How can I manage emissions uncertainty?

It is recommended that users of UKCP09 consider results from all three emissions scenarios so as to enhance the robustness of the resulting conclusions.

In detail

UKCP09 provides projections of climate change associated with three different SRES emission scenarios (labelled Low, Medium and High), which are each associated with a different plausible storyline about how the world might change, and thereby how the atmospheric concentration of greenhouse emissions may change, in the future.

The IPCC (2000) concluded that all the SRES emission scenarios are considered equally valid, but that they should not be treated as equally probable. They further concluded that it is not correct to associate probabilities with the different emission scenarios. The implication is that without probabilities attached, the climate projections associated with each emission scenario together provide an estimate of the associated emission uncertainty through providing a range of possible projections.

It is good practice where appropriate to consider all three of the emissions scenarios. For some uses, it is recognised that it may not be necessary (or practical) to consider all three. One such case is apparent from the observation that differences between the UKCP09 climate projections for the different emission scenarios only start to become significant after about 2040. As such, it may not be necessary to include all three sets of resulting projections when the timeframe for an impacts or adaptation assessment is limited to the period up to and including the 2030s (i.e. the 2020–2049 time period).

A second case is evident from the recognition that future emissions, and therefore climate forcing, may lie outside of the range depicted by the UKCP09 projections. The inclusion of climate projections associated with a Higher emission scenario, similar to that done within the Thames Estuary 2100 project may be necessary to explore potential impacts, especially where the consequences are high of what might be identified as a low probability future and more robust adaptation options are needed.

In practice the recommendation of using climate projections associated with all three emission scenarios may not always be feasible. This may lead to users developing their own approach as to which set of climate projections they will use, including using climate projections associated with one, a sub-set, or a combination of the available emission scenarios. Key when such an alternative approach is adopted is to note and clearly articulate along with the resulting assessment the underlying assumptions and rationale for the choice made. It is further recommended that the sensitivity of any conclusions drawn should be tested with respect to the choice(s) made.

Using climate projections associated with a single emission scenario will mean that the resulting impacts and adaptation assessment is dependent on the choice of the future selected and may not be robust should the future be very different from that chosen. When using this approach it should be kept in mind that the labels given to these emissions scenarios (high, medium and low) are relative and not absolute. The emission scenario labelled High does not represent a worst case or upper limit on future emissions and therefore future climate. In addition, that labelled Medium is not necessarily a middle-of-the-road emission scenario. 

One approach that has been suggested is to use the climate projections associated with the emission scenario that is thought to be closest to the most recent trend of global greenhouse gas emissions. As noted by Raupach et al. 2007, global CO2 emission in the period 2000–2004 were increasing at a rate faster than any of the SRES emission scenarios, including that which UKCP09 has labelled High. This observational evidence may suggest to some that using the climate projections associated with the High emission scenario would be a safe approach. If this method is chosen, it should be noted that this observational evidence is based on a 5-year trend, which may not be indicative of the long-term trend; and that the actual emissions may continue to grow beyond that represented by the High emission scenario.

Another alternative approach is to use the climate projections associated with two of the emission scenarios as they are believed to depict a range of interest or most concern. Of the possible pairings, High and Low, and High and Medium are often used under the assumption that future climate change is somehow bounded by the associated climate projections, or that they represent the basis for a safe strategy. Adopting this approach and defining the appropriate pairing should be based on the purpose of the underlying assessment and the user’s perceived vulnerability to climate change and willingness to accept risks associated with over-adapting or under-adapting (see UKCIP Risk framework).