地球大气层正在转暖，这种迹象日趋明显，这向人们提出了一个重大问题，即地球变暖在多大程度上应归咎于人类活动，又在多大程度上是自然原因造成的?气候变化科学家们正在寻求答案。经过对全球气候变化的全方位定期科学评估，科学家们断定全球气候受到了“显而易见的人为影响”

下面你将听到的是一段有关气候变化的讲话。
    The evidence that the Earth’s atmosphere is warming continues to accumulate. A big issue is thus raised concerning the extent to which the warming is due to human activity and natural causes. Climate change scientists are looking for the answer. Following the periodic full-scale scientific assessments of global climate change, they have concluded that there is a "discernible human influence" on the global climate.
    Although a reliable estimate of the magnitude of human imprint on climate still remains some distance off, human activities may have been the dominant factor in the global warming of recent decades. Human factors appear to be playing a part, stemming mostly from emissions of waste industrial gases like carbon dioxide, which trap heat in the atmosphere, and sulfate aerosols from industrial smokestacks.
    Will the climate change lead to the earth’s eventual extinction? We have no definite answer yet, at least not likely as far as our knowledge goes.
    The earth experienced a catastrophic extinction about 250 million years ago. In the space of a few thousand years, something terrible happened to our planet, something that wiped out 90% of earth’s ocean species and about 70% of those that lived on land.
    It was the worst extinction in the history of the earth, known as the Great Dying. It eliminated whole communities of coral reefs, forests, giant amphibians and ferocious reptiles, swarms of insects and the ocean’s ubiquitous triobites, those hard-shelled invertebrates that were never seen on the planet again.
    What caused that extinction? Any number of scenarios has been offered, ranging from the explosion of a nearby star to Ice Age cooling and greenhouse warming. None of them were entirely convincing.
    We tend to accept the most recent conclusion that the extinction was caused by the impact of an asteroid or comet or meteor, like the one that probably wiped out the dinosaurs 65 million years ago. In each case the damage would have been done not by the explosive collision itself but by the series of global disasters it triggered, for example, furious volcanic eruptions, a rapid heating of the atmosphere and the depletion of life-giving oxygen from the ocean. We can’t say exactly where this out-space visitor struck because the tectonic processes that rearrange the earth’ surface have since churned up most of the evidence. But we can estimate its size. It probably measured roughly half the size of the island of Manhattan and it would have slammed into the earth with the force of a magnitude-12 earthquake.
    Of course, this does not give us an excuse to underestimate the destruction that human harmful activities have brought to the earth’s climate change. We must realize that we are living the way beyond the limits of the capacity of this planet to sustain. There is almost nothing that we do that will not compromise the ability of humans to meet their needs in the future.
    Life ought to be enjoyable. But our destructive ways of life have serious consequences: new human diseases, strange animal and plant diseases, bizarre natural disasters, food shortages, etc. We must act now and channel our way of life in a healthy, sustainable direction.