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What is meant by probability in the UKCP09 probabilistic projections?

Horse-race

UKCP09 presents probabilities of different future climates. These probabilities are created by weighting future climate projections on how well they represent the past climate. So they can be seen as the relative degree to which each climate outcome is supported by the evidence available.

There are two types of probability; subjective and objective - UKCP09 is an example of subjective probability.

  • Subjective or inductive probability - a estimate based on the available information and strength of evidence, e.g. horse-racing odds or taking out insurance... and the UKCP09 probabilistic climate projections.
  • UKCP09 is NOT an objective or statistical probability - where all outcomes are accounted for, e.g. rolling dice or choosing a playing card at random

How to interpret these probabilities

CDF curve showing 50 and 80% probability levels

  • Correct interpretation of UKCP09 probabilities:

An 80% probability level is indicating that 80% of the model runs fall at or below that value and 20% of the model runs are above that value.

  • Avoiding common errors in interpreting UKCP09 probabilities:

An 80% probability level is not indicating that there is an 80% chance of the projection occurring in the future. In fact, UKCP09 gives no probability of a projection actually occurring in the future.

An 80% probability level is not indicating that 80% of the underlying climate model runs showed that particular outcome

The 50% probability level is not indicating the most likely projection but the value that has an equal probability of being exceeded and not being exceeded, as projected by UKCP09.

What is a probabilistic projection?

The probabilistic climate projections are based on the best available information but there are still uncertainties. These depend on the climate model used, climate observations, the modelling method and assumptions, and choices made during the modelling procedure.

A probabilistic climate projection:

  • Aims to allow users to make more robust decisions

By being clearer about the range of possible climate outcomes, i.e. the uncertainties, UKCP09 allows individual users to make decisions based on their individual / organisational risk profile, leading to more user-focussed decisions that should allow for more robust decision to be made. It may be likely that different systems, sites or operations will require a different level of adaptation. Thus, an organisation may choose to adapt its critical systems to handle with the biggest range of probabilities, but other processes may be less demanding.

  • Is a measure of strength of evidence in different future climate outcomes

A number of climate models (an ensemble) are used to create UKCP09.  All the climate models are checked by seeing how well they reproduce the past climate. If they are good at reproducing the historical climate, which we have measured and know about, then it is a good test of how well the models are likely to be at indicating the future climate.  The best-performing models are given a higher weighting, and this strength of evidence is the basis of the probabilities assigned to UKCP09.

  • Is dependent on the method used including assumptions & choices made

When modelling any future entity there are assumptions and choices requried throughout the modelling process.  It is important to put these assumptions and choices into context of the adaptation process.  Planners use projections of many different types in their decision making process eg demography, technology, economics etc, all of which have assumptions built into them.  Climate projections are in a similar vein.

  • Is based on the current evidence (models & observations)

Science is continually evolving and, with it, so are climate models. However, it is not feasible to wait until the ultimate climate model is developed as it will then be too late for mitigation and adaptation to climate change. We believe that with the current knowledge and information that UKCP09 provides, we have sufficient information to form a robust adaptation strategy now.

  • Does not reduce uncertainty, just makes it more transparent

The future climate will always be uncertain, no matter how good climate models are. UKCP09 is the first set of climate projections to make this uncertainty explicit. Having an indication of the range of uncertainty will allow users to make risk-based decisions about adapting to future climate.

For more information about probability in UKCP09 see Chapter 1 of the Climate change projections report and for further details on the methodology behind UKCP09 see Chapter 3.