Adding water tracers to the UK Earth System Model

What this project does

The project adds water tracers, including stable isotopes, to the UK Earth system model (UKESM2). These tracers follow water through the model’s hydrological cycle.

Why this matters

Stable water isotopes provide long-term records of climate change. Ice core measurements can reveal temperature changes over glacial-interglacial cycles.

Including isotopes in climate models helps establish relationships between isotopic concentrations and climate variables. It also allows evaluation of past climate simulations and improves understanding of the hydrological cycle.

How the project works

The model development project adds code to UKESM2 which will allow users to run with water tracers and isotopes.  This will allow studies into climate and hydrology:

• Relationship assessment: comparing isotopic concentrations with climate variables to improve interpretation of ice core data.
• Palaeoclimate evaluation: comparing model output with ice core measurements to assess model accuracy.
• Hydrological cycle analysis: tracking water pathways, including sources and movement, to better understand model processes.

Stable isotope properties: water molecules contain hydrogen and oxygen. Isotopic variations alter molecular mass from 18 to 22, providing a record that does not decay over time.

Credit: Lesleigh Anderson and Jeremy Havens, USGS

Science objectives

The project aims to:

• Track water and water isotopes through the atmosphere, land, ocean and sea ice model components

• Investigate precipitation sources and how they might change

• Evaluate palaeoclimate simulations against ice core data

Who is involved

The project is led by BAS as part of its National Capability programme (SURFEIT and PRESCIENT), with collaboration with the Met Office plus the SWAIS-2C, ECO-ICE and EU P2F projects.  The work started under the EU-TIPES project.

• The first stage of this work is to add a water tracer to the atmosphere and land surface component models. The tracer will track water through the hydrological cycle of the models.
• The development will be made so that the model can run with a number of different water tracers.
• Isotopic fractionation processes will then be added to allow a water tracer to become a stable water isotope.
• Water tracers/isotopes will also be added to the ocean and sea ice model components.

Update on progress:

• The water tracer code for the atmosphere and land surface components is now complete and being used to investigate sources of precipitation in the atmosphere model (McLaren et al., 2025).
• Isotopic fractionation processes have been added to the atmosphere component and the simulated water isotopes are currently being tested and evaluated.
• Work has started to add water tracers to the sea ice component under the ECO-ICE project.

External collaborators:

Jeff Ridley – Met Office

Publications:

McLaren, A. J., Sime, L. C., Wilson, S., Ridley, J., Gao, Q., Gorguner, M., Line, G., Werner, M., and Valdes, P.: Implementation of water tracers in the Met Office Unified Model, Geosci. Model Dev., 18, 8129–8142, https://doi.org/10.5194/gmd-18-8129-2025, 2025.

Preprints:

Ridley, J.K., McLaren, A.J., Sime, L.C. and Blockley, E.W.: The source of Arctic precipitation in HadGEM3-GC5. Authorea. March 03, 2026,  https://doi.org/10.22541/au.177255541.19417850/v1.

Datasets:

McLaren, A.; Sime, L.; Ridley, J. (2025): Water tracer precipitation output from the Met Office Unified Model (GAL9.0) for 1985-2014. NERC EDS Centre for Environmental Data Analysis, 10 June 2025. doi:10.5285/10ae416c4ccb4a90bdb5da0bbf68d4f9. https://dx.doi.org/10.5285/10ae416c4ccb4a90bdb5da0bbf68d4f9