For those who read the hieroglyphs of tree rings, bristlecone pines are a climatic record book. They have revealed long-term patterns of North American drought. Now they also are giving new insight into the climatic influence of volcanoes.
Frost damage in the annual growth rings of these long-lived North American trees often can be linked to specific volcanic eruptions.
Volcanic dust and sulfuric acid droplets thrown into the stratosphere can throw a veil over a hemisphere or even over much of the entire Earth. This can have a cooling effect and can influence weather for several years. By studying patterns of frost damage in trees at a given locality or over a sizable region of the Western US, Val-more C. LaMarche Jr. and Katherine K. Hirschboeck of the University of Arizona are able to pair short-term weather patterns with specific volcanic episodes.
LaMarche explains that linking volcanic dust veils with a cooling effect on climate is not new. ''What we have provided is a focus for certain kinds of climatological observations,'' he says. ''. . .It is one thing to know, in a general way, that the lower atmosphere cools. But that doesn't tell you much about the weather or climate on a local or regional scale.''
The bristlecone rings show both the year and the season (spring or fall) when frost damage occurred. Tracing the extent of such damage, LaMarche and Hirsch-boeck can then envision the patterns of wind flow and atmospheric pressure which produced these cold snaps. Thus, in a rough way, they are beginning to sketch some of the ''weather maps'' that have marked periods of volcanic cooling.
In a report of their work recently published in Nature, the researchers note that such frost damage appears in the bris-tlecone record at intervals of a few decades to a few hundred years for at least the past 4,000 years. It shows up in trees located over a region stretching from California to Colorado and down into Nevada.
Not all frost damage can be linked to volcanic eruptions. And there have been major eruptions with no obviously associated record in the bristlecone rings. Nevertheless, there are enough cases linking the two kinds of events for LaMarche and Hirschboeck to say that ''major eruptions are likely to be closely followed by notable frost events . . . at better than the 99.9 per-cent confidence level.'' In other words, there is less than one chance in 10,000 that the association of frost damage and eruptions is merely coincidental, to use LaMarche's informal figure.
During the past century or so, for which detailed re-cords are available, frost damage episodes can often be pinned down to within a few days and associated with specific weather maps.
The frost damage can also be used to help date ancient volcanic eruptions. Thus, as a research bonus, LaMarche and Hirschboeck say they may have pinned down the date of the eruption that devasted the Greek island of Thera. Some archaeologists have speculated that this cataclysm is reflected in the legend of the lost city of Atlantis.
Geophysicists have come up with a variety of dates for the eruption. The average is 1688 BC, give or take 57 years.
Frost damage in North American bris-tlecones, which may well be linked to that eruption, indicates a date of 1626 BC, or perhaps one or two years earlier.
LaMarche says this is only the beginning of a line of research which he hopes will be followed on a world basis. It also is relevant to concerns about the ''nuclear winter'' that might follow a nuclear war, although LaMarche and Hirschboeck are not looking into this themselves.
Studies reported last year by a number of scientists suggest that dust thrown up by nuclear explosions would bring winterlike conditions to much of Earth for many months. Much plant life might die, making recovery difficult even in areas not hit by any of the weapons. A better understanding of the weather patterns induced by volcanic dust veils should help scientists refine their estimates of the weather effects of nuclear war.
With a subject such as this, which is susceptible to fearful speculation, it is important to build as sound a scientific basis for thinking as is possible. This alone should be reason enough to pursue vigorously the line of research opened by LaMarche and Hirschboeck.
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