The rise of CO2 is unambiguously caused by human activity, principally fossil-fuel burning. This is clear from the numbers: We know how much fossil fuel is converted into CO2 each year and emitted into the atmosphere. The CO2 doesn’t all stay there because some enters the ocean and some is taken up by photosynthesis, which ends up in land plants and various types of bio-matter … as it happens, about 57% of the emissions have remained in the air.
Because fossil-fuel burning is not natural, the recent carbon increases in the atmosphere, oceans, and land biosphere cannot be natural either … few if any natural processes can release fossil carbon into the atmosphere as fast as we humans are doing it now via the extraction and burning of fossil fuels.
This graph shows the reconstructions of atmospheric CO2 back 800,000 years. At no point in this period were levels as high or increasing as fast. No surprise because it’s only recently that we humans have been burning billions of tons of fossil carbon each year.