Graph of yearly averaged sunspot numbers 1610 to 2000 and text follows below:
The Maunder Minimum represents the coldest phase of the Little Ice Age, when solar activity was particularly low and there was an almost total absence of sunspots. In a previous blog post I presented evidence for the LIA in Australia, suggesting 17th century global cooling. There was also a slight fall in atmospheric CO2 concentrations for the period between 1550 to 1800.
Drew Shindell of NASA’s Goddard Institute for Space Studies has attempted to explain the Maunder Minimum using a climate model. According to Shindell’s results less strong ultraviolet light was emitted by the sun, which in turn caused less ozone to be formed in the stratosphere. As a result the North Atlantic Oscillation became strongly negative, causing Europe to be unusually cold.
Read the story in more detail here.
Looking at the graph of yearly averaged sunspot numbers, we can discern a rising trend in solar activity following the Maunder Minimum, peaking in the 20th century, and remaining high despite a fall after 1950. Some solar scientists, including NASA’s David Hathaway are predicting a big fall in solar activity in the not too distant future, with an uncertain impact on global temperatures. Another Maunder type minimum on the way? We may not have to wait too long to find out.
Some notes about solar activity/the solar constant:
Measurements of the Nimbus-7 and Solar Maximum Mission satellites reported temporary large decreases of the solar constant of the order of a few tenths of a percent on a time-scale from days to weeks. Investigations show that these decreases were caused by ‘active’ sunspot groups with fast development and complex structure. This connection between the solar constant variation and the appearance of the active groups seems to be clearer in the maximum of the solar activity.
The intensity of the Sun varies along with the 11-year sunspot cycle. When sunspots are numerous the solar constant is high (about 1367 W/m2); when sunspots are scarce the value is low (about 1365 W/m2). Eleven years isn’t the only “beat,” however. The solar constant can fluctuate by ~0.1% over days and weeks as sunspots grow and dissipate. The solar constant also drifts by 0.2% to 0.6% over many centuries.
Samuel Langley and Charles Greeley Abbot of the Smithsonian recorded direct measurements of the solar constant (the level of the Sun’s radiation) over several decades. They concluded that this “constant” varies by about 0.3% on the short-term scale of several days and that on the longer term, the more active Sun is brighter by about 1%. [Hufbauer, 1991]
The proxy relationships observed during solar cycle 21 and the behavior of other sun-like stars [Baliunas and Jastrow, 1990] have been used by Lean et al.  to estimate the solar irradiance during the Maunder Minimum as somewhere between 0.15 and 0.35% lower than the present solar-cycle mean value. An independent estimate by Baliunas and Jastrow  gave a range of 0.1 to 0.7% based purely on observations of solar-like stars, discussed by Lockwood et al. . The use of other stars to infer solar variability has been questioned by Schatten , however, who has pointed out that the observed irradiance is likely to be a function of the heliographic latitude of the observer, being a minimum near the solar equatorial plane, where the Earth is located. Since other stars are observed at random latitudes relative to their spin axes, the variations observed might not be directly relevant to the local situation.
Baliunas and Jastrow  conclude that a reduction in irradiance of 0.4%, in the middle of their calculated range, would be enough to explain the cold average temperatures of the Little Ice Age, as estimated by Wigley and Kelly . Hoyt and Schatten  have used a variety of possible proxies for solar irradiance to estimate a value for the Maunder Minimum period that is about 5 W m-2, or about 0.36% below current values, in general agreement with other estimates. Rind and Overpeck  used a general circulation model to estimate the regional temperature changes caused by a decrease of solar irradiance by 0.25%, in the middle of the range estimated by Lean et al. . They found a global average reduction of 0.45C with no clear latitudinal variation, and with the largest effects over the continental land masses.