Among the worrisome（令人烦恼的，麻烦的） environmental effects of global warming is the thawing² of Arctic permafrost（永久冻土） ---soil that normally remains³ at or below the freezing point for at least a two-year period and often much longer. Monitoring changes in permafrost is difficult with current methods, but a study by University of Michigan researchers offers a new approach to assessing（评价） the extent of the problem. The new study approach, which relies on chemical tracers（化学指示剂） in stream water, is described in the journal Chemical Geology.

Overlying（上覆盖的） permafrost is a thin "active layer" that thaws⁴（使融解，暖和） every summer, and increases in the thickness of this layer over the years indicate thawing of permafrost. Both physical measurements and modeling suggest that active layer thickness has increased in some areas over the 20th century and that if present warming trends continue, increases of up to 40 percent could occur by the end of the 21st century.

Although the full effects of thawing are yet to be determined⁵, coastal⁶ erosion（侵蚀，腐蚀） and damage to the roads, buildings and pipelines⁷ that have been built on permafrost are likely outcomes. In addition, thawing permafrost may release the greenhouse gases carbon dioxide and methane⁸ into the atmosphere, triggering further warming and more permafrost thawing.

Currently, the main method for determining thaw¹ depth is with a graduated steel probe. "You stick it in the ground and see when it hits frozen material," said geochemist（地球化学家） Joel Blum, who with ecologist George Kling and former graduate student Katy Keller undertook（着手做，承担） the new study.

"We were studying the chemistry of soils in the area around Toolik Field Station in northern Alaska, and we found that once we got below the thickness that typically would thaw during summer, the soil chemistry changed dramatically," said Blum, who is the John D. MacArthur Professor of Geological Sciences. "Material that has not thawed⁹ since it was deposited by glaciers¹⁰ 10,000 to 20,000 years ago is now beginning to thaw, and when it does, it reacts strongly with water, which it's encountering for the first time. This soil is much more reactive（反动的，电抗的） than soils higher up that interact with soil water every summer."

In particular, the amount of calcium¹¹, relative to sodium¹²（钠） and barium（钡） , is higher in the newly-thawed permafrost, and the ratio of the strontium（锶） isotope¹³ 87Sr to its counterpart 86Sr is lower. The researchers wondered if these chemical signatures of increasing thaw depth could be seen in local stream water.

Kling, who is the Robert G. Wetzel Collegiate Professor of Ecology and Evolutionary¹⁴ Biology, has conducted research at Toolik Lake for many years and obtained stream water samples that had been collected over an 11-year period.

When the samples were analyzed¹⁵, "we saw really significant changes from year to year that were consistent with（与……一致） what you would predict from increasing thaw depth," Kling said.

Although the method can't reveal precisely¹⁶ how much permafrost thawing is occurring in particular localities（所在，聚居地） , it still can be a useful adjunct（修饰语，附属物） to current methods, Blum said. "We'd love to be able to say that we see an increase in thickness of, say, 1 centimeter over the entire watershed¹⁷（分水岭，流域） , but we simply can't say where in the watershed thawing is occurring. Nevertheless, we think it's important to monitor streams in Arctic regions to keep track of these kinds of changes and follow the rate of change."