Commenters have been drawing attention to a public letter written by healthcare professionals and climate scientists, which called on the Heartland Institute to reveal all its funding. This was cited in a Guardian article, which was later amended to show that the letter had been removed.
This appears to be it, here. In fact it looks as if the document just moved to a different URL, although I've sent it to Webcite just in case.
What motivates the Heartland Institute? As climate scientists and health professionals, we view the systematic sowing of unjustified doubt about climate science as confusing at best, and inhumane at worst.
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Given the disproportionate influence given to climate sceptics by the media, it is in the public, national, and global interest for all funding behind their activities to be revealed. This will help people to make up their own minds about the truth of the climate change threat, so that action can be planned on the basis of evidence rather than confusion.
Signatories include many familiar names including: