Search for a project

Interactions and feedbacks between biogenic aerosols and vegetation

Dr Alex Rap (SEE), Prof Dominick Spracklen (SEE)

Project partner(s): Dr Richard Ellis (Centre for Ecology & Hydrology, Wallingford )

Contact email: a.rap@leeds.ac.uk

This project will investigate the role of biogenic aerosol in vegetation-climate feedbacks, using state-of-the-art models to improve our understanding of biosphere-atmosphere interactions that may have important implications for future climate projections.

Background

Biogenic aerosols play an important role in vegetation-atmosphere-climate interactions (Carslaw et al, 2010). A key driver of this interaction is the fact that the terrestrial biosphere constitutes a large source of atmospheric aerosols, such as secondary organic aerosol (SOA) and biomass burning from wildfires. Once in the atmosphere, aerosols affect climate through their direct and indirect radiative effects (Rap et al, 2013). Strongly constrained by temperature, precipitation and radiation (Figure 1a), the vegetation responds to these changes in climatic drivers. This will in turn exert a strong control over the abundance and distribution of biogenic aerosol, therefore closing this important feedback loop.

Figure 1: (A) Distribution of climatic constraints to plant growth derived from long-term climate statistics (taken from Nemani et al., 2003); Satellite-driven global NPP trends between (B) 1982-1999 (taken from Nemani et al., 2003) and (C) 2000-2009 (taken from Zhao and Running, 2003).

In addition to their direct and indirect effects, a substantial contribution to the aerosol impact on climate is via their ability to influence biogeochemical cycles (Figure 2; Mahowald, 2011). This influence is exerted through changes in physical climate (temperature, precipitation, radiation) and through chemical deposition (nutrients, toxins). One such mechanism is the change in the surface radiation regime, i.e. reduction of direct and increase in diffuse radiation. The resulting effect on plant photosynthesis is a balance between inhibition due to reduction in total radiation and enhancement due to the diffuse radiation fertilisation effect (Kanniah et al., 2012). This occurs because under diffuse radiation conditions, light penetrates deeper into the canopy, illuminating leaves that may otherwise be in shade and thus enhancing photosynthesis overall. Previous studies demonstrated how big a role this effect can play in the terrestrial carbon cycle: Mercado et al. (2009) showed that diffuse radiation fertilisation from pollution aerosol induced a 25% increase in the global land-carbon sink in recent decades, while Rap et al. (2015) estimated that increased diffuse radiation from Amazon biomass burning leads to an increase in plant productivity equivalent to ~50% of the regional emissions from biomass burning. 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 (Figure 3).

Figure 2: Aerosol indirect effect through changes on biogeochemical cycles (green bar) compared to the aerosol direct and cloud albedo effects on climate (taken from Mahowald, 2011)

 Figure 3: Global distribution of year 2000 NPP changes caused by diffuse radiation fertilisation from SOA emissions.

A key uncertainty in understanding the full extent of these complex feedbacks is given by the fact that important mechanisms (e.g. the diffuse radiation fertilisation effect) are not currently represented in climate models. At Leeds, through established collaborations with scientists from CEH and the University of Exeter, 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 biogenic 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 UK Earth System Model (UKESM) simulations.

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

  1. A comprehensive assessment of regional and global BVOC and SOA emissions (with the associated uncertainties), both from process-based and from empirical models.

  2. Examining the role of biogenic aerosol in the observed NPP trends in recent decades (Figure 1b,c).

  3. Exploring the extent to which anthropogenic aerosol has affected the efficiency of these ecosystem feedbacks, considering the non-linearities in the system.

  4. Investigating the climate effects on these interactions during the last few decades (e.g. temperature and atmospheric carbon dioxide changes).

  5. Using future simulations to estimate how climate change is likely to affect these feedbacks.

  6. Assessing the role of these feedbacks in the terrestrial carbon cycle.

Potential for high impact outcome

There are still large uncertainties in our understanding of how the terrestrial carbon cycle has changed in recent decades and how it is likely to evolve in the future. With access to cutting-edge techniques and support from our world leading research groups, this project will improve our understanding of biosphere-atmosphere interactions and feedbacks that may have important implications for future climate projections. This will likely be of interest to both the general public and to policy makers working in climate mitigation and forest conservation. It is expected that findings of this project will be presented at national and international conferences and published in high impact journals.

Training

The student will work under the supervision of Dr. Alex Rap and Prof. Dominick Spracklen and will be a full member of two very active and supportive research groups in SEE, the Biosphere Processes Group and the Physical Climate Change Group. The student will also be part of the Leeds Ecosystem, Atmosphere and Forest (LEAF) research centre at the University of Leeds that brings together researchers from across the Campus. Through the high level specialist scientific training associated with this project, the student will develop a comprehensive understanding of vegetation-atmosphere interactions and will work with state-of-the art global land-surface and atmospheric composition-climate models. In addition, the student will learn how to communicate science and how to write high impact journal publications.

The successful PhD student will also have access to a broad spectrum of training workshops put on by the Faculty that include an extensive range of training workshops in numerical modelling, through to managing your degree, to preparing for your viva. A full list of training opportunities is available here.

Partners

The student will collaborate closely with Dr Richard Ellis from the Centre for Ecology & Hydrology (CEH) Wallingford and other CEH scientists working on the development of the next generation land-surface models.

Eligibility requirements

A good first degree, Masters degree or equivalent in a quantitative science discipline (e.g. Mathematics, Physics, Chemistry, Environmental Science, Geography, Engineering) and a keen interest in global environmental problems. While a substantial part of this project involves computer modelling, prior experience is not essential - we provide high level specialist scientific training during the PhD.

References and further reading

Related undergraduate subjects:

  • Chemistry
  • Engineering
  • Environmental science
  • Geography
  • Mathematics
  • Physics