The Fourth Assessment Report is made up of four sections:
One additional note - the IPCC reports use very formal terminology when discussing the likelihood of various events, and the degree of confidence in various predictions. Here is the terminology as defined in the Appendix of the Synthesis Report:
The first three reports were developed by three separate Working Groups, and are therefore also known as the WG1, WG2, WG3 reports.
Although the AR4 Synthesis Report was released after the WG1,2,3 reports, I'll be reading it first, as it functions as a summary of everything contained in the other reports.
The Synthesis Report has five sections:
Terminology Likelihood of the occurrence / outcome Virtually certain >99% probability of occurrence Very likely >90% probability Likely >66% probability More likely than not >50% probability About as likely as not 33 to 66% probability Unlikely <33% probability Very unlikely <10% probability Exceptionally unlikely <1% probability
The AR4 is full of sentences like this:
Terminology Degree of confidence in being correct Very high confidence At least 9 out of 10 chance of being correct High confidence About 8 out of 10 chance Medium confidence About 5 out of 10 chance Low confidence About 2 out of 10 chance Very low confidence Less than 1 out of 10 chance
"It is very likely that over the past 50 years: cold days, cold nights and frosts have become less frequent over most land areas, and hot days and hot nights have become more frequent. It is likely that: heat waves have become more frequent over most land areas..."
It's important to recognize that terms like "very likely" and "likely" in the AR4 refer to an explicitly quantitative probability.
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