| Rubidium, potassium and carbon are
three common elements used to date the history of Earth. The rates of
radioactive decay of these elements are absolutely regular when averaged out
over a period of time; nothing is known to change them. To be useful as clocks,
the elements have to be fairly common in natural minerals, unstable but decay
slowly over millions of years to form recognizable "daughter" products which are
preserved minerals. For example, an atom of radioactive rubidium decays to form an atom of strontium (another element) by converting a neutron in its nucleus to a proton and releasing an electron, generating energy in the process. The radiogenic daughter products of the decay-in this case strontium atoms--diffuse away and are lost above a certain very high temperature. So by measuring the exact proportions of rubidium and strontium atoms t A. They can be made into clocks. B. They are rich in content. C. Their decay is slow but regular. D. The products of their decay are the same. [简答题]
Forests remove carbon dioxide from the air, conserve soil and water, and are home to a variety of species. (46) They are also repositories of potentially valuable new products, such as pharmaceuticals, and as a source of building material and firewood they provide employment for millions worldwide. [简答题] In the atmosphere, carbon dioxide acts rather like a one-way mirror — the glass in the roof of a greenhouse which allows the sun’’s rays to enter but prevents the heat from escaping. According to a weather expert’’s prediction, the atmosphere will be 3°C warmer in the year 2050 than it is today, if man continues to burn fuels at the present rate. If this warming up took place, the ice caps in the poles would begin to melt, thus raising sea level several meters and severely flooding coastal cities. Also, the increase in atmospheric temperature would lead to great changes in the climate of the northern hemisphere, possibly resulting in an alteration of the earth’’s chief food growing zones.
The West Antarctic may be affected by only a few degrees of warming: in other words, by a warming on the scale that will possibly take place in the next fifty years from the burning of fuels.
Satellite pictures show that large areas of Antarctic ice are already disappearing. The evidence
我来回答: 提交
|
