Figure 13 Long-term variations of the three Milankovich orbital parameters: eccentricity (a), precession (effectively the Earth-Sun distance on June 21st (b)) and tilt (c) of the Earth.
The value of the solar constant (total amount of radiation incident on the earth in 1 year) is not just an estimate; it is determined for example from observations of the american Solar Maximum Mission in 1987. These measurements showed that although the solar constant S is not perfectly constant, the variation is less than 1% of the average value. Thus, the solar constant (S) is variable. Why?
The most logical factor is the variation of the intensity of the Sun. The radiation output from the Sun does vary on a wide range of time-scales, from days to millions of years. Beyond the very slow evolution of the Sun, i.e., as a hypothesis of stellar evolutionary theory, there is direct observational evidence for shorter-term variations in solar irradiance. Scientists have long tried to link sunspots to climatic changes. Sunspots are huge magnetic storms that show up as cooler (dark) regions on the Sun's surface. They occur in cycles, with their number and size reaching a maximum approximately every 11 years (Schwabe cycle). However, they are thought to have relatively little effect on Earth's climate. First, these variations are very small: less than 0.1%, and second, they are also too short-term to influence the more slowly responding parts of the climate system like ice-sheets, glaciers, ocean, etc. (see also Section 5.1).
The length of the Schwabe cycle (defined through the interval between successive sunspot maxima) varies between 8 and 12 years over a period of about 80 years (Gleissberg cycle). Statistical analysis shows a good match between the average surface temperature and the length of the Schwabe cycle. Lower-than-normal surface temperatures tend to occur in years when the sunspot cycle is longest, and visa versa. This close correlation could account for the average surface temperature changes from 1940 back to the 16th century, and could partly explain the slightly cooling phase between 1940-1970. The period known as the Little Ice Age corresponds to a minimum level of sunspot activity (the Maunder Minimum, 1645-1715), the estimated change in solar irradiance is a 70-year-long reduction of about 0.14%. Studies with climate models (Chapter 3) suggest that such a drop would neither be large enough, nor long enough to explain the observed cooling during the Little Ice Age. However, these climate models do not include the observed match between surface temperature and the length of the Schwabe cycle. But as long as the mechanism behind this correlation is not well understood, it cannot be incorporated in these climate models. Moreover, close correlation without a realistic mechanism does not prove anything. Recent studies about a possible mechanism focus on a connection between the global cloud cover and cosmic rays.
Figure 13 Long-term variations of the three Milankovich orbital parameters: eccentricity (a), precession (effectively the Earth-Sun distance on June 21st (b)) and tilt (c) of the Earth.
Another reason why S varies is because of changes in the average distance Earth-Sun. The eccentricity expresses to what extent the Earth's orbit around the Sun differs from a circle. The orbit is somewhat elliptical, and a higher eccentricity corresponds to a more elliptical orbit. The eccentricity varies regularly with periods of about 100.000 and 400.000 years (Figure 13).
The maximum change in S associated with variation in eccentricity is about 0.1%. However, the eccentricity has a stronger influence on the seasonal variation of short-wave radiation. When eccentricity is relatively high, the Earth receives more radiation on days when it is closer to the Sun (see Figure 14). This intensifies the seasons in one Hemisphere but moderates them in the other.
The eccentricity is just one of three orbital parameters that influence seasonal and latitudinal changes in short-wave radiation reaching the Earth. The angle of tilt of the Earth's axis of rotation (Figure 14) varies between 22° and 24.5° with a periodicity of about 40 000 years (Figure 13). The greater the tilt, the more extreme the seasons in each Hemisphere become. For example, the duration of the winter-darkness near a pole is determined by the tilt only.
Figure 14 Variation of the orbital parameters as seen from space. The ellipse of the Earth's orbit is somewhat exaggerated.
Precession is the change of direction in space of the Earth's axis of rotation. The axis changes direction with a periodicity of about 22 000 years (Figure 13). There is some correspondence between a top; the axis of rotation swings according to an imaginary cone perpendicular to the plane of the Earth's orbit (Figure 14). The precession affects the seasonality in each Hemisphere because it determines when the distance between the Earth and Sun is at a minimum or maximum. At present, the distance is a minimum in the Southern Hemisphere winter, so the Southern Hemisphere has on the whole slightly warmer summers and colder winters. The distance Earth-Sun in June 21st, as shown in Figure 13, is controlled by both precession and eccentricity. Active calculation of the solar radiation on an arbitrary place on Earth and time of year is not so easy. In practice the tilt of the Earth's axis of rotation becomes more important polewards.
All the above mentioned components of the orbit vary because of the gravitational attraction between the Earth and the other planets. The orbital changes are known as Milankovich orbital changes. He did put forward the theory that the periodic changes of climate between glacial and interglacial are related to the orbital changes of the Earth. They are only important for climate changes on very long timescales. Hence, when modelling the enhanced greenhouse effect, the solar constant can probably be handled as a true constant.
Links:
Papers by wilfried schröder in j atmosph terr phys, 1979 ; phys atmopsh.1988 ; j geom. geoelectr. 1992 ; acta geod. geophys. mont hung, 1992, incl. climatological part ; geopf internacional 1999
Book: Solar Variability and Geomagnetism, D-28777 Bremen, Science Edition