“How can forests suddenly die and emit carbon?”
Forests are one of the greatest carbon sinks available on this planet. However, with the changing climate, the conditions that led to their life are now changing. Induced heat stress and the spread of pathogens are causing entire forests to wither away and release stored carbon. This Forest Dieback can create a positive reinforcement cycle in which the more trees that die the worse the climate becomes, leading even more trees to die off. Two of the most extreme cases of this are in the Amazon rain forest and the Boreal forest, where large portions of the former is actually turning into a savanna.
Image credit upload.wikimedia.org