Search site

Probability in the UKCP09 projections

UKCP09 provides probabilistic climate projections. This is a major step forward in the way climate change projections are presented and used. To use them effectively it is important to have a good understanding of what they mean and how they are communicated.

What are probabilistic projections?

Probabilistic projections assign a probability to different climate change outcomes, conditioned on the scientific methodology used to produce them. They estimate, for example, the probability that temperature will rise by more than 3oC, or by between 3 and 3.5oC. UKCP09 constructs probabilistic projections from thousands of plausible climate scenarios, derived from a set of climate models.

One way that UKCP09 presents climate projections is using Cumulative Distribution Functions (CDF), showing the probability that change will be below some specific value. These are presented as graphs like the one below.

CDF curve of 10th and 90th Percentiles

The CDF shows the probability of the temperature increasing by less than the numbers shown at the bottom of the graph. The dotted lines are drawn at the 10% and 90% probability levels. Temperature change below the 10% probability level is very unlikely, as is temperature change above the 90% probability level. We have limited confidence about the changes beyond the 10 and 90% levels, because they are particularly sensitive to the assumptions and methodology used to construct the thousands of scenarios and resulting probability distributions.

The 50% probability level change is known as the central estimate. It is not the most likely outcome, it simply shows that half of the climate scenarios used to construct the probability distribution fell below that figure and half fell above it. So if, for example, the central estimate for temperature rise for 2050 was 3˚C then the evidence suggests that temperatures are as likely to increase by less than 3˚C as they are to increase by more.

How to interpret these probabilities

The probability presented in UKCP09 outputs is based on the weight of evidence and available climate science. The probability measures the degree to which a particular level of future climate change is consistent with the information used in the analysis. In UKCP09, this information comes from a large number of climate model simulations explicitly designed to capture uncertainties, and also accounts for how well different models replicate some key aspects of recent climate and historical climate change. Inevitably, however, the data provided does not account for uncertainties which cannot be quantified using model simulations, including the effects of earth system processes not yet included in the models, or of errors common to all models.

At the 10% probability level, only 10% of the climate model runs fall at or below that level, at the 90% probability level, only 10% of the climate model runs fall at or above that level.

For more information on interpreting probabilities in UKCP09 please see Chapter 1 of the Climate Change Projections Report, in the Reports & guidance section.

Need more help?

· For more details about the UKCP09 climate projections, please see the Climate Change Projection Report in the Reports and Guidance section.

· For examples of how other organisation have used UKCP09 projections please see the Case studies.

· For other queries please see our Help tab.