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INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE
WMO UNEP
Guidance Notes for Lead Authors of the
IPCC Fourth Assessment Report on Addressing Uncertainties

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Treatment of uncertainty
The IPCC uncertainty guidance note1 defines a framework for the treatment of uncertainties across all WGs and in this Synthesis Report.
This framework is broad because the WGs assess material from different disciplines and cover a diversity of approaches to the treatment of
uncertainty drawn from the literature. The nature of data, indicators and analyses used in the natural sciences is generally different from that
used in assessing technology development or the social sciences. WG I focuses on the former, WG III on the latter, and WG II covers aspects
of both.
Three different approaches are used to describe uncertainties each with a distinct form of language. Choices among and within these three
approaches depend on both the nature of the information available and the authors’ expert judgment of the correctness and completeness of
current scientific understanding.
Where uncertainty is assessed qualitatively, it is characterised by providing a relative sense of the amount and quality of evidence (that is,
information from theory, observations or models indicating whether a belief or proposition is true or valid) and the degree of agreement (that is,
the level of concurrence in the literature on a particular finding). This approach is used by WG III through a series of self-explanatory terms
such as: high agreement, much evidence; high agreement, medium evidence; medium agreement, medium evidence; etc.
Where uncertainty is assessed more quantitatively using expert judgement of the correctness of underlying data, models or analyses, then
the following scale of confidence levels is used to express the assessed chance of a finding being correct: very high confidence at least 9 out
of 10; high confidence about 8 out of 10; medium confidence about 5 out of 10; low confidence about 2 out of 10; and very low confidence less
than 1 out of 10.
Where uncertainty in specific outcomes is assessed using expert judgment and statistical analysis of a body of evidence (e.g. observations
or model results), then the following likelihood ranges are used to express the assessed probability of occurrence: virtually certain >99%;
extremely likely >95%; very likely >90%; likely >66%; more likely than not > 50%; about as likely as not 33% to 66%; unlikely <33%; very
unlikely <10%; extremely unlikely <5%; exceptionally unlikely <1%.
WG II has used a combination of confidence and likelihood assessments and WG I has predominantly used likelihood assessments.
This Synthesis Report follows the uncertainty assessment of the underlying WGs. Where synthesised findings are based on information
from more than one WG, the description of uncertainty used is consistent with that for the components drawn from the respective WG reports.
Unless otherwise stated, numerical ranges given in square brackets in this report indicate 90% uncertainty intervals (i.e. there is an
estimated 5% likelihood that the value could be above the range given in square brackets and 5% likelihood that the value could be below that
range). Uncertainty intervals are not necessarily symmetric around the best estimate.