  Here we study the prediction of even and odd numbered sunspot cycles
separately, thereby taking into account the Hale cyclicity of solar magnetism.
We first show that the temporal evolution and shape of all sunspot cycles are
extremely well described by a simple parameterized mathematical expression. We
find that the parameters describing even sunspot cycles can be predicted quite
accurately using the sunspot number 41 months prior to sunspot minimum as a
precursor. We find that the parameters of the odd cycles can be best predicted
with maximum geomagnetic aa index close to fall equinox within a 3-year window
preceding the sunspot minimum. We use the found precursors to predict all
previous sunspot cycles and evaluate the performance with a cross-validation
methodology, which indicates that each past cycle is very accurately predicted.
For the coming sunspot cycle 25 we predict an amplitude of 171 +/- 23 and the
end of the cycle in September 2029 +/- 1.9 years. We are also able to make a
rough prediction for cycle 26 based on the predicted cycle 25. While the
uncertainty for the cycle amplitude is large we estimate that the cycle 26 will
most likely be stronger than cycle 25. These results suggest an increasing
trend in solar activity for the next decades.
