final_program_abstracts[1]

11 IMSC Session Program
Bayesian estimation of the climate sensitivity based on a
simple climate model fitted to global temperature
observations
Friday - Plenary Session 6
Peter Guttorp, Magne Aldrin and Marit Holden
Norwegian Computing Center, Norway
Our aim is to estimate the climate sensitivity by modeling the relationship between
(estimates of) radiative forcing and global temperature observations in post-industrial
time. Complex general circulation models are computationally expensive for this
purpose, and we use instead a simple climate model of reduced complexity. This
climate model is deterministic, and we combine it with a stochastic model to do
proper inference.
Our combined model can be written as
yt = mt(xt-,θ) + f(zt) + nt
Here, yt is the observed temperature in year t and mt the corresponding output from
the simple climate model. Furthermore, the model input xt- is the unknown radiative
forcing in year t and previous years back to pre-industrial time. θ is a vector with
time-constant model parameters, whereof the climate sensitivity is the parameter of
main interest. The function f(zt) is an empirical motivated term added to the model to
account for the El Niño effect and zt is an El Niño indicator. Finally, nt is an
autoregressive error term representing model and measurements errors.
The output from the climate model mt is three-dimensional, consisting of the air
temperature on the northern and southern hemispheres and the global ocean heat
content. The temperature observations yt consist of three different time series of
global temperature observations from around 1850, and one series for the ocean heat
content from 1955. The three temperature time series are all based on observations at
various measurement stations around the earth, but differs by how these are weighted.
The “true” radiative forcing is unknown, but we use the most recent (temperatureindependent)
estimates with specified uncertainties as prior distribution. We also have
informative prior distributions for all parameters in θ except for the climate
sensitivity, for which we use a flat prior.
This work is done in cooperation with Center for International Climate and
Environmental Research in Oslo (CICERO).
Abstracts 295