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FEATURE: Finding unlikely connections – When soil meets sky in atmospheric science

Research from the Future Climate for Africa programme explores the links between soils, rainforests and rainfall patterns in the Ivory Coast – as Leonie Joubert explains.

Dr Conni Klein was stuck. She was working with a team of scientists to build a computer model that will help them understand how years of clearing-felling tropical forest in the Ivory Coast, and replacing them largely with mono-crop cocoa plantations might change how clouds form here during the monsoon period, and what this could mean for rainfallBut she was missing an important piece of the puzzle. She didn’t know what kinds of soil you typically find across this traditionally rain-forested area.

The German atmospheric scientist was a ten-hour flight away from the West African country, though, and tucked away in an office at the Centre for Ecology and Hydrology at the Natural Environment Research Council near Oxford in the United Kingdom. A quick internet search wasn’t going to give her the precise information she needed.

So she dialled up her friend and colleague Dr Bamba Adama, a meteorologist and climate scientist at the Université Félix Houphouët Boigny in Abidjan, to ask for his help.

Even though their home institutions are more than 5 000 km apart as the crow flies, Adama and Klein are both able to throw their intellectual weight behind this shared problem, because of the collaboration that started with the Future Climate For Africa (FCFA) programme. FCFA is a trans-disciplinary team of researchers from different continents that is grappling with some of the big questions of how climate change will express itself in different regions of sub-Saharan Africa.

As part of the West Africa* FCFA team, whose main focus is on the interplay between rising global temperatures and the behaviour of the monsoon in West Africa, Adama and Klein have been able to travel to expand their research and skills. Klein to travel to location in West Africa, and Adama to travel to both Italy and the UK to advance his modelling expertise.

This was also part of a process that supported the Ivory Coast government to bring super-computing capacity to the region which, according to Adama, will allow scientists in West Africa to run their regional climate models locally.

But why would two atmospheric scientists, who are concerned with physical processes taking place in the air 10 to 18km above the ground in West Africa, need to know what kinds of soils there are on the ground? What does soil type have to do with cloud formation or storm activity or changes in monsoon rainfall patterns?

‘The southern parts of Ivory Coast used to be covered mostly with tropical rainforest,’ Klein says. ‘These forests predominantly grow on infertile sandy soils. The soils aren’t high in nutrients because, having had such high rainfall over millions of years, this has leeched the soils of nutrients. So today the forest feeds itself by dropping its own leaves onto the ground. The leaf litter forms a layer of humus on top of the soil, which decomposes and releases nutrients that the trees’ roots can absorb.’

But these expansive bodies of trees do something else to keep themselves alive and thriving: they draw moisture from the soils, and release this into the air above their green canopy. When a canopy like this spreads out over hundreds of square kilometres, it has a significant impact on the climate: it keeps the air cool, and helps the atmospheric process of building clouds, which then drop their rain back down onto the ground. They’re not called ‘rain’ forests for nothing.

Around and around, these two systems go: the canopy dropping nutrient-rich leaves to the ground which feed the roots, and hence the forest; the leaves releasing moisture into the air and birthing rain, which waters the forest. 

But over the years, much of these forests have been clear-felled and replaced with mono-crop cocoa plantations.

Together with their research team, Klein and Adama need to answer an important question if they want to generate trustworthy and reliable information about how the climate has been affected by deforestation in this West African region: if you remove the tropical forests, and replace them with mono-crop cocoa plantations, you take away all that important leaf litter.

This loss of leaves might kick off a whole cascade of other natural processes. What happens to life in the soil, without the nutrients from the dead leaves? How long will the cocoa trees be able to live if the soil slowly dies? And if the soil dies, it’ll become drier and warmer. What will happen when rain lands on this soil? Usually, the spongey layer of dead leaves absorbs the water and slowly releases it over time, allowing it to trickle down into the ground beneath. But without this layer, the rain is more likely to rush off the top of an increasingly impermeable shell of hardened ground, or it will drain into deeper soil layers too quickly where it is less accessible to the trees’ roots.

With drier soils, and a thinned-out and less diverse tree canopy, what will happen to the transpiration processes above?

‘This is important for atmospheric scientists to know. Has this change in tree type impacted on the available moisture in the atmosphere, and the humidity in the soil? And what will happen in future, if deforestation continues as it has? With less transpiration, we can expect higher surface temperatures and less moisture in the atmosphere, so there’s less cooling. This could mean less rainfall. At the same time this may also drive an increase in temperature here and the formation of heat islands around the plantations, which could also create rain: when there’s enough atmospheric moisture and additional warming in the overlying air, warmer soils can result in greater cloud formation and torrential rain.

This means that replacing a rainforest with a mono-crop cocoa plantation could possibly create a negative feedback loop of drying and warming, or alternatively result in locally increased cloud formation driven by the stronger surface heat source.

The atmosphere responds to deforestation, Klein and Adama are sure of this. But what they don’t know for certain is which effect might be at work here. By building a model that can simulate the past transition from tropical cover to mono-crop cover, they might be able to better understand what the future impact will be on monsoon rainfall, if the remaining rainforests are cleared and replaced with mono-cropped plantations.

*African Monsoon Multidisciplinary Analysis (AMMA-2050) aims to address the challenges of understanding how the monsoon will change in future decades, to 2050, and how this information can be most effectively used to support climate-compatible development in the region.

This article was written by Leonie Joubert as part of a series that delves into the science that has been produced by various FCFA projects, and introduces some of the people behind it. It was first posted on www.futureclimateafrica.org.

Image: Cote d’Ivoire, credit UN REDD.

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