First - I do not know if this is the best site to pose this question. However, stack exchange does not appear to have an ecology sub-site. That said, this appears to be the most appropriate place post this question.
Consider this chart:
The chart was retrieved from here here.
I am curious as to whether anyone has any thoughts regarding this trend - note, however, that this is one chart among several that I came across on reputable site. All seem to depict this observed "capping" effect between roughly 2000 and 2010+. Unfortunately, I don't have much to offer right now since I am still looking into it.
Any input is appreciated.