A study of African and Amazonian forests has been published in Nature:
and summarised in a Guardian article by Fiona Harvey on 4th March 2020:
The detailed study of tropical forest trees has shown that, the uptake of carbon from the atmosphere peaked in the 1990s, when about 46bn tonnes were removed from the air, equivalent to about 17% of carbon dioxide emissions from human activities. By the last decade, that amount had sunk to about 25bn tonnes, or just 6% of global emissions.
This means that the property of these forests to act as carbon sinks is declining, mainly due to the clearance of land to grow crops. Forests lose their ability to absorb carbon as trees die and dry out from drought and higher temperatures, but the loss of forest area from logging, burning and other forms of exploitation is also a leading factor in the loss of carbon sinks.
Climate scientists have long feared the existence of “tipping points” in the climate system, which when passed will condemn the world to runaway global heating. There are many known feedback mechanisms: for instance, the melting of Arctic ice leaves more of the sea uncovered, and, as it is darker than the reflective ice, it absorbs more heat, thus leading to more melting.
These feedback mechanisms have the potential to accelerate the climate crisis far ahead of what current projections suggest. If forests start to become sources of carbon rather than absorbers of it, that would be a powerful positive feedback leading to much greater warming that would be hard to stop.