Climate scenarios
Every IMP run starts here. Before you can model a flood, you need to say what kind of world the rain is falling on.
In one sentence
A climate scenario is a plausible, hypothetical future built from a specific set of assumptions about human activity, greenhouse gas emissions and socioeconomic development.
They are what-ifs, not predictions
This distinction matters more than any other on this page, and it is the one most easily lost when results reach a non-technical audience.
A scenario does not claim to say what will happen. It answers a conditional question: given these assumptions, what follows? Two examples give the flavour:
- What would happen if fossil fuel emissions remained unchanged over the next 50 years?
- How would the climate evolve if strict policies were implemented to reduce emissions?
Both are legitimate questions. Neither is a forecast. When IMP results are presented, the scenario they rest on should travel with them.
The two families you will meet
Climate change impact assessment studies overwhelmingly use two sets of scenarios. They are not rivals so much as successive generations, and the newer set is broader in what it accounts for.
| Representative Concentration Pathways (RCPs) | Shared Socioeconomic Pathways (SSPs) | |
|---|---|---|
| Introduced | 2010 | 2017 |
| Accounts for | Future greenhouse gas levels only | Greenhouse gas emissions, plus global politics, policy and socioeconomic development |
| In short | The simpler of the two | The more recent, and the more complete picture |
RCPs are the simplest climate scenarios available. They consider future levels of greenhouse gases and nothing else.
SSPs are the more recent family. Alongside emissions, they factor in global politics, policy and socioeconomic development, which lets a study ask not just how much carbon but what kind of world produced it.
Where this feeds in
The scenario sets the backdrop. What the IMP actually needs, though, is weather: concrete rainfall and temperature values at a resolution fine enough to drive a flood or heat model.
Two components bridge that gap, and they are the rest of this chapter.
| Page | What it does |
|---|---|
| UKCP18 Local | Supplies high-resolution climate projections for the UK |
| Weather generator | Turns observations and projections into usable future weather |
| Further reading | The underlying data sources, for anyone going deeper |
Where this sits in the framework
Climate scenarios are the first step of the weather event component. See Weather event for how the full chain fits together, from scenario through to mapped hazard.