EARTH'S ICE AGES

Evening Post, Volume CXXII, Issue 67, 16 September 1936, Page 18

EARTH'S ICE AGES

RIDDLE OF THEIR CAUSE

IS THE SUN A VARIABLE

STAR?

ANSWER OF SCIENCE

For nearly a century there has been general agreement among geologists that remarkable variations of climate have occurred during what is known as geological time, writes the astronomical correspondent of the "Manchester Guardian." The famous Swiss geologist Louis Agassiz was the first to show that in bygone days extensive glaciation was widespread in what are now the temperate zones. Glaciers, as most people know, are formed by snow which collects on the top of high mountains and, sliding downwards, becomes packed more and more firmly together .until it is compressed into ice. Carrying with it sand, stones, and loose rocks, the glacier moves downwards till it reaches a point where the surrounding air is warm, enough to melt it. Flowing away as water, it leaves behind it the solid material which it has collected, and which is known as a moraine. Agassiz, while examining the rslopes of the Alps many tftindred feet below the present regions of glaciation, and in places where the heat is too great for the existence of masses of ice, found traces of huge moraines, indicating that in the past glaciers must have been much larger and must have existed at lower altitudes than is now possible.' He also found in these regions, what are called glacial scratches, and discovered huge pieces of rock on the Jura mountains of the same make-up as the Alps. These pieces of rock, he showed, must have been; at some remote date, carried right across Switzerland before coming to rest in the Jura region. Other geologists confirmed the work of Agassiz, and found traces of glaciation all over the temperate zones of the Northern Hemisphere. So geologists came to the conclusion that in the distant past the temperate zones were covered with ice, as Greenland is at the present time. This period was named the Ice Age or the Glacial Epoch. Modern geologists recognise four glacial epochs. The maximum of the first is dated about 500,000 "years ago, and of the last 50,000. Sandwiched in between these there were the three "interglacial" periods. THEORIES AS TO CAUSE. While there has been for many years general agreement as to the occurrence of these Ice Ages in the past, there has been nothing like unanimity as to their cause. Various theories have been put forward to account for them, but none has found universal acceptance. A recent number of the "Observatory," the semi-official monthly review of astronomy, gives an interesting summary of a discussion on the Tee Ages which took place in the rooms of the Royal Astronomical Society, prominent astronomers, meteorologists, and geologists taking part. Sir George Simpson, director of the Meteorological Office, pointed out that meteorology can give us no information concerning past climates. Only geology can do this, and it has done so. It remains for meteorology and astronomy to supply plausible hypotheses as to the cause of these climatic changes. : There are three theories, which may be called the geographical, the astronomical, and the "solar radiation" theories. The geographical theory suggests that climatic changes have been due to alteration in the height and extent of the continents and, as a result, changes in the volume and direction of ocean currents. Sir George Simpson rightly points out that this hypothesis might account for one glacial epoch, but it cannot explain a succession of almost identical changes. The astronomical theory postulates that climatic variations have been due mainly to changes in the eccentricity of the earth's orbit. The earth's orbit, as everybody knows, is in elipse with small eccentricity. All the planetary orbits, the earth's among them, are subject to changes in eccentricity, due to the mutual gravitational pull of the planets on one another. When the eccentricity is at its maximum value, the earth at perihelion—the point of its orbit nearest to the sun—will be 5,000,000 miles nearer to the sun than it is at present. AT A HIGHER FIGURE. .Dr. Croll, a famous Scottish geologist of the last century, found that in the course of the past 3,000,000 years there were a number of epochs when the eccentricity of our orbit was three or four times greater than at present. If the earth at such periods was at its nearest to the sun during our northern summer there would be a short hot summer, and a long, cold winter during which ice and snow would accumulate to-such an,-extent that the summer heat would'be; insufficient to melt it. The ■ theory: has been discredited on mathematical grounds, and various modifications have been suggested, but none of these has proved to be satisfactory. There remains the third theory, that the explanation, of the Ice Ages is to be found neither in variations in the level of the' terrestrial continents nor in changes in the eccentricity of the earth's orbit, but in. variations in the output of solar heat. Sir George Simpson inclines'. to 'this theory. An Ice Age, he remarks, would be caused not by a decrease but by an increase in solar : radiation. Such an increase would result in a general rise of temperature at the earth's surface, and this in turn would Cause an increase in the genera! circulation of the atmosphere and in the amount of cloudiness. • , AREA WOULD INCREASE. In consequence, the snow would accumulate to such an extent that the polar ice-caps and the ice-fields on mountains would increase in area. A still further output of heat on-the part of the sun would counterbalance this, however, and the ice would melt. "This," says Sir .George, "would produce an interglacial ■ epoch different from the one we are living in; it would be warm and wet, while our present interglacial epoch is dry and cold."

The Astronomer Royal, Dr. Spencer Jones, agrees that this third hypothesis is the most plausible explanation of the climatic changes which the earth has experienced, and that the sun is probably a variable star of long period. Measurements of the output of solar heat have been carried on over too short a period, to permit of any empirical test of the theory. There is a certain amount of evidence for minute variations in the light and heat of the sun. Indeed, it is likely that all stars are variable to a slight extent. "From the broad physical point of .view," says .Dr. Spencer Jones, "it should not be surprising if the sun's output of heat were variable; we should rather be surprised if it were not. A variation in the stellar magnitude of .the. .sun of .about half a magnitude would be adequate to account for glacial epochs; such variation is not large for a variable .star* and might :wclL occurs