can we "calibrate" the temperature sensor to compensate the body heat?

If you know how to account for all possible factors involved, like for example whether you wear sleeves or not, their shape and material, the humidity, wind, motion, sunshine, etc (all those will have great impact on the ratio between the body and the environment heat involved), then I recommend that you suggest it to Garmin at Submitting an Idea to Garmin

No. There are too many factors.

I am in the tropics where daily outdoor temperatures range from 26°C (78.8°F) on a cool day to 42°C (107.6°F) past noon. I have the found temp readings on my Enduro climbing consinstently with a TFA Dostmann thermometer. Perhaps in cooler climates the watch is influenced by body temperature, I just don't see that happening here. 

jan.shim said:

I just don't see that happening here. 

It depends on the type of the activity (besides others). On a bike, due to the rapid air flow, the sensor will be sufficiently cooled down by the ambient air (as long as you do not wear long sleeves of gloves over the watch), and reducing so the influence of the body heat. The situation will be different at slow runs (unless it is really windy), at hikes, or at indoor activities. And oppositely, it measures the ambient temperatuře in water or at high humidity, quite well, thanks to the high thermal conductivity of water (25 times higher than at air). As already written, there are so many factors that play a role, that it makes no big sense trying to calibrate it. If you really need to know the ambient temperature, get the Tempe sensor.