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Introduction7. projection of a climate-sensitive 'impact' variable;8. identification of adaptation needs.Different types of statements are subject to differentsources of uncertainty. As a general rule, the(sources of) uncertainty increases from observationsto attributions and projections, and from climatevariables to climate impacts and adaptation needs(see Figure 1.1). For example, observations of aclimate or climate impact variably (numbers 1and 4 above) can be made for short time serieswhereas statements about statistically significanttrends (numbers 2 and 5 above) require longer timeseries and the consideration of natural interannualvariability. With respect to projections, the futuretrajectory of GHG emissions is a relevant sourceof uncertainty for long-term climate and climateimpact projections but not for observations of thepast. Also, near-term projections (e.g. up to 30 years)show limited sensitivity to future GHG emissionsscenarios due to the long residence time of mostGHGs and the large thermal inertia of the climatesystem.Key messages are formulated so that it is clearwhat type of statement they make. Note that thetype of statement supported by a particular datasetmay depend on the spatial scale. For example, asignificant climate trend may be detectable at thecontinental scale (where year-to-year variabilityis low) but not in each region (where year-to-yearvariability is higher and regional factors may beimportant). For the sake of clarity, the combinationof different types of statements in a single messageis generally avoided.Appropriate choice of the level of precisionIt is useful to consider several different levels ofprecision (or quantification) in key messages, whichare ordered here from least to most precise (see alsothe IPCC uncertainty guidance (IPCC, 2005):1. existence of effect (but the direction isambiguous or unpredictable);2. direction of change or trend;3. order of magnitude of a change (e.g. indicatedby a semi-quantitative verbal statement);4. range or confidence interval;5. single value (implying confidence in allsignificant digits).Figure 1.1Cascade of uncertainties in climate impact assessments? ? ? ?EmissionscenariosCarbon cycleresponseGlobal climatesensitivityRegional cliamtechange scenariosRange ofpossible impactsNote:The length of the bars represents the magnitude of the uncertainty.Source: Ahmad et al., 2007, figure 2–2.44 Climate change, impacts and vulnerability in Europe 2012