The local derivative is most definitely not indicative of a general trend.
For comparison, I throw a ball down at the ground, and for the first 0.1 seconds it's moving down - can I conclude that the ball is going to keep moving down, even when it hits the ground? That's your logic.
Baseline is just the starting point of the x-axis. It doesn't matter.
Personally I'd set the baseline to 0 degrees. Because thats how temperature works.
But you can pick whatever you want. It does not matter what you x-axis starts at. It doesn't change the data.
Or you could just have no baseline. Just remove the X-axis line from the graph.