People have looked everywhere for alternative explanations and failed.
your statement is false, and it's a climate denialist talking point which was important enough such that climate scientists systematically examined and refuted it in the context of numerous pieces of peer-reviewed literature:
https://iopscience.iop.org/article/10.1088/1748-9326/7/4/044...
""Despite a high degree of consensus amongst publishing climate researchers that global warming is occurring and that it is anthropogenic, this discourse [that humans may not be responsible for some degree of climate change], promoted largely by non-scientists, has had a significant impact on public perceptions of the issue, fostering the impression that elite opinion is divided as to the nature and extent of the threat [when it is not]."
The article sited provides no data that would break down CO2 contributions based on natural phenomenon vs emissions. The only study I know of that does is the tree ring data paper and it has since been found to have significant mathematical errors and in need of revision based on the findings. This article is a sociology piece at best describing the natures of how denialist and right wing fanatics operate and how climate scientist tend to agree, but even with that there are social implications of funding, group think etc.