Uncertainty in climate change projections is a major problem for anyone planning to adapt to a changing climate. In projections of future climate change, this arises from three causes:
- Natural climate variability – arising from both external influences on the climate and internal chaotic climate processes.
- Modelling uncertainty – arising from incomplete understanding of Earth system processes and incomplete representation in climate models.
- Emissions uncertainty – arising from not knowing the amount of future global greenhouse gas emissions.
In UKCP09, emissions uncertainty is explored through three possible scenarios of future greenhouse gas emissions:
- Low emissions (IPCC : B1)
- Medium emissions (IPCC SRES: A1B)
- High emissions (IPCC SRES: A1FI)
- 10% probability level – the probability that the change will be less than that shown is 10%. We use the term to be less than to describe this.
- 50% probability level – the strength of evidence for the projected change is just as likely to be greater as it is to be less than the values shown. We call this the . It is not necessarily the most likely projection.
- 90% probability level – the probability that the change will be less than that shown is 90%. In other words, the change is very unlikely to be greater than shown.