Climate and Atmospheric Science (ICAS) PhD Projects
Climate impacts of pollutant deposition to the terrestrial biosphere
Supervisors: Dr Steve Arnold (SEE), Dr Dominick Spracklen (SEE), Dr Stephen Sitch (Geography)
Background and Motivation
The biosphere and atmosphere are intimately coupled aspects of the Earth system, and their interaction exerts strong control on atmospheric composition and climate. Vegetation plays a role in controlling energy and water vapour fluxes to the atmosphere, but also impacts climate through exchanging CO2 and releasing reactive organic compounds into the atmosphere. These reactive compounds (isoprene & monoterpenes) are produced naturally by vegetation and emitted to the atmosphere in large quantities globally. They impact climate since they affect the atmosphere’s ‘oxidising capacity’ – i.e. its ability to clean itself of emitted pollutants, including the greenhouse gas methane. In addition, they affect the abundance of tropospheric ozone (another greenhouse gas), and lead to the production of small organic particles (aerosol) in the atmosphere, which impact climate by scattering radiation or perturbing the properties of clouds.
Atmospheric pollution may affect the biosphere by several mechanisms. Pollutants can be taken up into the leaves of plants and trees through their stomata – small openings through which water, oxygen and CO2 pass during photosynthesis. This can directly damage plant cells and inhibit plant growth, as energy is allocated to repairing damage rather than accumulating biomass (e.g. during the uptake of the oxidising pollutant ozone). Additionally, pollutants deposited to soil can be taken up in root systems, or may impact the availability of nutrients to plants, resulting in limitation to plant productivity.
Recent research has shown that plant damage due to man-made increases in tropospheric ozone (which is formed in the atmosphere from industrial and transport emissions of nitrogen oxides), may have reduced the terrestrial carbon sink (uptake of CO2 from the atmosphere) by up to 1.3 PgCyr-1on average since pre-industrial times (Sitch et al., 2007). The consequent reduction in CO2 removal from the atmosphere by global vegetation may have resulted in a significant climate warming.
A European spruce forest destroyed by acid deposition in the 1980s (via independent.co.uk).
Acid deposition (or ‘acid rain’) damages vegetation mainly through its harmful effects on soils and foliage. This process was a major problem for Northern European forests and ecosystems during the 1970s and 1980s, mainly due to industrial sulphur emissions from Western and Central Europe. Since the 1990s, European emissions of sulphur have reduced substantially, however the rapid industrialisation of south and east Asia and their reliance on coal-fired energy generation, means that in many regions, sulphur emissions are rising. Consequently patterns of acid deposition are shifting globally, with both sulphuric acid as well as nitric acid due to increases man-made nitrogen oxide emissions, likely to have impacts on vegetation. The consequences of these historic changes and future projected changes in acid deposition on the terrestrial carbon cycle and climate have not been quantified.
Some of the largest uncertainties in the future global carbon cycle and vegetation evolution lie in the interaction between nitrogen deposition and CO2 fertilisation effects on vegetation. It is becoming clear that man-made perturbations to soil nitrogen loading may have implications not only for carbon assimilation, but that the effects of ozone damage may be exacerbated by increased nitrogen deposition.
Furthermore, the consequences for each of these processes and their combined impacts for emissions of biogenic hydrocarbons from vegetation are so far unknown.
Project Methodology and Objectives
In this project you will use state of the art computer models that simulate global sources, processing and deposition of gas-phase (e.g. ozone) and aerosol phase (e.g. sulphate) pollutants, and models of the global terrestrial biosphere and land surface. You will also exploit recently-developed Earth system model, which allows coupled simulation of the biosphere-climate-atmospheric composition system. You will synthesise observations from surface stations, aircraft and satellites to evaluate model processes and develop schemes to describe pollutant deposition effects on vegetation, and investigate consequent feedbacks on climate.
As part of the project, you will collaborate with the US National Center for Atmospheric Research & Colorado State University, and funding will be available for a series of collaborative research visits to Colorado.
Specific objectives of the project are to:
- Investigate historical regional changes in ozone and acid deposition over the past 30 years and model predicted impacts on land carbon sink, and compare with observations from deposition networks and carbon storage estimates.
- Assess sensitivity of ozone-induced carbon sink reductions to nitrogen deposition scenarios.
- Investigate how acid, ozone and nitrogen deposition have combined to impact forest ecosystems in different regions (boreal, tropical, temperate).
- Quantify the magnitude and sign of feedbacks in climate system driven by deposition impacts on vegetation emissions of biogenic hydrocarbons (isoprene, monoterpenes).
- Estimate future changes in deposition from the models and its impacts on projections for the terrestrial carbon cycle.
- Examine how these processes interact in a fully coupled Earth system modelling framework.
- Determine optimal locations for future aforestation efforts to maximize carbon storage as part of possible geoengineering strategy.
For this project you should have a 1st or 2:1 degree in a relevant physical science discipline (chemistry, physics, maths, meteorology). For informal enquiries and further information on this project, please contact Dr. Steve Arnold (s.arnold(at)leeds.ac.uk) or Dr. Dominick Spracklen (d.spracklen(at)see.leeds.ac.uk).