Main criteria and issues on constructing scenarios

Main criteria and issues in constructing scenarios

Climate change scenarios are the first step along the path to creating plausible representations of the future climate based on assumptions concerning future atmospheric composition, and on our understanding of the effect of increased atmospheric concentrations of, for example, greenhouse gases, particulates and other pollutants, on global climate.

A number of different methods exist to construct climate change scenarios, including techniques utilising arbitrary, analogue and global climate model (GCM) information. Each method has its own advantage and disadvantage. GCM-derived scenarios of climate change are by far the most common scenario type encountered, since they conform to most of the criteria proposed by the Intergovernmental Panel on Climate Change (IPCC) Task Group on Data and Scenario Support for Impact and Climate Assessment (IPCC-TGICA), an international body of experts convened to ensure consistency in climate impacts and adaptation research.

As suggested in Barrow et al., (2004), climate change scenarios have a number of roles to play in vulnerability, impacts, and adaptation (VIA) studies. In order to ensure that climate change scenarios are of most use for impact researchers and policy makers, Smith and Hulme (1998) as well as guidelines prepared by the TGICA group (IPCC-TGCIA, 1999) put forward the following:

Five criteria to aid scenario selection:

Consistency with global projections:

Scenarios should be consistent with a broad range of global warming projections based on increased concentrations of greenhouse gases. This range was given as 1.4°C to 5.8°C by 2100 in the IPCC Third Assessment Report (IPCC, 2001).

Physical plausibility:

Scenarios should not violate the basic laws of physics, which means that not only should the changes in one region be physically consistent with those in another region and globally, but that changes in the different climate variables should also be physically consistent.

Applicability in impact assessments:

Scenarios should describe changes in a sufficient number of climate variables on a spatial and temporal scale that allows for impact assessment. So for example, it may be necessary for scenarios to provide information about changes in temperature, precipitation, solar radiation, humidity and wind speed at spatial scales ranging from a single site to global, and at temporal scales ranging from daily to annual means. It is a relatively simple task to construct scenarios from GCMs which are at the same spatial resolution as the GCM, but construction of scenarios at finer resolutions (generally termed 'downscaling') which provide spatially- and temporally-coherent information for several climate variables is more problematic.

Representativeness:

Scenarios should be representative of the potential range of future regional climate change in order for a realistic range of possible impacts to be estimated.

Accessibility:

Scenarios should be straightforward to obtain, interpret and apply in impacts assessments.

As suggested, there are a number of ways in which climate change scenarios can be constructed, but those derived from global climate model (GCM) output generally conform better with the assumptions listed above than those constructed using synthetic and analogue techniques.

Synthetic scenarios (also known as .arbitrary. or .incremental. scenarios) are the simplest climate change scenarios to construct and apply. Their main use is in sensitivity analyses, i.e., in the determination of the response of a particular .exposure unit. (e.g., crop yield, stream flow) to a range of climatic variations. A synthetic scenario is constructed by simply perturbing an historical record for a particular climate variable by an arbitrary amount (e.g., by increasing precipitation by 10%).

In contrast to synthetic scenarios, analogue scenarios make use of existing climate information either at the site in question (temporal analogues, from paleoclimatic or instrumental information), or from another location which currently experiences a climate anticipated to resemble the future climate of the site under study (spatial analogues). More details of synthetic and analogue scenarios are given in Box 3.1 in the recent report published by Barrow et al. (2004). All useful information of all scenario types is also available in IPCC-TGCIA (1999) and Mearns et al. (2001).