A genuine glacier must be permanent. Generally, this implies that sufficient fresh snow must accumulate during the cold months to offset melting during the summer, although on a year-to-year basis, glaciers may expand or contract, depending on local and global climatic conditions. ① Today, most glaciers around the world are melting because of the warming climate, and it appears that the rate of melting is accelerating. ② This has been documented spectacularly in places such as the Alps, where historical records have been kept and dated sketches and photographs are available to compare with the present extent of ice. ③ Even over periods as short as a few decades, satellite images show that dramatic reduction of mountain glaciers has occurred in the Andes, the Himalayas, and elsewhere. ④ The various rocks and minerals contained in glaciers have become a popular subject of science projects in schools. ⑤ It is estimated that many small mountain glaciers will disappear completely within ten to twenty years unless there is a sudden and unexpected change in the present warming trend.
* offset: 상쇄하다