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Palaeometeorology
古气象学

Running rings round storms
看年轮解风暴

Sep 21st 2006
From The Economist print edition

Trees keep records of passing hurricanes
树木留下过往飓风的记录

STUDYING the past is a good way to understand the present, and may even illuminate the future. But the past does not give up its secrets easily. Hurricane scientists, for instance, would like to know about long-term changes in the frequency and strengths of the storms they study. （1）That would help to show whether the shifting pattern of hurricanes seen in the past few decades is cyclical, random or part of a trend that might be caused by global warming. Unfortunately, meteorologists have been keeping systematic tabs on the relevant data for only about 60 years. Before that, records are sporadic and anecdotal—and that is not enough to see the bigger picture.

Human records, however, are not the only sort available. Trees are popular with scientists who want to look at what happened a few hundred years ago, because their annual growth rings mean that their wood can be dated accurately. And Dana Miller, of the University of Tennessee, and her team have used that insight to search for hurricanes that humanity has failed to record. （2）Their results, just published in the Proceedings of the National Academy of Sciences, have identified a number of previously unknown storms that hit the south-west coast of North America. The trick they used to do this was to look at the isotopic composition of the oxygen in the wood of local trees.

Water contains two isotopes of oxygen, one of which has two more neutrons than the other, making it heavier. When a hurricane forms, it tends, initially, to rain water molecules containing the heavier isotope. At that point it is still over the sea. Conversely, the rain that falls from an old storm has more light oxygen in it—and that is the sort of rain that tends to fall on land.

When this rain enters the soil, some of it is taken up by trees and incorporated into their wood. （3）So, by measuring the ratio of the two isotopes in the rings of trees, and matching the result to the age of the ring, a history of hurricanes spanning the life of the tree can be reconstructed.

Dr Miller's trees of choice were longleaf pines on the Valdosta State University campus in Georgia. Some were still alive, and some were in the form of stumps left from logging that took place on the site at the beginning of the 20th century.

To test her method, Dr Miller looked first at the period from 1940 to 1990 (America began making accurate hurricane records in 1940). She was able to identify all 18 years when storms had affected the university campus in that time, though the method also suggested a storm in one year (1943) when there had been none.

（4）Going back further into history, she looked at 1855-1939. Here the records for landfalls are still good, although storms were not tracked accurately before they landed. Again, the method identified all known years with storms (21 in total), and indicated one other year when a hurricane had passed. That may or may not have been a false positive. Going back further still, to 1770-1854, the records are much more patchy. In this case the trees suggested hurricanes had passed by in 25 years,whereas only ten stormy years had been recorded.

Clearly, the method is not perfect, as the false positive in 1943 shows. But over a period of centuries such errors will come out in the statistical wash. （5）And, by extending the method to other sites (Dr Miller now has data from South Carolina and Florida, as well as Georgia), it should be possible to look for long-term patterns in the frequency of hurricanes and to answer questions that are of interest to insurance companies and environmentalists alike.

答案揭晓：

（1）That would help to show whether the shifting pattern of hurricanes seen in the past few decades is cyclical, random or part of a trend that might be caused by global warming.

这一研究将能说明过去几十年间，飓风所表现出的模式转换（shifting pattern）是周期性的，随机性的，还是因全球温室效应所造成的影响。

（2）Their results, just published in the Proceedings of the National Academy of Sciences, have identified a number of previously unknown storms that hit the south-west coast of North America.

他们刚刚刊登于《美国国家科学院院刊（Proceedings of the National Academy of Sciences）》的研究成果揭示了许多此前不为人知的，曾袭击北美西南海岸的风暴。

（3）So, by measuring the ratio of the two isotopes in the rings of trees, and matching the result to the age of the ring, a history of hurricanes spanning the life of the tree can be reconstructed.

因此，通过测量树的不同年轮（ring）中两种氧同位素的比例，并将结果比对年轮所显示的树龄，就可再现树木一生中所经历的所有飓风。

（4）Going back further into history, she looked at 1855-1939. Here the records for landfalls are still good, although storms were not tracked accurately before they landed.

回溯到更早时期，米勒博士检测了1855年到1939年这段时期内的风暴。虽然此期间内风暴着陆之前的情形无据可查，但其着陆后的情况记载完好。

（5）And, by extending the method to other sites (Dr Miller now has data from South Carolina and Florida, as well as Georgia), it should be possible to look for long-term patterns in the frequency of hurricanes and to answer questions that are of interest to insurance companies and environmentalists alike.

而且，将此方法沿用于其他地点（米勒教授现已掌握了南加利福尼亚州、弗洛里达州以及乔治亚州的数据），可揭示长期内飓风发生频率的模式，并能解答诸如保险公司和气象人员所感兴趣的那些问题。

posted on 2006-10-10 18:59 Daisy 阅读(1795) 评论(1)  编辑  收藏 所属分类: 【英美报刊】 网摘收藏