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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 : 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 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.