Denialists make many false claims but one of them is that the conspirators -- consisting of thousands of scientists and government agencies all over the world -- are keeping the data that supposedly support their hoax a secret. This is so preposterously false it isn't even laughable.
Gavin Schmidt discusses transparency and reproducibility in climate science here, with links to all of the data you could possibly want. As Schmidt writes:
A small selection of climate data sources is given on our (cleverly named) “Data Sources” page and these and others are enormously rich repositories of useful stuff that climate scientists and the interested public have been diving into for years. Claims that have persisted for decades that “data” aren’t available are mostly bogus (to save the commenters the trouble of angrily demanding it, here is a link for data from the original hockey stick paper. You’re welcome!).He goes on to discuss some of the challenges in replicability and reproducibility, which in the case of climate modeling have to do with technical limits of computing power more than anything else. However, it is the fact that models are in broad agreement and the historical fact of climate change is absolutely proven. NASA's GISTEMP data is publicly available here. They'll even give you software to analyze the data yourself! Have at it! Also here!
Of course, denialists don't actually understand anything about the subject and can't legitimately or honestly critique the work of scientists who do actually know what they are doing. So they just spout bullshit out of ignorance. And make fools of themselves.