Background: At least in the USA, there are a lot of people who believe that anthropogenic climate change is a hoax.
One day an astronaut in the ISS is doing a live Q&A and taking pictures of the earth, and sees clouds forming a very clear message:
IS (NOW) A HOAX
After he takes a picture (it turns out nice, with the message clear and easy to read), the clouds quickly go back to the formations they were in before.
The picture quickly goes viral and is seen by pretty much everyone who uses the internet or watches TV. The authenticity of the picture is easily verified - the live broadcast the astronaut was doing showed the message, and a few weather satellites managed to capture at least part of the message.
People watch the clouds fervently, but no further messages appear. There are also no immediate changes to the climate. All attempts to determine the source of the message will fail - there's no way to tell if it was an entirely random event, an act of God, or a message from aliens.
In the years after the message was seen, climate trends start reversing. After decades (maybe as long as a century), the climate looks much like what we know of the climate in the 1600s or so. Once again, there's no way to determine how this happened - there's no evidence that makes it possible to tell whether it is an entirely natural occurrence, divine intervention, or an act of benevolent aliens.
How would skeptics respond to the implication that they had been wrong but are now correct, and how would believers respond to the implication that they were correct but now are not?