Tropical forests can still store ‘high levels’ of carbon under 2C of warming


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.”

Evergreen

Around one quarter of all of the carbon stored on land can be found in tropical forests.

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.”

World view

For the study, the...

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