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Handling uncertainty in UKCP09

All climate change projections are subject to uncertainty. The degree to which, for example, temperature or rainfall, for example, might change in the future cannot be predicted exactly. One of the major advantages of the UKCP09 projections is that they give a measure of uncertainty over a range of possible outcomes.

The projections provide a comprehensive representation of uncertainties currently known to be important drivers of future climate change, and understood sufficiently well to be included in climate models. Inevitably, the data provided does not account for uncertainties which cannot be quantified using climate model simulations, including the effects of earth system processes not yet included in the models, or of errors common to all models.

Understanding uncertainty in the UKCP09 projections

Uncertainty in climate projections stems from;

  • The complexity of the climate system,
  • Natural climate variability, and
  • Uncertainty over the future pathway of greenhouse gas and aerosol emissions.

UKCP09 deals with uncertainty around the levels of greenhouse gas emissions by presenting 3 possible emissions scenarios - low, medium and high - all of which are assumed to be equally plausible. For each emissions scenario, the projections are presented as probability levels for a range of possible outcomes, based on thousands of plausible climate scenarios derived from climate model simulations. These are the Intergovernmental Panel on Climate Change (IPCC) scenarios B1, A1B and A1FI respectively. More information on the nature of these emissions scenarios can be found in the IPCC's SRES report.

To deal with other types of uncertainty UKCP09 presents the projections as probability levels for a range of possible outcomes. These probabilities are expressed in percentages 10%, 33%, 50%, 67% and 90%. The 50% value is the median or 'central' estimate of change.

For example, you may be interested in finding out how average temperature might change in London in the 2050s under a high emissions scenario. In that case the UKCP09 projections show a 10% probability that it will rise by less than 1.3˚C and a 90% probability that it will rise by less than 4.6˚C. So it is very likely that average London temperature will rise by between 1.3˚C and 4.6˚C by the 2050s under the high emissions scenario. The 50% (central estimate) value is 2.7˚C. This means that average London temperature is as likely to rise by less than this amount, as it is to rise by more.

To learn more about how to interpret probability in UKCP09 see the probability in UKCP09 page

Dealing with uncertainty in decision-making

Uncertainty around climate change projections does not negate the need for action. Many measures that will increase resilience to climate change can be justified without requiring absolute certainty in climate projections. Organisations and businesses looking to adapt should consider adaptation options that will be beneficial in a wide range of likely future climate scenarios.

To make robust decision-making in the face of climate uncertainty it is important to assess a range of adaptation options by considering their sensitivities under a wide range of future climate scenarios. This approach helps to identify those choices that are robust under these scenarios and hence are 'no or low regrets'.

'No regret' or 'low regret' adaptation options are those that have a high chance of success in the light of uncertainties in climate change. Assessing whether an investment is no/low regrets involves testing its performance against potential future scenarios. UKCP09 data can be used to test adaptation options in this way.

Other adaptation options may be 'climate change justified'. These options do not give acceptable returns unless some degree of climate change materialises. Using UKCP09 to see if the UK climate is likely to pass certain climate thresholds can give you an idea of whether an adaptation option is likely to be 'climate change justified.

UKCP09 can also help you to assess which adaptation options might be 'climate change risky'. These options give good returns without taking climate change into account, but give low returns if climate change materialises.

For more information about decision-making under uncertainty please see this paper from the UK Climate Impacts Programme (UKCIP): 'Climate adaptation: Risk, uncertainty & decision-making' [link to Pdf 1.3MB on external site].

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.