At the beginning of the twenty-first century, the earth is at a critical point in its history. People ? ? ? change are changing (1) it. They ? ? ? pollute are polluting (2) the air and water. They ? ? ? cut are cutting (3) down forests and ? ? ? build are building (4) houses, roads and factories everywhere. As a result, global temperatures ? ? ? rise are rising (5). Thousands of species of plants and animals ? ? ? are becoming become (6) extinct, and the sea level ? ? ? is rising rises (7). Lately the sea levels ? ? ? rose have risen (8) 20 cm. Scientists ? ? ? believe are believing (9) the sea levels could rise 50 cm by the year 2010. This could put many heavily populated coastal areas at risk.
Along with the rising air temperature, the ocean temperatures ? ? ? rise are rising (10). It ? ? ? has brought brought (11) about changes in weather patterns with more frequent and more severe storms. Rising ocean temperatures arc one of the reasons for the death of coral reefs. Reefs are a vital part of the ocean ecology, as they ? ? ? provide are providing (12) homes for 65 percent of the world's fish. Scientists predict that unless the situation ? ? ? changes will change (13) the reefs will be gone. When the reefs die, so do the fish.
Scientists also ? ? ? noticed have noticed (14) that some animals ? ? ? have moved moved (15) to new areas where the temperatures are cooler.
Why are the temperatures rising? The burning of fossil fuel ? ? ? releases is releasing (16) carbon dioxide into the atmosphere. C02 always ? ? ? was has been (17) part of the atmosphere. However, the amount of C02 released into the atmosphere ? ? ? increased has increased (18) enormously. The forests that once ? ? ? absorbed have absorbed (19) C02, ? ? ? are being are cut down. The result is a thick blanket that ? ? ? is covering covers (21) the earth, making it warmer.