CESM Paleoclimate Working Group Development Priorities
April 2014
A. Coupling across components and understanding interactions

Coupling of isotopic water and carbon tracers across components will allow better
diagnostics on how the CESM water and carbon cycles perform.
Isotopic water tracers can be used to identify and track moisture sources of air masses,
examine the stratospheric-tropospheric exchange of water vapor, and provide additional
diagnostics of the global hydrological cycle and cloud processes. Carbon isotopes (14C, 13C)
are used as diagnostic tracers in ocean and land carbon-cycle modeling. Measurements of
naturally-produced radiocarbon (14C) in deep-sea sediment cores have been used to infer
deep-ocean ventilation rates for past and present climates. Adding radiocarbon into CESM
will help validate ocean-model circulation and deep-water mass formation processes in the
present-day ocean, and will provide a solid test for ocean ventilation rates in the paleo-ocean.
Carbon isotopes, measured in atmospheric CO2 and oceanic dissolved inorganic carbon, have
been used to constrain carbon exchanges between the atmosphere, ocean and terrestrial
biospheres. The oceanic distribution of 13C is complex, depending on the effects of biology,
temperature, gas exchange, and circulation, and thus provides a severe test for model
performance.
B. New parameterizations/processes

Adding isotope tracers to components of the CESM.
Specifically, adding isotopic water and carbon tracers to the ocean component (pop2), the
atmosphere component (cam5), the land component (clm), and isotopic water tracers to the
sea ice component (cice) and coupling them. This will be useful for diagnosing biases in the
model, as it opens up a new datasets to compare with. It also allows for more direct
comparisons to paleoclimate records, which are often in the form of isotopic measurements,
which are then interpreted as a climate change signal (warming, cooling, wetter, drier, etc.).
The comparison provides validation for the interpretation and use of the proxy. This activity
ties directly to the paleoclimate developments outlined in the CESM science plan
(paleotracers).
C. High-resolution and new dynamical cores

Exploring the effects of varying horizontal resolution in CAM5 on the simulations of
the isotopic water cycle.
D. Addressing biases/shortcomings

Comparing isotopic water fluxes to observations provides additional information that
can help elucidate the source of model biases.
E. Software development

The promulgation of passive isotopic water and carbon tracers throughout all active
and data component models of the CESM requires software development within all
model components.
Code development is also taking place in the atmospheric data model component as well in
order to drive the ocean, land, and sea ice components with observed data sets before
coupling for testing and spin-up purposes. Depending on the model component, software
development can be extensive or pervasive. This is especially true in CAM because isotopic
water differentiates from bulk water due to fractionation at every phase change, and is
intimately coupled to cloud and convective processes. In the POP model, restructuring of the
Ecosystem module and the development of an ecosystem driver was necessary to allow the
addition of biotically coupled carbon tracers. Additional passive trace element tracer modules
(geotracers) are also planned for development over these two years within the ocean
component (Pa, Th, Nd). Additionally, isotopic water tracers will be incorporated within the
land and runoff models to complete the isotopic water cycle.

Toolkit for setting up Quaternary and Deep-time coupled simulations with CESM1.2.
Develop an online Paleo User Resources site to provide the community with background
on the CESM code and dataset modifications that may be necessary to address scientific
questions about past Earth climates with the CESM model, including useful tools that
are designed to modify the forcing and input files for paleoclimate applications.