（1996年度　本試験　英語　第５問 ）

Several years ago, certain scientists developed a way of investigating the nature of the atmosphere of the past by studying air caught in the ice around the North or South Pole. According to their theory, when snow falls, air is trapped between the snowflakes. The snow turns to ice with the air still inside. Over the years more snow falls on top, making new layers of ice. But the trapped air, these scientists believed, remains exactly as it was when the snow originally fell.

To find what air was like three hundred years ago, you use a drill in the shape of a hollow tube to cut deep into the layers of ice. When you pull up the drill, an ice core made of many layers comes up inside it. Then, back at the laboratory, you count the layers in the core ? each layer represents one year ? to find ice formed from the snow that fell during the year to be studied. Using this method, these scientists suggested that the amount of carbon dioxide (CO2), one of the gases which may cause global warming, had increased greatly over the last two hundred years.

A Norwegian scientist, however, pointed out that there might be a problem with this method. He claimed that air caught in ice does not stay the same. In particular, he said, the quantity of CO2 does not remain stable, since some of it is absorbed by ice crystals, some enters water, and some locks itself up in other chemicals. If this were true, then there could have been more CO2 in the past than we thought. Even so, measurements taken over the past thirty years show that CO2 has increased by over ten percent during this short period.

(291 words)