I met a climate scientist the other day...

...the conversation went sometihng like this...

Scientist: Look at my marvellous study. Do you see that squiggly line? It shows that temperatures 1000 years ago were not very high compared with today because tree rings show smaller amounts of growth then. The earth is much hotter now than it has ever been.

Me: Fascinating. What do those same tree rings show about recent history, say the last 50 years?

Scientist: Well, they show very little growth over the last 50 years.

Me: You mean just like the periods in the past where you claim they indicate low temperatures.

Scientist: Well, yes

Me: So, doesn't that imply that the tree rings indicate that temperatues over the last 50 years aren't higer than 1000 years ago?

Scientist: No, because I take them off the chart and you don't see them, so we can just ignore that.

Me: But isn't that, sort of, cheating?

Scientist: Of course not. It is a "trick" - that is a technical scientific phrase meaning a special adjustment.

Me: But doesn't this hide a decline in the tree rings that would indicate temperatures in the last 50 years aren't particularly high compared with 1000 years ago?

Scientist: It is very technical, but there is something in the data that has caused the decline which isn't related to temperature, so it makes sense to leave it out - hide it if you will. We call call this problem the "divergence problem"

Me: What is this "something" that is effecting the data over the last 50 years? What is the cause of this "divergence problem"?

Scientist: We don't know.

Me: Might this same "something", that you don't know about have affected tree rings in the past?

Scientist: We don't know.

Me: Doesn't this make your analysis all a bit, what is the technical term, ropey?

Scientist: Not at all, we can employ this trick. You see if we hide it then you can't see the decline.

Me: Didn't we cover this...?