Weather and Climate

Scientists have established a climate correlation with the 10.5- to 11.2-year cycle of sunspots (Figure 2.27). For convenience, the sunspot recurrence is called the 11-year solar cycle. During the Sun's cycle, the number of sunspots increase and then decrease, and the solar regions of spot appearances move from higher to lower solar latitudes. The reason for the climate response has been ascribed to a change in the balance of solar radiation. Sunspot regions are cooler, giving off less radiation than the average solar surface. However, when there is a maximum in sunspot occurrence, the regions of the Sun's surface near the spots are more active. Studies have shown that, for unknown reasons, during a number of consecutive cycles either the cooler spots or the activity regions remain dominant. The radiation changes reaching the Earth affect the climate in such a way that for many solar cycles some continental regions are dryer or wetter, cooler or hotter, in tune with the 11-year cycle. For example, the sunspot cycle appears in the amplitude of the annual flooding of

FIGURE 2.27 ► Sunspots and their nearby regions of granulation. A part of the Earth's recurring climate change can be ascribed to a modulation of the solar radiation output associated with the 11-year cycle of sunspots and their adjacent active regions. Geomagnetic disturbances on Earth have a similar 11-year cycle.

FIGURE 2.27 ► Sunspots and their nearby regions of granulation. A part of the Earth's recurring climate change can be ascribed to a modulation of the solar radiation output associated with the 11-year cycle of sunspots and their adjacent active regions. Geomagnetic disturbances on Earth have a similar 11-year cycle.

the Nile River and in the occurrence of special Ethesian winds over Athens, Greece.

Researchers have studied the increase in area of solar coronal holes (regions of singular solar field polarity) that occurs most often during sunspot minimum. They have found a correspondence with the cooling of the Earth's lower atmosphere by about 2 degrees below that region's temperature at sun-spot maximum. However, some scientists have questioned whether a cause-effect relationship exists for this correlation because so many other solar changes are transpiring.

FIGURE 2.28 ► Mesa Verde Indian sites were abandoned because of an extended drought that lasted two full sunspot cycles.

Tree-ring dating (the measurement of the annual growth-ring spacing along a radius of the tree's trunk) tells us that in Mexico and southwestern United States, beginning in 1276, there occurred two consecutive 11-year solar cycles of dry climate. Because low crop yields could not meet the population demand, near the end of that long drought period the Anasazi Mesa Verde Indians abandoned their cliff dwellings and migrated to other more fertile farmlands. These ancient dwellings (Figure 2.28), preserved by their unique cliff locations in southwest Colorado, are now both a major tourist attraction and stark evidence of the sunspot-climate connection.

We shall see, in Chapter 4 that the solar-terrestrial outbursts of particles and fields coincide with the 11-year sunspot activity cycle. Geomagnetic storms, recorded at world magnetic observatories, result from these solar-terrestrial disturbances and represent the flow of a variety of strong currents of particles through locations prescribed by the unique design of the Earth's field in space and the upper-atmospheric ionosphere. Some scientists have looked at these processes as triggers for weather changes on Earth. Unique low-pressure areas in the polar regions often develop a few days following magnetic storms. Intense electric currents are guided along the Earth's field lines at high latitudes and cause heating in the high atmosphere and a world-traveling pressure wave. Originating in the auroral region, an infrasonic (much below audible frequencies) pressure wave moves in the atmospheric region near the Earth to low-latitude locations (see Section 4.1.4, p. 111).

Was this article helpful?

0 0

Post a comment