather or hazard-based. They tell you what is happening regarding rainfall levels, wind speeds, rising river levels and drought conditions. That information matters, obviously. However, on its own, it often leaves people to figure out the consequences by themselves.
Impact-based early warning tries to close that gap. Instead of only forecasting the hazard, it asks what that hazard is likely to mean for different people and places, so we can answer questions like “who is exposed? Who is vulnerable? What kind of impacts are likely, and where?”
The goal is to move to forecasting what the weather will do, which changes the warning itself. Where a standard early warning might say: “50 millimetres of rainfall expected” an impact-based warning might instead say: “flooding is likely in low-lying neighbourhoods. Roads may become impassable. Households in flood-prone areas should prepare to evacuate.” Both warnings are about the same hazard event but the level of usefulness is completely different.
That difference comes from adding another layer of information, which is knowledge about risks. Risk includes exposure, meaning who or what is in harm’s way, as well as vulnerability, indicating who is most likely to be affected and why.
Such information is necessary, because the same storm does not affect everyone equally, for instance. Or, a flood warning lands differently if you live in a well-protected neighbourhood than if you live in an informal settlement with poor drainage and no safe evacuation route, for example. Impact-based early warning systems try to account for that reality instead of treating populations as uniformly exposed and vulnerable.
Why does this matter?
A warning people do not understand or that they cannot connect to their own situation often does not lead to action. In practice, that can mean communities receive information that is technically accurate but still not actionable. In that, timing matters, but it is not the only aspect. Relevance matters too.
That is where impact-based approaches become important. They connect weather forecasts with actual conditions on the ground, such as infrastructure, poverty, mobility, housing quality, access to information, presence of vulnerable groups and other relevant factors. Sometimes, vulnerabilities are not immediately visible.
In many parts of the world, women and girls, persons with disabilities and displaced populations often face very different risks during floods, droughts and other hazards, for example. Not because the hazard itself is different, but because access to resources, information, transport or decision-making power is uneven. So, a warning system that treats everyone the same can still leave people behind.
Another important shift is that impact-based early warning is not only about predicting impacts, but also about enabling earlier action. That could mean farmers changing planting schedules before drought conditions intensify, relocating livestock, preparing evacuation routes or positioning emergency services in advance, for example.
However, for warnings to work, they also need to be accessible and actionable. People need to actually receive and understand them. This could be through radio, text messages or local networks, in formats and languages they can use. Also, warnings need to include realistic guidance. Telling people to evacuate only helps if they have the means, information and somewhere safe to go.
All in all, the idea is straightforward, with warnings becoming more valuable when they help people make decisions before a disaster unfolds. Or, in other words, they can help prevent hazards turning into disasters. As climate extremes become more frequent and less predictable, that distinction matters more each day.
