Tropical forests absorbing more carbon dioxide than previously thought

We have another reason to love the rainforest, and not just for the plethora of exotic species, beautifully lush forests, and zip-lining vacations, either!  New research out of the Jet Propulsion Laboratory combines a host of modeling data backed by experimental measurements to find converging evidence that forests closer to the Equator are absorbing much more CO2 than boreal forests in the Northern Hemisphere.  These findings highlight another reason to improve protection against deforestation and suggest that tropical forests could delay significant climatic changes due to global warming if we work to keep them around.

Figure courtesy of windows2universe.org

Figure courtesy of windows2universe.org

The carbon cycle plays a large role in determining future climate change, but it also contributes a great amount of uncertainty due to our lack of understanding in how its feedback systems operate.  The cycle consists of carbon emitted into the atmosphere by animal respiration, industrial emissions, etc., balanced by terrestrial uptake from the atmosphere, largely due to photosynthesis in plants that requires CO2.  This latter uptake from the atmosphere could lead to a large negative feedback on climate change.  This is because increased CO2 in the atmosphere leads to changes in plant stomata and enzymes that allow for more CO2 to be absorbed.  Previous data has shown that photosynthetic rates increase with more CO2 in the atmosphere, and this rate increases to a greater degree in hotter climates.  So the tropics should be very important for this negative feedback system!

Figure courtesy of theguardian.com

Figure courtesy of theguardian.com

However, increased CO2 also leads to higher temperatures and possibly drier regions more susceptible to forest fires, leading to loss of forest and decreased terrestrial uptake, a positive feedback that would lead to even more CO2 in the atmosphere.  Thus, understanding how these two feedback loops – one negative and one positive – balance is crucial to create accurate models for future climate change.  Previous theoretical work has suggested that the tropical forests should be a large CO2 sink, but experimental studies have not supported this.  Enter the current study to clear up some of the mystery…

To understand the role of tropical forests in terrestrial uptake, Schimel et al combined data on atmospheric CO2 levels from many different sources, matching biosphere and atmospheric models to available experimental data.  A large reason for the difficulty in pinning down the role of tropical forests in CO2 uptake is the huge degree of uncertainty (read: large standard deviation) in many theoretical predictions.  Schimel et al attribute this to huge fluxes in land use, leading to big rises and falls in the amount of forest, that occur over the same years that terrestrial uptake was measured.  Therefore, it’s very hard from that data to extract exactly whether the tropical forests are taking in more CO2 or if land use is the dominant effect.

Because many of these past theoretical studies included results with large degrees of uncertainty, the authors then only kept those models that appeared to best reproduce ground-based experimental measurements.  This method provided a rigorous process to choose models that converge with experimental findings and reduce the uncertainty seen in previous theoretical studies.  The main result, shown below, uses data averaged over nine theoretical models that converge with experimental data.  The CO2 effect, in black, quantifies how much uptake increases with increasing CO2, and the gross primary production (GPP), in red, measures the amount of biomass.  A larger GPP basically indicates more forest!  Both are plotted as a function of latitude (x-axis) to determine where on the planet uptake is greatest:

Figure courtesy of [1]

Figure courtesy of [1]

The plot shows two main points.  First, the CO2 effect on terrestrial uptake is dominated by tropical forests near the equator at latitudes near zero.  There is also a smaller peak in the northern hemisphere, around 50 degrees latitude, corresponding to the boreal forests that spread over huge tracts of land in Canada and Siberia.  Second, there is a strong correlation between the shape of the CO2 effect in black and GPP in red, indicating that the CO2 effect is strongest in regions with more forest!  This correlation is important, as it strongly suggests that forests and the reason for the CO2 effect, due to the increased photosynthetic rates with more CO2 in the atmosphere (although correlation is not causation!)

This is the first clear evidence from theoretical models backed by experimental measurements that tropical forests play a huge role in terrestrial uptake, much larger than previously known.  The boreal forests had previously been predicted to be the major sink, and, although this new data does show they play a role, it argues that tropical forests are more important.  This is to be expected from the basic knowledge that photosynthetic rates increase in hotter climates, leading to more CO2 uptake, but never before has it been shown so clearly through theoretical modeling.

Scientists predict that around 30% of all anthropogenic CO2 emissions are absorbed by forests.  These new results show how important the tropical forests are to continue to uptake all this CO2 – continued deforestation will lead to more CO2 emissions just sitting around in the atmosphere without the trees to absorb them.  From an optimistic point of view, successful conservation policies in the tropics that leave most of the rainforest intact would provide us with a valuable CO2 sink and possibly delay global warming changes, giving us more time to adapt, prepare, and develop carbon-neutral energy sources.

References

1)

ResearchBlogging.org

Schimel D, Stephens BB, & Fisher JB (2014). Effect of increasing CO2 on the terrestrial carbon cycle. Proceedings of the National Academy of Sciences of the United States of America PMID: 25548156

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