Reality Check
Penultimate Amazing
Obviously I did not make myslf clear enough.It's true of global warming because global warming is a fast enough change that 20 years of data is, indeed, usually enough to show it, but it's not true of all trends everywhere. This is the mistake r-j has been making all along.
It is true that global temperatures have significant trends when looking at periods of > 20 years.
It is (AFAIK) also true that local temperatures should have significant trends when looking at periods of > 20 years.
As you know, in both cases it is the noise in the data that determines whether a trend is statistically significant. I suspect that local temperatures usually have less noise than global temperatures, especially over land where the effect of sea temperature oscillation may be less. However you are right - the significance of local trends will vary.