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Probability
Probability is a way of expressing knowledge or belief that an event will occur, and is concept most people are familiar with in everyday life. In mathematics, there are two types of probability. Most people are familiar with objective probability, which is the likelihood of an outcome based on empirical observations and information. The second type of probability, subjective probability, is where the likelihood of an outcome is based on the strength of belief in different outcomes.
In detail
There are two types of probability:
Objective probability is the expected frequency of occurrence of an outcome based on observations of a large number of independent trials carried out under the same conditions for which all outcomes are accounted for. For example, knowing that that a coin has only two sides, it can be shown that the average probability of tossing a ‘head’ is 50% in the long run.
Subjective probability is a measure of the degree to which a particular outcome is consistent with the information considered in the analysis (i.e. strength of the evidence). In this case, all possible outcomes might not be known at the outset, and probabilities may change as new information is discovered or interpreted differently.
Probabilistic climate projections are based on subjective probability, as the probabilities are a measure of the degree to which a particular level of future climate change is consistent with the evidence considered. In the case of UKCP09, a Bayesian statistical framework was used, and the evidence comes from historical climate observations, expert judgement and results of considering the outputs from a number of climate models, all with their associated uncertainties.
The methodology that generates the probabilities is based on large numbers (ensembles) of climate model simulations, but adjusted according to how well different simulations fit historical climate observations. As such, the probabilities provide information on the consistency of future climate outcomes with the evidence considered which can be used to support decisions related to impacts and adaptation options.
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One important consequence of the definition of probability used in UKCP09 is that the probabilistic projections are themselves uncertain, because they are dependent on the evidence used, including how the methodology is formulated. Section 1.1.1 of the UKCP09 Climate change projections report explains what is meant by probability in UKCP09.
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An overview of how probabilistic projections are created and presented in UKCP09 is described in Box 3 of the UKCP09 Briefing report. It is also recommended users read the UKCP09 Guiding Principles.
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More details are provided in Chapter 3 of the UKCP09 Climate Change Projections report, including an overview of the methodology in Section 3.2.12, and discussion about the interpretation of UKCP09 probabilistic projections in Section 3.3.
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A discussion of sources of uncertainty in the UKCP09 probabilistic projections is available in Annex 2.4.
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The UKCP09 User Interface allows the UKCP09 probabilistic projections to be provided as either future absolute climate or as change from the 1961–1990 baseline climate.
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