Water isotopes in UKESM2

We will add water tracers (including stable water isotopes) to the UK Earth System Model (UKESM2) which will track through the model’s hydrological cycle. This work started under the EU Tipping Points in the Earth System (TiPES) project (https://www.tipes.dk/) and is now part of a BAS National Capability International research programme, Surface Fluxes in Antarctica (SURFEIT – https://www.bas.ac.uk/project/surfeit/).

There are many benefits for including water tracers/isotopes in an earth system model for both present day and palaeoclimate simulations. These include:

1. Assessment of relationships between isotopic concentrations and climate variables

Measurements of stable water isotopic content in ice cores can provide important long-term records of past climate changes. For example, temperature records over glacial-interglacial cycles have been inferred from the isotopic content of ice cores drilled in Antarctica. In order to infer climate records from isotopic ice core measurements, a relationship between the two quantities needs to be established. This is typically calculated using the spatial correlations of present day observations. Climate model simulations, that include water isotopes, allow these relationships to be better investigated and understood, thus, enhancing our ability to produce accurate climate records.

2. Palaeoclimate model evaluation

Including water isotopes in model simulations of past climates allows the model output to be compared to ice core measurements, which provides a method for model evaluation.

3. Enhanced understanding of a model’s hydrological cycle

Another value of adding water tracers and isotopes to a climate model is that they can be used to gain a better understanding of some of the complex hydrological processes within the model. For example, non-isotopic water tracers can be used to track water originating from a specified source region, allowing water pathways to be investigated in an atmosphere model.

A water molecule consists of two hydrogen atoms and one oxygen atom. ‘Water isotopes’ are water molecules that contain isotopes of hydrogen or oxygen. As the diagram on the left shows, the different combination of stable isotopes used to form the water molecule causes the molecular mass to vary from 18 to 22.  Note, stable isotopes do not decay over time.