We have to read the essay, or at least a great number of paragraphs, as a whole, in order to understand everything in context. I have just done that. What your sentence means is: 1. The earth has existed for a long time, but human records of its climate only cover a relatively small percentage of the earth's history. 2. It is believed that the earth's climate has changed over time, over the millenia. We want to know how and why it has changed. 3. It is believed that there have intrinsic factors of nature that have brought about these changes (we call these "natural causes"). There have also been human factors. 4. Even though human records are limited (both in terms of completeness and scientific quality), they should nevertheless, up to a point (that is, to a certain extent), tell us something about how great the changes have been, in what ways, and how the natural causes have combined and interacted to bring about these changes. To this admittedly limited extent, the records are useful in helping us study and understand climate change. ----- The original essay is not very well written, but there are many other essays on climate change which are even worse - much much worse. Sadly, many scientists and scientific writers, and indeed many native English speakers, do not write good or clear English. That is the reality. There is very little we can do about it. We live in a terrible world. I don't know which is worse, climate change or bad English.

I agree with Ben. The words sound good but there is no coherent meaning. Please don't try to make too much sense of it.