Environmental Engineering Reference
In-Depth Information
KEY POINTS
1
Climate has been changing since Earth's atmosphere first formed. We can find
evidence of this in the rocks and unconsolidated sediments which have accumulated
over millions of years.
2
Some of this change has apparently been the result of the movement of continental
crust. More recently Earth has experienced much colder periods, interspersed with
warmer phases when continental movement has been so small that its effects would be
insignificant. There is evidence of warmer and cooler periods even during the last
millennium.
3
For earlier periods we have to use indirect or proxy evidence, but since the seventeenth
century instruments have been used to measure the elements of climate.
4
The causes of the changes are not fully understood and are likely to vary, depending
upon the time scale. We can distinguish external and internal factors. The orbital
variations of Earth about the sun are important external factors and do appear to have
been significant in triggering the recent Ice Ages. Internally, the changing nature of
Earth's surface, both land and sea, and atmosphere can have major effects, amplified
through positive feedback. Human impacts on climate have now become so marked
that we cannot be sure that the natural cycle of Ice Ages, which has lasted for the last 5
million years or so, will continue.
