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Carbon Brief Staff

22.07.2013 | 12:10pm
ScienceScientists examine how Africa’s rainforests will fare as the climate changes
SCIENCE | July 22. 2013. 12:10
Scientists examine how Africa’s rainforests will fare as the climate changes

For most people, the rainforests of Africa don’t conjure up the iconic images of brightly-coloured birds and snaking rivers that the Amazon does. And yet Africa holds the second largest area of rainforest in the world.

So how is it we hear so little about these forests and how they might fare under climate change? A new collection of papers in the journal Philosophical Transactions of the Royal Society B hopes to deepen understanding of Africa’s rainforests by bringing together the latest research on their past, present and future.

We’ve pulled out some of the highlights to find out what rising greenhouse gas emissions could mean for the region.

The African rainforests

The African continent is home to the second biggest area of rainforest in the world – about   30 per cent of global rainforest cover. As this   satellite data from 2005 shows, most of the rainforest is located in central Africa, but some also grows in west Africa and on the island of Madagascar:

Africa Distribution

These rainforests are home to an abundance of plants and animals and provide food, fuel and shelter for local communities. They’re also important for the global climate: the trees lock up an enormous amount of atmospheric carbon through photosynthesis, help regulate atmospheric circulations patterns and provide moisture to the atmosphere.

Climate change is one of many factors which could affect rainforests’ abilities to perform these critical functions in the future.

Dr. Simon Lewis from the University of Leeds is one of the scientists involved with the new research. He explained to Carbon Brief that there’s a delicate balance in how these forests will respond to climate change between carbon dioxide, rising temperatures and changes in rainfall:

“The key factors for forest trees are how much they respond positively to more carbon dioxide in the atmosphere, versus how detrimental higher temperatures will be to plant growth. Overlying this are the impacts of any changes in rainfall.”

Temperatures are projected to rise over the coming century. But changes in rainfall and the long term effect of more carbon dioxide is less clear.

Temperature rise

Scientists can use information about what the climate’s been like in the past to anticipate how African rainforests might respond to rising temperatures.

Studies show there have been warm and wet periods in the past which caused rainforests to spread north – leading to the greening of the Sahara desert. But where warm spells were accompanied with less rainfall, the rainforest shrank – replaced instead by grassy landscapes called savannas.

Direct human interference – clearing land for agriculture, for example – has also affected how the rainforest responded to climate change in the past.

That’s likely to remain the case. At the moment rainforests in the region are relatively untouched, with lower rates of deforestation than their counterparts in South America and south-east Asia. But economic and population growth could place increasing pressure on Africa in the future.

On the other hand, that these forests have already been somewhat degraded could prove to be useful if some of the less adaptable species have been sifted out. So although the usual caveats apply, it’s possible that African rainforests could be more resilient to future climate change.

So changes in other factors, like human influence, means scientists can’t draw too many conclusions from what happened in previous warmer spells.

Drier or wetter?

Until recently, there wasn’t much research focused on what rising greenhouse gases and temperatures could mean for the future of the African rainforest. While limited modelling now exists, its hard to say for certain how rainfall patterns might change over the region.

A number of climate models suggest the eastern limits of Africa’s rainforests could get a bit wetter, while west African rainforests may see considerably less rainfall – especially in the dry season. Africa’s rainforests have only just enough rainfall to be viable. If conditions get much drier, rainforests could be replaced by grasslands.

Over recent decades there’s been evidence of a drying trend in African rainforests, but scientists studying the trend say its likely to be due to natural fluctuations in the Indian and Pacific ocean temperatures which affect the climate. A separate group of scientists studying drought in central African rainforests found no evidence they had changed as a result of manmade climate change.

So there’s a mixed picture from models. There’s also a data problem. Across the entire central African basin there are just 10 rain gauges collecting on-the-ground data, making it difficult to model how rainfall might change in the future.

Plant food

Alongside changes in temperature and rainfall, scientists have to factor in the effect of more CO2 in the atmosphere.

Rising carbon dioxide levels will directly affect how African rainforests change over the coming century. More CO2 is thought to stimulate plant growth and encourage plants to use water more efficiently. These abilities could help trees outcompete nearby grassy habitats, meaning there’s a chance the rainforests could expand.

A more efficient rainforest with greater mass could lock up more atmospheric carbon, and indeed research suggests this is already happening. To some extent, that might help limit the effects of rising greenhouse gas emission, as Dr Lewis explained:

“Undisturbed intact rainforest is absorbing carbon dioxide from the atmosphere, acting as a partial brake on the rate of increase in atmospheric carbon dioxide concentrations”

There are limits to this. The rate at which the rainforest can lock up carbon is slowing, and that’s expected to continue over the course of this century. Scientists say there are other factors – like the availability of nutrients – which will also limit how much rainforests can grow.

A picture of resilience?

There’s a lot of research to cover here, but to sum up: There are a number of reasons why its hard to predict the fate of African rainforests under climate change. Scientists are fairly confident temperatures in tropical Africa will rise, but future changes in rainfall are uncertain – making it hard to know if rainforests might expand or shrink. Other factors like deforestation, logging and fires will also help determine the forests’ resilience to future climate change.

Looking at past climates shows African rainforests have in the past shrunk once a threshold of warmer, drier conditions was crossed. But if that doesn’t happen, African rainforests may be pretty resilient to climate change.

As the scientists involved with these new papers note, most research and policy attention has so far focused on the Amazon. Africa’s rainforests have gone relatively understudied. They hope this new research will be a starting point for addressing that deficit.

Many of the scientific papers referenced in this blog came from the September 2013 edition of Philosophical Transactions of the Royal Society B. This was a theme issue entitled “Change in African rainforests: past, present and future”.

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