Dust plumes spewed out by volcanoes, long thought to cool the climate, may well have little weather effect, a new study contends. ``Previous suggestions of large cooling during the first few months after volcanic events appear to be unwarranted,'' researchers Clifford F. Mass and David A. Portman say in the June issue of the Journal of Climate, published by the American Meteorological Society. Volcanic eruptions have often been blamed for cooling of the climate in the past, based on the theory that the dust they inject into the atmosphere would block sunlight and reduce the normal warming of the land and sea surface. Indeed, the cold weather in 1816, known to historians as ``the year without a summer,'' has been widely blamed on a series of volcanic eruptions that occurred in various parts of the world between 1812 and 1815. In their study, Portman and Mass said prior analyses may have been confused by temperature changes that involve a periodic warming and cooling of sea and air temperatures, a pattern known as the El Nino-Southern Oscillation. After subtracting those effects from the historical records, they concluded that ``only the largest eruptions are suggested in the climatic record and that a modest cooling is observed for one to two years after these large events.'' They defined modest cooling as about one-tenth to two-tenths of a degree Celsius in average temperatures. That would amount to 0.2 to 0.4 degrees Fahrenheit. The authors found little hint of any effect on air pressure or rain or snowfall. Considering that eruptions throwing dust into the air should interfere with radiation from the sun, they asked themselves: ``Why is the volcanic effect on surface climate so small?'' The answer, they concluded, amounts to an altered form of the greenhouse effect _ the currently widely discussed threatened heating of the planet. ``Volcanic stratospheric aerosols can drastically reduce the direct solar beam,'' they concluded, an effect which should lower the temperature by reducing the warming from the sun. But at the same time these chemicals and dust also block infrared rediation from the ground and ocean what would normally be radiated back into space, cooling the planet. Because volcanic clouds often shade only a small portion of the surface and have only a negligible net effect on the radiation balance, they result in little measurable climate change, concluded Mass, of the University of Washington in Seattle, and Portman of Atmospheric and Environmental Research Inc., Cambridge, Mass. Records which seem to indicate cooling following eruptions could reflect ordinary variations in climate, they conclude. In fact, a more detailed analysis of the records indicates that in some cases gradual cooling reported following eruptions had actually begun beforehand. As for 1816, Mass and Portman conclude that it was an unusually cold summer in North America and parts of Europe, but the degree of cooling was not unprecedented. The cold year could have resulted from consistent wind patterns bringing arctic air south, they said. The greenhouse effect which has become a source of concern in recent years results from an accumulation of carbon dioxide and other gases in the atmosphere, largely a result of burning fossil fuels and other industrial processes. Like volcanic dust, these gases trap the earth's heat, preventing it from radiating out into space and cooling the planet. But unlike the volcanic dust, the greenhouse gases do not balance things by blocking incoming sunlight.
Two scientists say volcanic eruptions may have little effect on climate cooling. The cold weather in between 1812 and 1815 has been blamed on a series of eruptions worldwide 1816 , but the periodic warming and cooling effect of Volcanic , which brought earth , was also involved. Only the largest eruptions effect a modest cooling for one or two years . Volcanic dust clouds drastically reduce the solar beam but also trap the arctic air south 's heat, unlike greenhouse gases which do not block sunlight. Also, volcanic dust has a negligible effect because it shades a small portion of the earth 's surface.