Quantifying Gross Primary Production of the Earth’s biosphere using isotopes of carbon dioxide in the atmosphere
Anthropogenic carbon dioxide (CO2), released to the atmosphere as a result of fossil fuel burning, cement manufacture and land use change, is the main cause of global warming. Only roughly half of anthropogenic carbon emissions remain in the atmosphere, while the rest is taken up either by the oceans or by vegetation on land. Whilst carbon uptake by the oceans is relatively well understood, this is not the case for land carbon uptake. The magnitude of this carbon sink is therefore not well constrained and its sensitivity to climate change is poorly understood. The entry point of carbon into land vegetation is via plant photosynthesis. This carbon flux is called gross primary production (GPP, e.g. in units of (gC m-2 day-1)). In contrast to the net carbon gain of plants, GPP is difficult to measure and current estimates of global GPP differ by up to 50%. Understanding of how GPP changes in a warmer, high-CO2 world is important to predict the future role of land vegetation for the global carbon cycle and therefore for the climate itself.
Recently a new approach to measure GPP and its variation with climate has been proposed and pursued. The method is based on multiple isotopes of carbon dioxide: C17O16O and C18O16O. The technique takes advantage of a phenomenon called ‘mass-independent fractionation’. During chemical reactions, as well as processes like diffusion and evaporation, the speed of conversion differs between different CO2 isotopes, causing changes in the isotopic ratios. This is known as fractionation and for almost all processes, fractionation of the heavier isotope is twice as large as for the lighter isotope.
This means that the variable D, equal to 17R – 0.5*18R, where 17R is the isotope ratio C17O16O/CO2 (and similarly for 18R), does not change during fractionation processes. An exception to this rule is due to chemical reactions in the stratosphere involving ozone (O3), which result in a positive stratospheric D anomaly. Meanwhile, as CO2 is taken up by plants via photosynthetic exchange, the D anomaly resets to near zero. The value of D in the troposphere is therefore balanced between the high stratospheric D and zero, which can be used to estimate the magnitude of GPP (e.g. Koren et al. 2019).
The purpose of this project is to develop the capability to simulate carbon isotopes for estimating GPP and apply it to existing and upcoming data to improve estimates of GPP and its sensitivity to climate. We will use the UK Earth System Model (UKESM) and the land surface model contained within it (the Joint UK Land Environment Simulator, JULES) to simulate the distribution of the isotopes of CO2 in the atmosphere. These simulations will be compared to direct measurements of D that have been made around the globe and the model will be used to investigate the relationship between the distribution of CO2 isotopes in the atmosphere, plant productivity and climate variations. Furthermore, the earth system model can then be used to improve our predictions for the future impact of climate change on GPP and the carbon cycle. Use of a full earth system model to assess GPP through biospheric D exchange is a novel and robust approach.
The key research questions are:
- How can measurements and simulations of CO2 isotopes be used to better constrain the magnitude of global and regional GPP and therefore the biospheric carbon cycle?
- What is the atmospheric distribution of these isotopes, and how and why does its mass flux at the surface vary due to variations to GPP?
- What does this tell us about how the future impacts of climate change will alter the global carbon cycle?
The PhD student will use state-of-the-art global earth system and land surface models, together with the latest measurements of carbon isotopes and other related species to answer the above research questions.
Potential for high-impact outcome
Effectively quantifying the biospheric contribution to the global carbon cycle is a vital component of cutting-edge climate science, and the methods developed for this project have not previously been applied in regard to the problem. This novel approach, combined with the use of the state-of-the-art models and latest measurements, will mean that there are significant opportunities to produce important and high-impact scientific output during the PhD.
Training and Research Support
The successful applicant will join the Atmospheric Composition Group, part of the Institute for Climate and Atmospheric Science (ICAS). They will also be strongly links with the the Ecology and Global Change research group of the School of Geography. These are large and vibrant research groups with long records of producing world-leading research and members meet regularly, and will be available to discuss results and to provide support to all aspects of the research project. The student will have access to training courses to provide both specific and transferable skills to be applied as necessary. There will be many opportunities to present research at national and international conferences, and students are encouraged to do so regularly.
Applicants should have a good first degree (1 or high 2:1), Masters degree or equivalent in a physical or mathematical discipline such as Physics, Mathematics, Meteorology, Climate Science, Environmental/Geophysical Sciences, Engineering or Computer Sciences. The applicant should also have a keen interest in biosphere/atmosphere modelling, although previous experience is not required as our training will equip the student with the necessary skills.
Koren, G., Schneider, L., van der Velde, I. R., van Schaik, E., Gromov, S. S., Adnew, G. A., et al. (2019). Global 3‐D simulations of the triple oxygen isotope signature Δ17O in atmospheric CO2. Journal of Geophysical Research: Atmospheres, 124, 8808– 8836. https://doi.org/10.1029/2019JD030387