Volcanoes and World Climate-- Harry Wexler, SCIENTIFIC AMERICAN 1952
THE EASTERN U. S. has just had another mild winter, which again raises the perennially fascinating subject of weather trends. Two years ago George H. T. Kimble, director of the American Geographical Society, called attention in an article in this magazine to the fact that winters in the Northern Hemisphere have been growing warmer for some time ("The Changing Climate"; SCIENTIFIC AMERICAN, April, 1950). That the earth has longterm spells and seesaws of climate is attested both by recorded history and by the vastly longer geological records of the ice ages.
(The news clipping is from the St. Paul Globe, 17 August 1902)
Theories about the causes of long-run fluctuations in climate are almost as numerous and various as the kinds of weather we have. This article will discuss one theory that seems to gain support from recent events in the world's weather. But to place it in the context of thinking on this subject, let us first consider very briefly some of the other leading theories.
To begin with, there is the theory long favored by some geologists: that major changes in the earth's climate have been due to geological revolutions which altered the planet's topography. The main objection to this idea is that we have apparently had large climatic changes during· the last few thousand years in regions of the earth where the topography has been stable. In recent years a very different idea has been growing in popularity: namely, that climatic shifts are caused by variations, one wav or another, in the radiation the earth receives from the sun, which of course is the prime source of our weather. One school of thought suggests that climatic swings may be due to fluctuations in the sun's energy output. To this there are two objections: 1) variations of the magnitude suggested (10 per cent) have never been observed by the high-alttude instruments that measure solar radiation, and 2) such large variations are also ruled out theoretically by the steady carbon-cycle process by which the sun produces energy. Another theory is that fluctuations in the sun's ultraviolet radiation, related to sunspot activity, may affect our weather, but this has not been proved, either theoretically or statistically.
Then there is the theory recently described by George Gamow ("Origin of the Ice"; SCIENTIFIC AMERICAN, October, 1948). This theory attributes the earth's ice ages and long-range climate trends to wobbles in the planet's spin on its axis and to periodic shifts in its path around the sun, which would change the amount of solar radiation received by the earth's Northern and Southern Hemispheres, respectively. This theory has been answered devastatingly by the British meteorologist Sir George Simpson. He pointed out, among other flaws, that geological evidence indicates the ice ages occurred in both hemispheres simultaneously, not alternately, and that the theory fails to explain the absence of ice ages for a long period before the beginning of the Pleistocene Epoch 700,000 years ago.
THE HYPOTHESIS I wish to discuss here is that explosions of volcanoes may play an important part in shaping long-term variations in the world's climate. It is not a new theory, but there are new reasons now for reviving and reconsidering it. The beginning of the story goes back to a spectacular event almost 69 years ago.
On August 27, 1883, the island of Krakatoa in the Dutch East Indies blew up in a huge volcanic explosion. It threw into the air some 13 cubic miles of rock, dust and ash. Great clouds of the fine volcanic ash rose 20 miles or more into the atmosphere and drifted across the seas and continents of the earth. Many people alive today remember vividly the colorful results. All over the Northern Hemisphere the dusty haze made the sun and moon look blue, purple or green, and produced gorgeously-hued sunsets and rose-colored twilights that lasted more than an hour after sunset.
Three months after the explosion the great drifting dust pall arrived over Europe. During the whole of the last week of November, 1883, it startled and delighted Europeans with its displays of color. But at the Montpellier Observatory in the south of France astronomers noticed something even more remarkable: the sun's radiation, as recorded on their instruments, dropped suddenly from 30 per cent above normal to 20 per cent below normal.
The solar energy received at this Observatory remained 10 per cent below normal for three years after the explosion, as the Krakatoa dust pall hung in the air. In the next three decades there were sew'ral other major volcanic explosions, and observers found that these, too, cut down solar radiation at various places over the world; for example, the pall of ash from the explosion of the Katmai Volcano in the Aleutian Islands in 1912 brought about a 20 per cent reduction in the sun's radiation in Algeria, many thousands of miles away.
The remarkable sunlight-reducing effect of the Krakatoa dust pall led two Swiss scientists, the cousins P. and F. Sarasin, and later the famed U. S. meteorological physicist W. J. Humphreys, to investigate the question whether volcanic dust could influence the earth's weather. Dust in the air scatters, reflects and absorbs the sun's radiation. If volcanic ash in the upper atmosphere could intercept sunlight and appreciably reduce the amount reaching the ground, it might well cool the area below it. Investigation showed that while a dust blanket shuts out some of the sun's radiation, it does not hold in the longer heat waves radiated back from the ground. Humphreys calculated that a volcanic dust pall might reduce the ground-level temperature by several degrees. This alone would mean a noticeable change in climate. But in addition dust clouds would reduce the "sunfall" by different amounts at different places and different latitudes. And since differences in sunfall between the tropics and higher latitudes are what drive the atmospheric clIrrents responsible for weather, volcanic dust clouds could well have far-reaching effects on the world's climate. Bv increasing temperature differences' behveen regions they might accelerate air circulation currents and produce cold, stormy winters and cool, cloudy summers.
Dust particles can float in the air for an amazingly long time. Even the heavier particles take several months to settle, and the finest ash, if thrown high enough, may remain suspended in the atmosphere for many years. A long series of volcanic explosions that built up the dust pall, making the winters stormy and cold and the summers cool, might nourish glaciers and bring on an ice age. On the other hand, in a period of vol· canic inactivitv the earth's climate would gradually grow warmer as volcanic ash settled down and the air cleared.
