Medieval Maximum. The complicated climatic effects of varying IR, visible, and UV radiation are not properly understood in terms of a global climate model for Earth, nor do we understand the nature and cause of the variations at the Sun.
An immediate question concerns the substantial increase in the general level of activity of the Sun through the 20th century, with an increase in brightness inferred to be nearly 0.1%. Unfortunately, this has occurred on top of the observed increased abundance of greenhouse gases, particularly carbon dioxide, in the atmosphere, the observed warming of the ocean surface waters charged with dissolved carbon dioxide, and the burning of fossil fuels that releases carbon dioxide into the atmosphere. There is no evident way to disentangle the climatic consequences of these separate processes, nor to distinguish which is cause and which is effect. On the other hand, as already noted, U.S. policy making on global change requires an understanding of solar variability and its effects on climate, so the problem is urgent and baffling.
We clearly need observations of the extreme phases of solar activity as exemplified by the Maunder Minimum and the Medieval Maximum. We may reasonably expect the Sun to pass through such phases over the next 1,000 years, but millennial research is not timely. Therefore, we turn to the only source of information available—monitoring a large sample of distant solar-type stars.
Fortunately, the effort has already been started in a modest way by a dedicated group of nighttime observers who have carried on the monitoring of the chromospheric activity of other stars, begun over 30 years ago by O.C. Wilson. For the last decade they have monitored the brightness of several nearby solar-type stars, with the interesting discovery that one of the stars is resting in a Maunder Minimum state, while another slid through a weak activity maximum into the Maunder phase. In addition, they have found an approximate universal linear relation between changes in brightness and the activity level that applies to the Sun and other middle-aged late main-sequence stars. These observers have also found that the brightness of one of their stars declined by about 0.4 percent over a few years of sharply declining activity, suggesting a similar possibility for the Sun at some point in time, presumably accompanied by significant terrestrial climatic effects.5 The observations also show that early in its life (during the first 3 × 108 years), a star like the Sun is fainter, rather than brighter, during times of high activity, suggesting a preponderance of sunspots over faculae. This is yet another dimension to the physics of the rotating convecting star.
Lean, J., Skumanich, A., and White, O. 1992. Estimating the Sun's radiative output during the Maunder Minimum. Geophys. Res. Lett. 19: 1595-1598.