/ 25 July 2021

Afforestation can hinder fight against global warming if done wrong, study says

Drcongo Environment Economy Agriculture Silviculture
Restoration: Illegal charcoal production and logging are the main threats to the conservation of Virunga National Park in the Democratic Republic of the Congo. (Alexis Huguet/AFP)

Forests offer a multitude of benefits to society, including the capture, storage and regulation of water flows and the improvement of air and water quality. But if afforestation is not done correctly it can hamper efforts at mitigating the effects of climate change.

A study published by the scientific journal nature notes that forests play a key role in tackling climate change, thanks to their capacity to sequester carbon, a process by which carbon dioxide is removed from the atmosphere and held in a solid or liquid form.

The study says preserving and expanding forest cover is essential to this, but warns that changing the land cover can further affect the climate system through biophysical effects. 

“Trees should only be planted in areas that naturally support forest — planting on grasslands, peatlands, or in tundra ecosystems can have unintended consequences that enhance warming,” said Bonnie Waring, a senior lecturer in the faculty of natural sciences at the Grantham Institute in the United Kingdom.

Afforestation can alter the cloud regime, with potential repercussions on the hydrological cycle, a reference to the continuous movement of water as it makes a circuit from the oceans to the atmosphere to the Earth, according to the study in the nature journal.

“We show that for 67% of sampled areas across the world, afforestation would increase low-level cloud cover, which should have a cooling effect on the planet,” it says. 

“We further reveal a dependency of this effect on forest type, notably in Europe where needleleaf forests generate more clouds than broadleaf forests.”

The study says simplistic approaches to tree restoration that do not properly account for the complexities of the plant–atmosphere interactions can underestimate the amount of land needed and overall provide a dangerous and misleading message.

“Planting trees is not a simple solution and can have various climate impacts and mitigation efficacy depending on how and where they are planted,” it says.

“Ultimately, it is becoming increasingly evident that land-based climate mitigation through afforestation, forest restoration or avoided deforestation should be based on the comprehensive assessment of both the biogeochemical and the biophysical processes triggered by forest cover change.”

The study makes a global-scale observational assessment of where afforestation could lead to an increased cloud formation by exploiting satellite data products. 

It shows that afforestation generally leads to an increase in low cloud cover over most of the world, and predominantly in the warmer months of the year.

Waring says it is important to consider how forests will affect the total amount of carbon stored over time.

She co-authored a briefing paper on the pros and cons of using trees to fight climate change. 

The authors found that, while trees and forest ecosystems help limit global warming by reducing the concentration of carbon dioxide in the atmosphere, ultimately, tree planting is not a silver bullet to averting climate change.

“Only rapid reduction of greenhouse gas emissions can halt the ongoing rise in global temperatures.” 

Chris Gilili is an Adamela Trust climate and economic justice reporting fellow, funded by the Open Society Foundation for South Africa