Reports & guidance: Uncertainty

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 SRES: B1)
• Medium emissions (IPCC SRES: A1B)
• High emissions (IPCC SRES: A1FI)

UKCP09 provides probabilistic climate projections – different future climate outcomes that have different strengths of evidence associated with them. Probability levels used in the reports are:

• 10% probability level – the probability that the change will be less than that shown is 10%. We use the term very unlikely 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 central estimate. 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.