The tropical forest carbon cycle and climate change – a summary

Our scientific heritage and extensive work in the tropical forest is what sets us apart in our industry; we are proud that our senior team has published over 100 scientific papers.

Here you will find a summary of the first of several papers we will share, along with links to the full document.

We begin with a paper first published in Nature by our Chief Scientist Professor Ed Mitchard et al.

LINK: Mitchard, E 2018, ‘The tropical forest carbon cycle and climate change’, Nature.

Summary

This paper focuses on what we know about how much carbon is contained within tropical forests, the rate at which this might be changing, and how climate change may impact this rate of change in the future. This knowledge is crucial because tropical forests can both take in carbon from the atmosphere, thus reducing the impacts from increasing atmospheric carbon dioxide (CO2) levels caused by fossil fuel burning, or may themselves contribute carbon to the atmosphere as forest is cut down and degraded, thus amplifying the effects of fossil fuel use.

Types of forest loss

Forests can be a source of carbon to the atmosphere in one of three ways.

First through deforestation, which is the total loss of trees in an area, usually for the purpose of establishing some form of agriculture. The area of forest lost can be measured by satellites, although estimating how much carbon is being lost is more difficult. Degradation, which is much harder to measure, is the partial loss of trees in an area. The limited data available suggests that, globally, degradation may release approximately twice as much carbon to the atmosphere as does deforestation. Finally, small areas of tropical forests are developed on carbon-rich peat deposits. Data on these are even more sparse, although it is likely that this peat is losing carbon at present, potentially significantly so.

The tropical forest as a carbon sink

On the other hand, forests are also a sink for atmospheric carbon, drawing CO2 out of the atmosphere and storing it in woody matter known as biomass. The regrowth of trees in previously deforested or degraded landscapes contributes to this, as does the growth of trees in undisturbed forests. This is primarily due to the fact that the increase in atmospheric CO2 levels makes photosynthesis easier.

Quantifying data

Quantifying the current rates of tropical forest carbon loss and gain is difficult for several reasons. In general, this is because the data are either accurate but very localised (e.g., direct measurements from small field sites), making them hard to generalise from, or the data are broad averages over large areas (e.g., most satellite datasets), which obscure important details. Despite this, and with large uncertainties, at present tropical forests appear to remove approximately as much carbon from the atmosphere as they contribute. It’s important to note that in unusually warm ‘El Nino’ years tropical forests contribute much more carbon than they remove.

Rate of tropical forest loss

Understanding the current rates of loss and gain is important, but knowing how and why these rates are changing, and how they are likely to do so in the near future, is even more important. Satellite data suggest that the rate of tropical deforestation is increasing, and it is likely that this trend will continue. Given that deforestation and degradation frequently occur together, it seems likely that the rate of degradation also is increasing.

Future behaviour

Predicting the future behaviour of undisturbed forest as a carbon sink is complex. The increased atmospheric CO2 that drives most climate change also causes ‘CO2 fertilisation’, which makes it easier for trees to grow and store carbon. However, the warming that results from this additional CO2 will likely cause more trees to be killed by drought and fires, as well as increase the rate of CO2-releasing forest respiration. Which of these effects will dominate is hard to say, and different methods give different answers. 

The scenario in degraded forests is similar, because as the amount of degradation increases, there is ever more degraded forest that could potentially regrow, consuming carbon at a faster rate than in the original, undisturbed forest. However, these regrowing trees are subject to the same potential negative impacts from drought, fire, and increased respiration. There also is evidence that, eventually, most degraded forest becomes fully deforested, and remains that way.

Conclusion

Considering all the evidence, it seems likely that in the future the major factors causing forests to lose carbon will remain stable or increase, while the ability of forests to remove carbon likely will decrease. Quantifying this using computer models remains a major challenge, and the main limitation is knowledge about forest behaviour that must be gained from field experiments.

While this picture is bleak, the latest models do not fully reflect the potential impacts of government policy at national and international level. Strong commitments to reducing, and ultimately reversing, degradation and deforestation could dramatically change the current trends. Rather than adding carbon to the atmosphere and exacerbating climate change, tropical forests have the potential to be the most important mechanism for removing it, helping to mitigate climate change impacts. For this to become a reality, better maps of how forests are changing are required, as are field experiments to measure how forests are responding to increasing atmospheric CO2 levels. 

Read the full paper here