In general, uncertainty ranges for results given in this Summary for Policymakers are 90% uncertainty intervals unless stated otherwise, 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. Best estimates are given where available. Assessed uncertainty intervals are not always symmetric about the corresponding best estimate. Note that a number of uncertainty ranges in the Working Group I TAR corresponded to 2-sigma (95%), often using expert judgement. (page 2)
In this Summary for Policymakers, the following terms have been used to indicate the assessed likelihood, using expert judgement, of an outcome or a result (page 4):
- Virtually certain > 99% probability of occurrence
- Extremely likely > 95%
- Very likely > 90%
- Likely > 66%,
- More likely than not > 50%
- Unlikely <33%>
- Very unlikely <10%
- Extremely unlikely <5%
- Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. This is an advance since the TAR’s 2001 conclusion that “most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations”.
In this Summary for Policymakers the following levels of confidence (page 5) have been used to express expert judgments on the correctness of the underlying science:
- very high confidence at least a 9 out of 10 chance of being correct
- high confidence about an 8 out of 10 chance of being correct.
- The understanding of anthropogenic warming and cooling influences on climate has improved since the Third Assessment Report (TAR), leading to very high confidence that the globally averaged net effect of human activities since 1750 has been one of warming
Reference: Intergovernmental Panel on Climate Change IPCC pdf report