We use "nature" in at least three related ways, to mean:
- the intrinsic properties of things, as in "it's in the nature of copper to conduct electricity," and sometimes what is normal for them. We might say in this way that it's not in the nature of pigs to fly, and that it is unnatural for pigs to have five legs, meaning just that it is abnormal;
- the universe as it is, especially with respect to its own intrinsic properties, as in "Nature includes everything that exists." Here, "nature" contrasts only with what's supernatural, if it contrasts with anything at all;
- the parts of the universe which are not created or produced or significantly modified or impacted by humans. In this sense, it is simply the opposite of "artificial" or "artifact," which means something created by us. In this sense of nature, what we call "nature" is a subset of nature in the second sense.
Human creations and our destructive effects on the non-human world are natural in the second sense and are not natural (or unnatural) in the third sense. The first sense is not especially relevant. The potential confusion here arises from equivocation, which means using the same word in two different ways.
We see that there is something in common among these different usages, in that the second two are related to what I think is the oldest meaning—the first, about intrinsic or essential properties. The connection is that we sometimes think of intrinsic properties as properties not changed by us. For example, an oak tree isn't blue unless someone paints it.
So, these meanings are related. However, I think when people use "nature" clearly, they make it plain that they are using it in one of these ways. The trouble is that some people are sloppy about usage. This word "nature" is very easy to be sloppy with because it sounds appealing, and so it gets used to advertise manufactured cleaning supplies, and advertisers want you to notice connotations, not clear meanings.
This is a nice example of how trying to understand the various ways people use words helps us distinguish genuine philosophical puzzles from things that needn't puzzle us once we're clear.