National Geographic : 1990 Oct
More volcanic eruptions, throwing fine dust and gases high into the stratosphere, might operate against the greenhouse, cooling the earth temporarily. But the best computer models suggest that to bring on marked cooling, volcanic explosions far more violent than those of Mount St. Helens in 1980 or Krakatoa in 1883 would have to occur every five years for as long as a cen tury. The resulting dirty air and acid rain would be worse for life on earth than global warming. If a return to ice-age conditions rather than greenhouse warm ing sounds farfetched, it was thought a serious possibility as recently as the mid-1970s. The nine major interglacial periods of the past million years have each lasted scarcely 10,000 years before the cold returned-and it has now been longer than that since the last great continental ice sheets melted back. And even though global temperature has been rising since the start of the industrial age, from 1940 until 1970 it leveled and even declined slightly in the Northern Hemisphere. Earthmovers replaced tugboats on the Mississippi River in 1988, as drought brought record low water that grounded barges near Memphis (above). The drought slashed U. S. corn pro duction 30 percent, boosting the volume of futures trading at the Chicago Board of Trade (right). Wichita farmer Larry Steckline and son Greg (left) lost $80,000 of wheat. Many scientists believe that while it didn't directly cause this devastation, global warming will make droughts and storms more fre quent and severe. J. Murray Mitchell, Jr., senior climate researcher of the U. S. Weather Bureau and later of NOAA's Environmental Data Ser vice, was one of those who documented that downward drift. Now retired, he told me recently: "We thought natural forces, such as volcanic activity or perhaps variation in the sun's radi ance, might be at work. But we still don't know whether it was a real change or just a quarter-century-long twitch in the climate cycle." OES THE SUN BLAZE absolutely uniformly, sending always the same amount of light, heat, and other radiation into space? Is its total radiance constant, as has long been assumed, or does the energy received by the earth vary, even minutely? The question is crucial in today's climate studies. From astrophysical evidence the sun is thought to have been 25 to 30 percent dimmer when the earth was young-three and a half billion years ago. Pondering how life could have developed Is Our World Warming?