Biosphere Daily Earth Science Environment Repost

When do forests become a carbon liability?

So, we have a system we think we understand. Fossil fuels burn, thus releasing greenhouse gases into the atmosphere. Carbon dioxide (CO2) is a major component of these greenhouse gases. But the forests on our planet offset the atmospheric CO2 buildup by using the CO2 in photosynthesis. Thus, sequestering the carbon in new wood and in the soil. However, when this balancing act fails, then forests become a carbon liability.

A recent article in Nature highlights the declining capacity of African rainforests to sequester carbon. This trend in Africa compares with established research on the Amazon Rainforest, which demonstrates the loss of carbon sequestration capacity in this South American setting.

The work in both of these studies is not based on loss of forest due to logging and farming. These studies focus on established old-growth forests and decades of research recording data from defined research plots. The results are not encouraging. Both the African and South American rain forests show a decreasing capacity for carbon sequestration over the past 25 years.

Amazonian rain forests are predicted to be carbon neutral by 2035. So, they will release as much carbon as they sequester. After 2035 they may become a carbon liability, releasing more CO2 into the atmosphere than they sequester. 

Carbon balance

The carbon balance of a forest depends on three factors. The first factor is how much biomass is added annually in terms of wood and permanent vegetation. This added biomass represents sequestered carbon. The second factor focuses on the amount of carbon sequestered in the forest soils. The third significant factor is the amount of carbon released back into the atmosphere as trees die, and bacteria, insects, and fungi consume their biomass.

 When factor three is larger than factors one and two, then the forest is a carbon liability. There are several aspects of tropical rainforests that make them prone to move towards being carbon liabilities. Carbon sequestration in forest soils is much lower in tropical rainforests than in temperate rainforests or boreal forests.  So, most of the tropical rainforest carbon sequestration capacity is in added forest biomass. 

Tropical rainforest growth is very sensitive to moisture and temperature. So, under hotter and dryer climatic conditions, forest growth slows down. Less biomass is added, and thus less carbon is sequestered. Hotter and dryer conditions will also increase stress on existing forests resulting in more trees dying. The net result is that carbon sequestration decreases with time, and carbon loss increases. Eventually, these forests become net producers of CO2, not carbon sinks.   

Other types of carbon liabilities

Forest fires represent another pathway for a forest to become a carbon liability. The fire itself results in an immediate, massive release of stored carbon into the atmosphere. Then there is the longer-term effect of the slow decay in the remaining deadwood. This phase represents an extended period of carbon release. Immediately after the fire, there is no live forest to act as a carbon sink. So, the short-term net effect is a carbon liability where more carbon is released into the atmosphere each year than is sequestered.

Studies in Yellowstone National Park indicate that after a forest fire, a 40 to 50-year period occurs before new-growth carbon storage exceeds the volume of deadwood carbon in the forest ecosystem. All indications are that climate change is bringing warmer and dryer conditions to some of the planet’s tropical rainforests. Also, dry seasons are longer and hotter in many temperate-zone forests like the western USA, leading to an increased frequency of forest fires. 

Our current CO2 budgeting may be overestimating the carbon sequestration capabilities of some key forested areas. This overestimation represents a carbon liability in the long run.


Tongass National Forest: The good, the better, and the beautiful (Source: ArcheanWeb) – Also:

Forests as a pathway for terrestrial carbon sequestration (Source: ArcheanWeb) – Also:

Carbon sequestration in soils (Source: ArcheanWeb) – Also:


The Congo rainforest is losing ability to absorb carbon dioxide. That’s bad for climate change. (By Daniel Grossman; Washington Post) – Also:

Asynchronous carbon sink saturation in African and Amazonian tropical forests (By Wannes Hubau; Nature research) – Also:

Sink or Source? Fire and the Forest Carbon Cycle (By Joint Fire  Science Program) – Also:

Feature Image: Gabon (By Nik06) (Modified) –  – This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license. –

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.