SUCH IS the theory. Humphreys and his contemporaries were unable to find evidence that the volcanic dust pall actually had the predicted effects on climate. The records at weather stations around the world failed to show any consistent or significant cooling of the globe after the Krakatoa explosion or other great volcanic eruptions. This is not surprising, however, even assuming the theory to be correct, because there were too few weather stations to tell whether an over-all temperature change had occurred. Even today it would be difficult to determine the world-wide temperature mean, for with all our weather stations large areas of the earth, notably the oceans, are still not covered. It is risky to base conclusions on the temperature records over a small area. Regional temperatures, as everyone knows, vary a great deal. For example, the winter of 1948-49 was unusually cold in the Western half of the U. S. (five degrees below normal) and unusually warm in the Eastern half (five degrees above normal) .
The scattered temperature readings surveyed after the Krakatoa explosion therefore prove nothing. Nevertheless, the lack of corroborating evidence discouraged the volcanic climate theory, and it was dropped.
Two recent developments make it seem worth reviving. One is the discovery that dust particles can act as nuclei to form ice crystals in subfreezing air saturated with water vapor. Vincent J. Schaefer has found that particles of volcanic ash are effective nucleating agents at below-freezing temperatures such as exist in the high atmosphere. It therefore seems likely that floating ash may came clouds to form in supersaturated air. A pall of volcanic ash thus might increase cloudines.s in the atmosphere, which would add to the effect of the dust itself in reducing our sunshine.
The second development is the striking fact that since 1912 no major volcanic explosion has occurred in the Northern Hemisphere and during this period the winters have been growing steadily warmer in large parts of the Hemisphere. The average winter temperature at Spitsbergen, for instance, is now 18 degrees higher than 40 years ago. None of the other theories can explain this marked warming up: there have been no continental upheavals, no astronomical vagaries of the earth's axis or orbit, no appreciable changes in the output of energy by the sun. Nor is the warming related to sunspots, for the trend has marched on through three full sunspot cycles. The one conspicuous change has been that whereas during the 150 years before 1912 volcanoes erupted in one great explosion after another in the Northern Hemisphere, since 1912 they have been comparatively quiet.
Presumably as the dust of the Krakatoa and other explosions settled and the atmosphere cleared, the Northern Hemisphere has been receiving more solar radiation during the past 40 years. This. would account for the warmer weather in middle and low latitudes. In the Far North the clearing of the atmosphere would not itself make winters warmer, because during the long Arctic night there is no sunlight anyway. But the Northern winters may have been warmed by warmer air from the lower latitudes, turned northward by new circulation patterns. The increased solar heating of the earth at middle latitudes, combined with the lack of such an effect in the more northern latitudes, would speed up the westerly winds over the continents, e.g., North America. When the winds exceeded a certain critical speed, they would break down into large eddies over the oceans and adjacent land areas. Such eddies would transport large amounts of heat from the middle to the north latitudes.
While the Northern Hemisphere has been free from major volcanic outbreaks, the Southern Hemisphere has had at least two in recent years-in 1921 and in 1932 in the Southern Andes. If the volcanic theory is correct, these should have affected the Southern Hemisphere's climate. There is some evidence that in the last 30 years or so temperatures have decreased and the ice has grown at the higher latitudes in the Southern Hemisphere. The British-Scandinavian meteorological expedition, which has just completed its observations in the Antarctic, is expected to throw new light on this important question.
THERE IS one major objection to the volcanic theory of climatic change: the geological record shows no consistent connection between periods of volcanic activity and ice ages. Geologists look for evidence on this point by analyzing cores from the bottom of the ocean which contain sections of successive strata of deposits. It has been found that the glacial deposits of an ice sheet sometimes do lie on top of deposits of volcanic ash, indicating that an ice age followed a period of great volcanic activity, as the theory predicts. But often there are ice ages without any apparent evidence of preceding volcanic activity, and, contrariwise, there are heavy deposits of volcanic ash that were not followed by ice ages.
As to the first of these discrepancies, Humphreys calculated that the amount of ash required to cut solar radiation by 20 per cent, if discharged by volcanoes each year for 100,000 years, would make a layer of dust only one fiftieth of an inch thick-hardly enough to detect in an ocean-core sample.
The fact that extensive volcanic activity sometimes failed to produce ice ages also can be explained. It would take a special kind of volcanism to yield the ash required to reduce solar radiation substantially for long periods. Only a catastrophic explosion would create such a dust pall; ordinary eruptions would not suffice. To produce major effects on climate, two important conditions would be necessary: 1) the volcano must throw high into the atmosphere a fine material that will float there for many years, and 2) the ash particles must be of the type that are efficient as nuclei for forming clouds. The clouds would help screen out the solar radiation. Volcanic activity, however intense, that does not meet these conditions might have little or no general effect on the climate.
It would be foolish to argue that volcanic explosions are the sole cause of long-range climatic variations, but this is the one theory for which we have been able to find any kind of check by direct observation. Its chief strength is the established fact that a single volcanic explosion can substantially reduce the amount of solar radiation reaching the earth for as long as three years. -
Harry Wexler is Chief of the Science Services Division in the U. S. Weather Bureau.
JC L'Angelle UNR Spring 2023, NYU Summer 2023
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