5.2 The secular trend

Since the Maunder Minimum there seems to have been a steady increase in sunspot cycle amplitudes (Wilson, 1988). This is readily seen in the yearly Group Sunspot Numbers plotted in Figure 33View Image and in the cycle amplitudes for Group Sunspot Numbers plotted in Figure 23View Image. Hathaway et al. (2002) found a correlation coefficient of 0.7 between cycle amplitude and cycle number. This linear trend is not so apparent in the International Sunspot numbers plotted in Figure 23View Image. It obviously cannot continue forever but may represent an important characteristic of the solar cycle that should be represented in viable models for the cycle. Radioisotopes also show this recent trend (Solanki et al., 2004) and indicate many upward and downward trends over the last 11,000 years.
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