Exploring the climate side effects and co-benefits of an expansion to UK forests

In order to limit warming to 1.5 degrees Celsius above the pre-industrial period, global carbon dioxide emissions must reach net-zero by around the middle of the 21st century and become negative thereafter [1]. A net-zero emission level requires that any remaining emission sources are balanced by processes that remove carbon dioxide from the atmosphere.

In June 2019, the UK Government passed legislation that commits the nation to reach net-zero greenhouse gas emissions by the year 2050. The Committee on Climate Change estimated that under an ambitious scenario this reduction is potentially achievable [2], assuming that at least 30,000 hectares (1 hectare is approximately the size of an international rugby pitch) of new woodlands are created per year [3, 4]. This level of woodland creation would increase the UK's canopy cover from its current level of 13% to over 17%.

Trees and other vegetation take carbon dioxide from the air as they grow, but they also impact the climate and the composition of the atmosphere in numerous other ways by influencing the concentration of a number of pollutants (i.e., particulate matter and ozone).

This project will combine a hierarchy of computer models, spanning from emissions accounting tools to 3D atmospheric chemistry models, with observational data to develop a framework within which the impacts of tree planting and landscape restoration can be explored to answer questions such as:

a) How much of a contribution to reaching net-zero greenhouse gas emissions can afforestation and landscape restoration in the UK realistically provide?
b) How sensitive is this plan to the level of climate change that the UK experiences in the future?
c) What are the co-benefits (e.g., removal of particulate pollution via deposition onto vegetation) of this level of tree planting?
d) Are there any counter-productive side-effects (e.g., production of ozone due to emission of biogenic volatile organic compounds from vegetation) associated with this level of afforestation? How do these side-effects change as emissions from other sectors (i.e., transport) decline?
e) How sensitive are these side effects and co-benefits to the nature of woodland expansion (i.e., the impact of predominantly conifer plantations versus more natural broadleaf woodlands)