The world’s tropical rainforests will still be able to soak up “high levels” of CO2 in the future, but only if they are left intact and global warming is limited to 2C above pre-industrial levels.
This is the finding of a new Science study, which draws on data from almost 600 tropical forest sites spread across the world.
However, if temperatures exceed 2C – the warming threshold set by countries under the Paris Agreement – the ability of tropical forests to store CO2 will decline rapidly, the study finds.
The results suggest there is “an opportunity, if we really reduce emissions, to keep forests within a safety zone”, the lead author tells Carbon Brief. “But the negative side is that if we push warming past a point…then the loss of carbon will be faster.”
Forest trees take in CO2 from the atmosphere when they carry out photosynthesis – the process where plants absorb CO2 in order to build new materials, such as shoots, roots and leaves. This means that, as long as forests remain intact, they can act as long-term “sinks” of CO2.
However, climate change threatens the ability of forests to store carbon.
One reason for this is that temperature plays a key role in moderating photosynthesis. The optimum air temperature for photosynthesis in tropical rainforests is estimated to be around 23-28C. Further increases in temperature will likely cause the rate of photosynthesis to decline, but the total extent of this decline – and its impact on carbon storage – is still uncertain.
Climate change could also raise the risk of drought in some tropical forests. Droughts can quickly kill forest trees, causing them to release their carbon stores as dead trees rot.
However, at the same time, rising CO2 levels could also boost forest carbon storage. This is because CO2 is a key ingredient in photosynthesis and so, if more CO2 is in the atmosphere, plants could carry out photosynthesis at a faster rate. (This phenomenon is known as the CO2 fertilisation effect.)
Understanding how all of these factors will shape forest carbon storage in the future is a key aim for climate scientists.
The new study addresses this question by looking at how climatic drivers such as temperature and rainfall could affect the ability of tropical forests to store CO2 in the long term.
It finds that, over long timescales, temperature becomes the most influential factor on tropical forest carbon storage, explains lead author Dr Martin Sullivan, a lecturer in statistical ecology at Manchester Metropolitan University. He tells Carbon Brief:
“We find that daytime temperature is the most important factor in terms of affecting the amount of carbon that forests can store.”
For the study, the authors aimed to get a better understanding of how climate change could affect tropical forest carbon storage in the long term – on the scale of hundreds of years.
However, it was not possible to do this by simply looking back at historical records of carbon storage in tropical forests, Sullivan explains:
“The one problem with studying forests is that, even with decades of monitoring, we can only just start to see changes in carbon storage emerging. This is because trees are really long-lived.”
Because of this, the researchers instead studied the relationship between carbon stocks and factors such as temperature and rainfall in tropical forests growing in different climates across the world. “We’re looking at variation in space as a proxy for time,” explains Sullivan.
The researchers then used this information to make projections about how future climate change could affect long-term forest carbon stocks.
The map below shows the spread of the tropical forest plots – sites where individual trees are regularly monitored – included in the analysis. These include 273 plots in South America (green), 239 plots in Africa (orange), 61 plots in Asia (purple) and 17 plots in Australia (pink).
From their spatial analysis, the researchers find that the most influential negative factor on forest carbon stocks in the long term is increases in maximum daily temperature.
This is likely because, once temperatures exceed a certain threshold, the rate of photosynthesis starts to decline rapidly, Sullivan says.
The chart below shows the relationship between forest carbon stocks and – from top to bottom – daily minimum temperature, maximum temperature, precipitation in the driest quarter of forests, cloud cover, wind speed, soil texture and soil fertility.
(On the chart, a value below zero indicates a negative relationship between the climate factor and carbon stocks, whereas a value above zero indicates a positive relationship.)
The relationship between carbon stocks and (top to bottom) daily minimum temperature, maximum temperature, precipitation in the driest quarter of forests, cloud cover, wind speed, soil texture and soil fertility. Source: Sullivan et al. (2020)
The chart also shows how increases in minimum temperature have a positive relationship with carbon stocks.
This is likely because, as the very coolest temperatures get warmer, they get closer to the optimum temperature for photosynthesis. However, this is outweighed by the negative impact of rising maximum temperatures on photosynthesis and, therefore, carbon stocks.
Increases in precipitation (rainfall) in the driest tropical forests were also associated with greater carbon stocks, the chart shows. This is because drier forests receive more of a boost to plant growth from greater rainfall, according to the study.
The authors used their findings on the relationship between forest carbon stocks and maximum daily temperature to make projections for the future.
The research finds that global warming of 2C could cause tropical forests to lose between 35.3 and 49bn tonnes of carbon. (For comparison, the world emitted 36.6bn tonnes of CO2 in 2019, which is equal to 10bn tonnes of carbon.)
The carbon losses would be greatest in the Amazon rainforest, followed by rainforests in Asia and Africa, the research finds.
This estimate only considers the impact of temperature rise alone on carbon stocks and not the other impacts of climate change such as drought, Sullivan explains:
“It’s possible, if you add drought on top of that, that there will be a little bit more of an effect.”
However, the authors estimate that the CO2 fertilisation effect “will partially or wholly offset the effect of this temperature increase” on global tropical forest carbon stocks.
Because of this, the authors conclude that tropical forests will remain a resilient carbon store under 2C of temperature rise. In the their research paper, they say:
“Although many tropical forests are under severe threat of conversion, our results show that, in the long run, tropical forests that remain intact can continue to store high levels of carbon under high temperatures.”
It is worth noting, however, that the CO2 fertilisation effect may not be as large as some estimates suggest, says Prof Anja Rammig, a researcher of land-surface interactions from the Technical University of Munich, who was not involved in the study. She tells Carbon Brief:
“There are several reasons why this CO2 effect may not be as strong as models expect in the future. If that is the case, then the carbon losses from tropical forests will be larger.”
One factor that could limit the CO2 effect is nutrient availability, she says. That is, if forest soils run out of key nutrients such as phosphorus, this might limit the extent to which rising CO2 levels will be able to boost photosynthesis and, therefore, carbon storage.
The study also estimates that, if temperatures rise exceeds 2C, carbon losses from tropical forests could start to accelerate. Sullivan explains:
“We have an opportunity, if we really reduce emissions, to keep forests within this safety zone. But the negative side is that if we push warming to a point where daytime temperatures are going way up then the loss of carbon will be faster.”
The resilience of forests to warming will also depend on humans bringing an end to rampant tropical deforestation, he adds:
“Trees that can cope with these new warmer climates will need to be able to move to new areas. As we’re fragmenting forests, we’re impeding that movement.”
The findings “align well” with a research paper published in Nature in March using the same datasets, says Prof Simon Lewis from the University of Leeds and University College London, who is an author on both papers. (Lewis is also a contributing editor for Carbon Brief).
The Nature paper found that the ability of tropical forests to remove CO2 from the atmosphere on a year-to-year basis is already in decline. Lewis explains:
“The new study asks a different question, assessing what the really long-term impacts of environmental factors are on intact tropical forests…Yet despite the decades versus hundreds of years difference of focus, the results are similar: Both studies show that high temperatures are negative for tree growth and droughts are important in increasing tree mortality, and that Amazonia is more vulnerable than African forests.”
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