A climate forecast useful for government policy-making would need to predict a lot of things:
(1) future global average temperatures,
(2) effects of global temperature changes,
(3) effects of alternative government policies, and
(4) whether the best government policy could be successfully implemented.
For the past 30 years we have used computer models that are unable to accomplish (1).
IPCC (UN) climate change reports are nothing more than the personal opinions of a group of activists and scientists.
Those personal opinions could have been written on the back of envelopes, but that would not be persuasive.
So the personal opinions were transformed and obscured by by complex mathematics, computer models and writing.
But the IPCC reports always say the same thing:
Without any scientific proof, we believe adding CO2 to the air, by burning fossil fuels, is going to end all life on Earth, through runaway global warming, unless you do as we say without question!
The bottom line is that unaided judgmental forecasts by experts are personal opinions.
They have no predictive value, whether presented in words, spreadsheets, or complex mathematical models.
Among the reasons for this are:
People can't assess complex relationships through unaided observations.
People confuse correlation with causation.
People have difficulty using evidence that contradicts their initial beliefs.
Agreement among experts is weakly related to accuracy.
Complex computer models offer many opportunities for errors, and the complexity makes errors difficult to find.
Concerns about changes in average global temperature are based on the false assumption that the Earth is currently at an optimal temperature, so any changes are undesirable.
Many policy decisions are based on forecasts by experts.
But forecasts that happen to be made by scientists are not necessarily scientific, or correct, forecasts.
Especially forecasts about a subject where there are so many unanswered questions, such as climate change!
“Today’s scientists have substituted mathematics for experiments, and they wander off through equation after equation and eventually build a structure which has no relation to reality."
Nikola Telsa, inventor and electrical engineer, 1934