Hello all. I think screamer spawn percentage increases with the biome difficulty, but it's just based on my experience on day 11 of a relatively easy (low difficulty level) playthrough.
Here's my story: I had a base in Forest, and was farming Rekt missions. I got a Tier IV mission to my North, which was in Wasteland. I cleared the POI. _After_ I killed the last zombie, two screamers appeared (IIRC I had used iron tools but not much -- I don't remember whether or not I left a cooking fire on at the base, and I had one active dew collector -- which IMHO should not generate heat). I killed the two, which were infected, but they summoned two more, plus a horde. Eventually, I fled.
Near zero screamer spawning in Forest, but a ridiculous experience in Wasteland. So I think that anyone playing on Wasteland Only has an intense screamer experience that may not have been playtested. Anyone spending the early days in Forest or Burnt Forest might have a different experience.
Yes, my forest base is also my horde base. I have a desert base that has little protection and has had no problems.