If sunspot activity is markedly reduced over several solar cycles, that indicates the sun's output is lower. I.e., Maunder Minimum and Little Ice Age. They certainly know that. Any other effects don't have an experimental evidence as a forcing factor. Cloud cover - yes. It's the main uncertainty. I certainly hope it starts ramping up soon.
And to think all these ice core data and tree ring data really gives us an accurate picture of previous climates is just bazaar.
IN this marketplace, some data is more meaningful and interpretable than others. Ice core data is extremely reliable as a CO2 concentration indicator and quite reliable for temperatures (stable oxygen isotopes, particularly from polar ice). Tree rings are certainly more problematic as temperature proxies but still indicate climate variation related to both temperature and hydrologic conditions.
Though we are philosophically different on this issue, don't dismiss what scientists can learn just because you don't want them to learn something you don't want to know.
There are people who strongly disagree with that assessment. Here is one for instance.