Caveat Bettor, to answer your post, I would say that:

#1. The average temperature of the Planet Earth is difficult to get. Think of the difficulty to gather the data from the different points (Virginia, the North Pole, South Africa, the oceans, the French Riviera, the Canadian big lakes, Indonesia, etc.).

#2. The scientists don’t have a perfect global model, yet.

TAKEAWAY: I understand that many traders would believe in “global warming” and buy up such an event derivative, but where would they get advanced indicators on future temperatures?? Past data is not of help if you don’t know how to interpret them. You don’t just buy up a global warming event derivative just because temperatures have risen in the past and that you believe religiously in global warming.