"90 percent of the variation" is misleading when comparing the levels of one time-series against another. It's very easy to find two time-series that regress almost perfectly on one another because both steadily increase. Looking at first-differences is more informative about possible causal relations. The image Cyan posted is effectively two data points from a first difference perspective: both went up and then both went down.

Another graph from Nevin's website is slightly more persuasive:

There you can see 4-6 corresponding changes in trend. Still not that impressive, but maybe enough to start looking more closely.