Biosphere-atmosphere interactions and feedbacks via radiation changes

This project will investigate the role of radiation changes in vegetation-climate feedbacks, using state-of-the-art models to improve our understanding of biosphere-atmosphere interactions with important implications for future climate projections.

Background

Natural aerosols play an important role in vegetation-atmosphere-climate interactions. The terrestrial biosphere and the oceans are a large source of atmospheric aerosols, such as secondary organic aerosol (SOA), biomass burning from wildfires, dimethyl sulfide (DMS) from plankton, and sea-salt. Once in the atmosphere, these natural aerosols affect climate through their direct and indirect radiative effects. On the other hand, the abundance and distribution of natural aerosol is controlled by changes in climate. For example, biogenic aerosol emissions from land vegetation are strongly constrained by changes in temperature, precipitation and radiation, therefore closing this part of the feedback loop.

The aerosol-induced change in the surface radiation regime, i.e. reduction of direct and increase in diffuse radiation is also an important mechanism contributing to the aerosol impact on climate. Plant photosynthesis is more efficient under diffuse radiation conditions, essentially due to deeper canopy light penetration. Previous studies demonstrated the importance of this mechanism in the terrestrial carbon cycle, i.e. diffuse radiation fertilisation from pollution aerosol induced a 25% increase in the global land-carbon sink in recent decades. In addition, current work from our group indicates that SOA formed through atmospheric oxidation of plant-emitted volatile organic compounds is also an efficient diffuse radiation fertiliser.

However, there are still many uncertainties in the current understanding of these complex feedbacks, as important mechanisms such as the diffuse radiation fertilisation effect have so far been missing from climate models. At Leeds we now have the ability to represent these interactions in a more comprehensive way than ever before. This project will therefore provide an exciting opportunity to employ state-of-the-art global models to further our knowledge in this area.

Objectives

The central objective of this project is to analyse and quantify the role of natural aerosol in the various vegetation-atmosphere-climate feedback mechanisms. The approach will likely involve a combination of global aerosol, radiation and vegetation models, together with simulations using the new UK Earth System Model (UKESM).

While relatively flexible to allow for your interests, the studentship is likely to involve:

• A comprehensive assessment of regional and global natural aerosol emissions (with the associated uncertainties), both from process-based and from empirical models.
• Examining the role of natural aerosol in the observed NPP trends in recent decades using both on-line and off-line modelling.
• Exploring the extent to which anthropogenic aerosol has affected the efficiency of these ecosystem feedbacks.
• Investigating the effect of temperature and atmospheric carbon dioxide changes on these interactions during the last few decades.
• Using future simulations to estimate how climate change is likely to affect these feedbacks.
• Assessing the role of these feedbacks in the terrestrial carbon cycle.