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Climate change projections Uncertainties

  • Modelling uncertainty – arising from incomplete understanding of the climate system, and our inability to model it perfectly.

An estimation of modelling uncertainty is quantified through the use of a perturbed physics ensemble (PPE), which explores variation in model parameters related to atmospheric and oceanic processes, the Sulphur cycle, and the . In addition, downscaling uncertainty is explored by generating a PPE from the regional climate model HadRM3. Further, to capture differences in the way that different international global climate models represent the physics of climate, a multi-model ensemble was used, thus incorporating structural error (discrepancy) into the probabilistic projections.

This treatment of modelling uncertainty in UKCP09, is a significant advance from UKCIP02, which had no estimation of modelling uncertainty. For more information on the way in which modelling uncertainty is addressed in UKCP09 see Chapter 2 and Chapter 3 of the Climate change projections report, and Annex 2. 

  • Natural climate variability – arising from both internal and external factors on the climate system, are addressed differently in UKCP09.

The process of varying climate model parameters in the PPE not only explores modelling uncertainty, but also accounts for the effects of natural internal factors on climate variability.

The effect of natural external factors on climate variability cannot be formally incorporated in the UKCP09 projections, so this source remains as an uncertainty.

This is a significant advance from UKCIP02, which explored natural climate variability in a more limited way. See Chapter 2 and Chapter 3 of the Climate change projections report for more information.

  • Emissions uncertainty – arising from not knowing the amount of future global greenhouse gas emissions

Emissions uncertainty is explored by presenting probabilistic projections of future climate change for three emissions scenarios: low (IPCC : B1), medium (IPCC SRES: A1B), and high (IPCC SRES:A1FI).  It is not possible to assign a probability to the different emissions pathways, however.

This is the same approach to emissions uncertainty as used in UKCIP02.

  • Contributions to uncertainty from downscaling

Downscaling for 300 km to 25 km makes a similar contribution to each of the sources of uncertainty (i.e. roughly 1/6th of the overall uncertainty). As such, adding the noise from downscaling does not widen the PDFs significantly because there are lots of other uncertainties already included.

The uncertainty in UKCP09 is quite wide at the 300 km scale because of that introduced through internal variability, timescaling, modelling uncertainty, structural uncertainty and that related to the . Generally all components contribute and none dominate. Uncertainty accumulates by adding up the variances – a consequence of this is that if you already have a large variance then you have to add a variance of a similar magnitude to make uncertainty increase appreciably.

For more information on uncertainty see Chapter 1, Chapter 2 and Chapter 3 of the Climate change projections report.