Because there are so many elements that go into climate and the change in climate
A variety is used to measure the change in the climate system because there is so much that is incorporated with the climate, that not one tool would be able to be used. There are many tools to measure different parts of the climate system, so that way a more accurate reading could be made. For example, we can say that the climate is warming up, because based off of the results, of rain, snow, and the temperature. If it was simply the temperature, then we wouldn't get any results, just that its a little warmer than last year.
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