问答题 As evidence that the Earth's atmosphere is warming continues to accumulate, scientists are making slow progress toward an answer to the big question raised by the evidence: (89) How much of the warming is due to human activity and how much to natural causes?
The UN Intergovernmental Panel on Climate Change, (90) the group of scientists widely considered the most authoritative voice on the subject, has already concluded that there is a "discernible human influence" on the global climate. Now the panel is deep into another of its periodic full-scale scientific assessments of global climate change.
(91) Some experts on the problem say the human imprint on climate is becoming clearer, and may even have been the dominant factor in the global warming of recent decades. Not everyone agrees and virtually all experts say that in any case, a reliable estimate of the human imprint's magnitude still remains some distance off.
A number of influences, both natural and manmade, cause the planet's temperature to vary. The natural ones include changes in solar radiation, and sulfate droplets called aerosols cast aloft by erupting volcanoes, which coo] the atmosphere by reflecting sunlight.
(92) The human influence stems mostly from emissions of waste industrial gases like carbon dioxide, which trap heat in the atmosphere, and sulfate aerosols(硫酸盐悬浮颗粒)from industrial smokestacks.
(93) Human factors appeared to be playing a part, but no judgment has been made on whether that part was big, small or in between.