]]]]]]]]]]]] THE SUN IN THE "GREENHOUSE" [[[[[[[[[[[ (5/24/1989) By Albert Arking [``Albert Arking is head of the climate and radiation branch of the National Aeronautic and Space Administration's Goddard Space Flight Center.''] [From The New York Times, 23 May 1989, p. A29:2] [Kindly uploaded by Freeman 10602PANC] Greenbelt, Md. Stories in the news media give the impression that a consensus exists among scientists, and Government scientists in particular, that the global warming of the 1980's is primarily the result of increased ``greenhouse'' gases in the atmosphere. There is no question that these gases are increasing rapidly and that their overall effect, other things being equal, is to increase global surface temperatures. However, the magnitude of their impact is the subject of considerable debate within the scientific community. Moreover, there is strong evidence that another factor is having an important influence on our climate: solar activity. The recent upward trend in global temperatures has been the subject of intense scrutiny. The current warming trend goes back only 12 years, however. Before that, temperatures were decreasing or unvarying between 1940 and the late 1970's -- although this period was one of strong growth in world energy consumption and fossil-fuel burning. Furthermore, the 50 years prior to that -- from 1890 to 1940 -- was a period of significantly less fossil-fuel burning, yet the Earth warmed up by more than 1 degree Fahrenheit. That represents about twice the amount of the recent warming. These changes in temperature do not at all follow the pattern of the changes in the greenhouse gases. Indeed, while these gases are a potentially important factor, there are other possibilities to consider. Volcanic eruptions, for example, have long been recognized as influencing changes in climate. Volcanic eruptions produce sulfur dioxide, which becomes droplets of sulfuric acid in the stratosphere, forming a global haze. This haze partly screens the Earth from the sun's rays, and cools the planet. To effect a warming trend, there would have to be a lengthy period in which the rate and intensity of volcanic eruptions is substantially below normal. This has certainly not been the case in recent years. Another factor -- variations in the sun's output of radiant energy -- has been identified in many studies over the years. But these variations have not been given the recognition they deserve because it was not fashionable to think that the sun's power could vary. (Indeed, it is still common to call the energy from the sun ``the solar constant.'') However, satellite measurements over the past 10 years have confirmed that such solar variations occur. They reveal that during the last solar cycle -- the 11-year period of growth and decline of solar activity, typically manifested by large variations in sunspots, solar flares and other phenomena -- the sun's power varied correspondingly. The amplitude of the observed variation was small (about 0.1 percent during the course of one solar cycle). But once the principle is established that the sun's output can vary over the course of one sunspot cycle, the possibility that over decades or centuries the variations can be much more than 0.1 percent must be given serious consideration. In fact, there have been substantial changes in solar activity over the centuries, and they correlate with temperature changes on Earth. The facts are striking: First, the period of lowest solar activity occurred between 1600 and 1700, when sunspots practically vanished during the later half of the century. That was also the coldest period in the last thousand years, sometimes called ``the little ice age.'' Second, more or less simultaneous with the cooling that took place from the 1940's to the late 1970's, there was a substantial reduction of solar activity. And from the late 1970's to the present, as the temperature of the Earth rose sharply, solar activity also reversed and started back up again. This pattern of changes in global surface temperatures over the centuries suggests a strong solar influence. Perhaps increased greenhouse gases also played a role in the recent warming trend, but it is too early to judge their relative importance and draw conclusions about the future. [The following is not part of the original article.] More: Lean, J. ``Contribution of Ultraviolet Irradiance Variations to Changes in the Sun's Total Irradiance''. Science 244:197-200 (14 April 1989). * * *
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