Indeed it is complicated. However, it is my understanding thete are tree over the world from many different historical and current environments that can allow for an analysis that gives a pretty good picture of global warming caused by other factors than human industrial innovations.
In addition, there seem to be a dearth of studies about natural “healing” of the changes that indusrialization might cause.
Well it's too bad for your understanding that such is NOT what was done. The data was cherry picked, particularly at the Yamal River, where Mann had a choice of 300 cores and picked only one.
Here is a summary.