As anyone who has tried to start a campfire knows, starting a fire is harder than it looks. The focus of sunlight is so precise; it burns away any flammable material as the earth spins and the dot moves thus preventing the card from burning up.
Essentially, nothing. The paper has a coating to prevent it from getting soggy. The recording card can remain in the sunshine recorder for several days without damage. What is more important is to change the card at proper times when the sun is shining to prevent over burn of yesterday’s data with todays.