The secular variation of the Earth's magnetic field is itself subject to temporal variations. We investigate these with the aid of the coefficients of a series of spherical harmonic models of secular variation deduced from data for the interval 1903-1982 from the worldwide network of magnetic observatories. For some studies it is convenient to approximate the time variation of the spherical harmonic coefficients with a smooth, continuous, function; for this we have used a spline fitting. The phenomena that are investigated include periodicities, discontinuities and correlation with the length of day. The numerical data we present will be of use for further investigations and for the synthesis of secular variation at any place and at any time within the interval of the data—they are not appropriate for temporal extrapolations.