Alastair McKinstry: Exoclimes: the diversity of planetary scientists
I'm just back from ExoClimes 2010: Exploring the Diversity of Planetary Atmospheres.
An excellent conference: the PDFs of the talks and posters are now online, and they are putting the videos of the
talks up soon. But in particular the organizers deserves thanks for bringing exoplanetary scientists and observers
together with climate modelers doing Earth (and Mars, Titan, Venus, ...) models.
Peter Cox on model complexity
The last talk on Friday was by Peter Cox on
Climate change and exoplanet sciences that was far better than expected for the "graveyard shift". One theme of the conference was the need for a 'heirarchy' of models, from simple energy-balance models to full circulation (GCM) models: using progressively more complex models to understand more bits of whats going on. Exoplanet workers mostly use simpler models, progressing now to GCMs, while Earth modellers are moving beyond GCMs to "Earth system" models including biology, etc. Peter pointed out the two styles of work: the exoplanet modelers are short of data, and risk being too speculative. We know little of what the planets are like, and concentrate on implementing physics in the models to see what they might be like. Earth modelers on the other hand are if anything swamped with data: the tendency here is to make the model fit the data, by adjusting parameters until it does so. The danger of this approach is that the model will then not work away from current present-Earth conditions.
Tim Lenton pointed out some work that was done with the Met Office model, where they took the radiative transfer part of the model and tested it for
other planets, and paleo-Earth conditions. The model blew up : it wasn't capable of x2 or x4 current CO2 levels. (This has since been corrected).
Over dinner there were interesting discussions on the different styles within the communities.
While the underlying GCMs used come from the Earth sciences,
its quite common within the exoplanetary community for a researcher to work on all parts of the model: dynamics
one day, radiative transfer the next. In Earth climate work people have become more specialized and someone
is a 'radiative transfer' person, and won't touch other parts of the code
(even if they can follow them in the huge codes we have today!).
On the other hand, there is a greater
tradition of model inter-comparison in Earth sciences, where we compare the model outputs to each other
for some known test cases (
Held & Suarez, the
CMIP5 project, etc.) Apart from some initial work by
Emily Rauscher, little has been done on this in exoplanetary models; it was agreed more of this would
be a good idea.
Radiative transfer (the interaction of 'sunlight' with the atmosphere, where it gets absorbed, scattered and re-radiated)
in particular seems to be an area that could benefit from this.
In this middle ground Francois Forget showed the work on the LMDZ model and
applying GCMs to terrestrial planets.
They've successfully applied this model to Mars, Titan, and partially to Venus (a much tougher problem, due to its heavy clouds
giving a long radiative timescale). There are problems with correctly explaining super-rotation though.
This is where the atmosphere rotates faster than the planet: on Venus for example the planet rotates every 243 days,
while the clouds rotate around the planet every 4 days.
Sebastian Lebonnois described
the possible mechanisms for Venus and Titan; Johnathan Mitchell so did some interesting work on this recently.
Different regimes are involved for different rotation rates of the planet.
Ralph Lorenz pointed out the lack of "real paleo-Earth" climate work at the moment. While geology has inspired
a lot of work on the
atmospheric composition,
what with the different gas mixtures (meaning earth-model radiative transfer codes don't work) and
the faster dynamics meaning super-rotation could apply (Earth's day was about 8 hours long in the Archean era), we don't have a model of the climate yet.
It looks like we should treat Earth as an exoplanet.