Natural and human effects on clouds and climate over the ocean

Aim of the PhD project

This PhD project aims to improve our understanding of how emissions of natural and pollutant aerosol particles over marine regions affects climate. The project brings together very advanced climate and weather-scale models (lead supervisor at the University of Leeds) with a wealth of new and upcoming chemical measurements (co-supervisor at the University of York).

Background to the science

The project explores two important ways in which aerosol particles affect climate. Firstly, natural trace gases produced by biotic processes in the ocean are known to enter the atmosphere and create gas phase sulfur species and aerosol particles that modulate the natural properties of clouds, such as cloud reflectivity. This process has been explored for many years, but some recent fundamental changes in our understanding of the chemical reactions and aerosol formation processes means that the influence of natural emissions on clouds and climate needs to be reassessed. Secondly, emission of gas phase chemical species and aerosols from ships is known to have a strong effect on climate by forming visible “ship tracks” in clouds and by adding to the amount of aerosol over wider areas of ocean regions. In 2020 international regulations reduced the sulfur emissions from shipping by 80%, which led to a detectable effect on ship tracks. However, our understanding of the effect of this emission reduction on aerosols and clouds over wider ocean regions needs to be quantified.

What the research involves

The project will use state of the art climate and weather models together with existing and new gas phase chemical measurements from the University of York, as well as many aerosol and cloud measurements. The co-supervisor at York has developed a new instrument that can measure sulfur dioxide at considerably lower concentrations than previously possible. This makes it possible to investigate the natural sulfur cycle in marine regions and how it affects aerosols. It also enables the effect of shipping sulfur reductions to be measured in shipping areas that are now much cleaner than they were before 2020. The project will involve analysing measurements from two shipping-related aircraft campaigns over the Atlantic and English Channel, as well as upcoming measurements of natural sulfur species and aerosol over the Atlantic.

The project will involve further developing and running advanced weather and climate-scale models. The global model is the Met Office Earth System (climate) model, which can also be run at very high spatial resolution (so-called zooming) over specific regions so that shipping corridors can be resolved and compared to the measurements. Model simulations will be evaluated against existing and new measurements, as well as satellite data, to quantify the role of natural and shipping emissions in regulating the climate state of the North Atlantic and other regions.

This PhD presents an excellent opportunity for a student who wants to develop capability in advanced climate modelling with a strong connection to measurement scientists. Although much of the required measurement data already exists, we anticipate providing opportunities to get involved in related field campaigns and to develop a good understanding of advanced atmospheric measurement techniques.