- The side effects of large-scale forestation initiatives could double plant water stress through increasing demand
- A research team led by the University of Sheffield used computer models, which simulate the land, ocean and atmosphere, to investigate the impact of forestation on the water cycle under future climate scenarios
- Results of the study found large-scale forestation could result in a surface cooling of up to 2°C at low latitudes by 2100, due to a 10 per cent increase in evaporative cooling
- Combining forestation with other climate mitigation strategies is vital for more effective long-term climate action
The side effects of large-scale forestation initiatives could have serious consequences for water availability, a pioneering study has found.
The research, led by scientists at the University of Sheffield in collaboration with NCAR, WWF and University of Washington, has raised significant concerns about the potential unintended impacts of large-scale forest expansion as a nature-based solution to climate change.
Published in Biogeosciences, the study revealed how expanding forest cover-while beneficial for capturing carbon dioxide-may lead to water scarcity and impact water security in some of the world's most vulnerable regions. Central Africa, a region with low adaptive capacity to climate change, could see water availability reduced by 15 per cent
The study found that large-scale forestation could result in a surface cooling of up to 2°C at low latitudes by 2100, driven by a 10 per cent increase in evaporative cooling. However, this cooling effect comes with trade-offs.
Building on previous work examining the atmospheric and radiative effects of forest expansion, this study focuses on the implications for water availability. The research found that in certain regions, particularly in the tropics, plant water stress more than doubles when forest expansion is implemented, raising concerns about the feasibility of these initiatives and their potential to compromise water security.
This reduced water availability could negatively impact agriculture and food production, particularly in regions reliant on groundwater and rivers.
Dr James King, from the University of Sheffield's School of Biosciences and lead author of the research, said: "Our findings suggest that large-scale forest expansion plans may have more complex side-effects than previously thought if they are realised in the next few decades.
"While forestation can contribute to local cooling, the increased water demand could have serious consequences for water supply, especially in tropical and subtropical grassland areas. This calls into question the viability of forest expansion as a one-size-fits-all solution for climate change mitigation.
"We argue that while 'nature-based solutions' such as forest expansion are important and valuable tools in the fight against climate change, they need to be stress-tested using simulations to uncover potential negative side effects. This research highlights that relying on a single mitigation strategy is risky, and there is no substitute for reducing greenhouse gas emissions."
Dr Maria Val Martin, University of Sheffield UKRI Future Leaders Fellow and senior author of the study, said: "Our study shows that large-scale forestation can offer regional cooling benefits and contribute to climate change mitigation, but it also has trade-offs. Expanding forests enhances evaporative cooling and lowers temperatures, but it also doubles water demand in some tropical regions, raising concerns about water availability in the future. It underscores the importance of carefully considering the biophysical impacts of reforestation and afforestation strategies, in addition to their CO2 removal potential."
Using advanced computer models simulating the Earth's land, atmosphere, and oceans, the researchers analysed the global effects of current forest expansion plans. Beyond capturing and storing carbon, trees also significantly increase water transfer from soils into the atmosphere. While this has a cooling effect on the Earth's surface, it can also dry soils and rivers, particularly in tropical and subtropical regions, which may affect local food and water security.
The study comes at a time when tree planting initiatives are gaining momentum worldwide as an easy-to-understand and widely promoted solution to climate change.
The research group's next steps include investigating how forest expansion might be affected by wildfires in a warming world, as well as examining the consequences of wetland restoration for climate mitigation. The team is also studying how combining different climate strategies may affect air quality, particularly in the UK.
