“It is most likely a combination. “
With Man’s contribution in the 1 parts per billion range!
Maybe, may not. There are a variety of natural factors that could explain the rise from the 70’s through 1998, but none explain it all. I have read many articles (in links here and elsewhere) that proclaim that it is all positive PDO or AMO or lower cosmic rays or (etc), but none of those articles can quantifiably explain the rise. A combination of those might, or a combination of those and rising CO2.