78 / Basics of Environmental Science
These axial variations alter the area illuminated by the Sun. If the axis were at right angles to the
ecliptic, for example, giving an obliquity of 0°, the half of the Earth facing the Sun would be lit
evenly. Day and night would always be the same length and there would be no seasons. Tilt the Earth
much more, on the other hand, say to an obliquity of 60°, and over almost the whole of each hemisphere
the Sun would never set in summer or rise in winter.
Dramatic climate change occurs when the three cycles coincide, and the Milankovich solar radiation
curve, which combines the three, is used to make long-term climatic predictions (deschutes.gso.uri.edu/
~rutherfo/milankovitch.html). It is this that allows climatologists to assert that a cooling trend which
began about 6000 years ago will continue, leading us into a new ice age (HIDORE AND OLIVER,
1993, pp. 370–371), although the solar influence may be overridden by that of greenhouse gases if
these continue to accumulate in the atmosphere.
In the shorter term, the solar output itself also varies. The first person to relate this to climate change
was the British astronomer Edward Walter Maunder (1851–1928). Like many astronomers, he was
interested in sunspots (es.rice.edu/ES/humsoc/Galileo/Things/sunspots.html), dark ‘blotches’ on the
surface of the Sun that come and go in a cycle of about 11 years. Checking through old records of
sunspot activity, in 1893 he discovered that very few sunspots were reported during a period of 70
years from 1645 to 1715, and for 32 years, it seems, there were no sunspots at all. He published a
paper describing his findings in 1894, but it attracted little attention, any more than did earlier papers
challenging the idea of the constancy of solar output, published by Maunder and by the German
astronomer Gustav Spörer (1822–95). Today, the period during which sunspots were much reduced
in number is known as the ‘Maunder minimum’. Its significance extends far beyond the realms of
solar astronomy, because the 1645–1715 minimum Maunder identified coincides with the peak of
the ‘Little Ice Age’, when average temperatures were about 1°C lower than they had been previously
(LAMB, 1995, pp. 69 and 321). More recently, the American solar astronomer John A.Eddy checked
the Maunder and Spörer findings, added more of his own, and found a correspondence between solar
Figure 2.28 Variations in axial tilt (obliquity of the ecliptic) in degrees