Ranges of plausible future climate

Climate science provides a range of possible regional climate change responses for any given increase in greenhouse gases. This is indicated by the range of differing results obtained from multiple climate models, although considerations other than climate model results, such as climate observations and model evaluation, may expand this range. The range in regional response to climate change also arises from the consideration of multiple emissions scenarios (i.e. a range of possible futures emerges from consideration of futures with low, intermediate and high emissions).

Here this range in regional response is not usually reduced to a single best estimate (based on the model mean or median), and instead users are encouraged to consider the full range in their applications. The choice of climate change projections information used in decision-making is dependent on the context in which the decision is taking place. For example, in a particular decision-making context, the risk posed by an extreme of the range of projected climate change, even though of low likelihood, may be the climate of greatest relevance.

Presentation of projections will differ by climate variable

Projection information on the website is presented in a number of ways (e.g. statements on the direction of change, quantitative ranges of area average change, sets of maps of spatially detailed change from GCMs or downscaling models).  The choice of presentation methods varies between variables because of differences in the availability of relevant modelling results, the nature of the results and confidence in those results.  Important guiding principles are to maintain emphasis on the range of plausible climate change, and to not highlight detailed aspects (such as fine spatial detail) of the climate model simulations available to us where confidence in this detail is low.

‘Projections’, not ‘predictions’

Descriptions of future regional climate are usually referred to as ‘projections’ (or ‘scenarios’, see below).  This term as used here implies a range of change in a climate variable which represents uncertainties such as the differences between the results of various climate models.

A climate change ‘projection’ is also conditional on the choice of a particular future greenhouse gas emission scenario.  This conditionality is the primary reason a projection is not called a ‘prediction’.

Climate scenarios

A ‘climate scenario’ is a physically consistent description of future climate, usually across a range of variables. This means that an individual future climate sitting within the range of future climates described by a set of projections is considered to be a future climate ‘scenario’.  Such a scenario is also called a ‘climate future’ within the Climate Futures web tool available on this website. A single simulation from a climate model can also provide a scenario.