Compared to the carbon dioxide that humans produce, Mother Nature routinely transfers 40 times as much CO2, and 24,000 times as much water vapor (Earth's primary greenhouse gas), back and forth between the atmosphere and the Earth's surface, every day.
Scientists have simply assumed that these natural processes have been in balance for centuries. But, what if there have always been some small — but natural — imbalances in those large up-and-down flows that slowly change over time? In that case, our measured increases in greenhouse gases and global temperatures might well turn out to be more natural than manmade, lost in the noise of natural variability.
Can I prove any of this? No — not yet, anyway. But neither have any scientists produced one single scientific paper showing that Mother Nature isn't the dominant source of what we are seeing. Mankind is one possible explanation, and our measurements of natural variability in the climate system on time scales of decades to centuries are simply not good enough to find out how many natural sources of variability are also out there.