Climate Change: 2013 IPCC Update

1. How are uncertainties handled by the IPCC?

Although the body of knowledge on the climate system is growing, and the confidence in projection is growing as well, there are still many uncertainties in climate science. The Intergovernmental Panel on Climate Change’s treatment of uncertainty has becmedium confidenceome more and more sophisticated from one Assessment Report to the next, but the rapid growth and considerable diversity of climate research literature presents ongoing challenges.

An integral element of the 5^th assessment report is the use of a specific uncertainty language. The degree of certainty in key findings in this assessment is expressed as a level of confidence that is based on the type, amount, quality, and consistency of evidence and the degree of agreement in the scientific studies considered.

Where appropriate, findings are also formulated as statements of fact without using uncertainty qualifiers, but when these are needed, there are two metrics that are used in this report for communicating the degree of certainty in key findings:

• Confidence in the validity of a finding, based on the type, amount, quality, and consistency of evidence (e.g., data, mechanistic understanding, theory, models, expert judgment) and the degree of agreement. Confidence is expressed qualitatively and reflects the judgements of the authors of the chapter about the validity of findings.

Confidence increases toward the top-right corner as suggested by the increasing strength of shading. Generally, evidence is most robust when there are multiple, consistent independent lines of high-quality. {Figure 1.11}

• Quantified measures of uncertainty in a finding expressed as a level of likelihood. The likelihood statements in this report can be used to express an estimate of the probability of an event occurring, or of an outcome, such as a projected change lying in a given range. Statements made using the likelihood scale may be based on statistical or modelling analyses, elicitation of expert views, or other quantitative analyses.

The following terms have been used to indicate the assessed likelihood:

When they are used in this summary, these statements are indicated in italics. More...

Term*	Likelihood of the outcome
* Additional terms (extremely likely: 95–100% probability, more likely than not: >50–100% probability, and extremely unlikely: 0–5% probability) may also be used when appropriate.
Virtually certain	99–100% probability
very likely	90–100% probability
Likely	66–100% probability
About as likely as not	33–66% probability
Unlikely	0–33% probability
Very unlikely	0–10% probability
Exceptionally unlikely	0–1% probability