Plants absorb CO2 but plants also release CO2, ie they die and
decompose sooner or later or end up burned. For Plants to absorb all
the increase of CO2 humans produce the overall mass of living plant
matter has to rise. A Lot. This doesn't appear to be occurring. The
assumption that they will "catch up" with the increase is, like most
skeptic arguments pretty poor and not in line with observed facts. It
only sounds reasonable in the absence of knowledge of the subject.
Ken