I am comparing the correlation coefficient of variables during a period spanning 1963 - 2001 with the correlation coefficient of the same variables during a period spanning 2002 - 2005. Obviously, the first correlation coefficient is based on almost 10 times as many observations as the second. Can this explain the fact that the first correlation coefficient is higher than the second?
Its not the number of observations in itself that's matter, but a low number of observations do not represent the population very well. With observations from a longer timespan eventual trends will be clearer, and could produce spurious correlation.
You need to look at plots! Is there a visually obvious trend? Is the variance constant?