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Phil. Trans. R. Soc. A
Vol. 365, No. xx, 2007; Pages:
2103–2116

Probabilistic climate change predictions applying Bayesian model averaging

BY SEUNG-KI MIN*, DANIEL SIMONIS AND ANDREAS HENSE

Climate Research Division, Environment Canada, 4905 Dufferin Street, Downsview, Ontario, Canada M3H 5T4.


Abstract

This study explores the sensitivity of probabilistic predictions of the twenty-first century surface air temperature (SAT) changes to different multi-model averaging methods using available simulations from the Intergovernmental Panel on Climate Change fourth assessment report. A way of observationally constrained prediction is provided by training multi-model simulations for the second half of the twentieth century with respect to long-term components. The Bayesian model averaging (BMA) produces weighted probability density functions (PDFs) and we compare two methods of estimating weighting factors: Bayes factor and expectation–maximization algorithm. It is shown that Bayesian-weighted PDFs for the global mean SAT changes are characterized by multi-modal structures from the middle of the twenty-first century onward, which are not clearly seen in arithmetic ensemble mean (AEM). This occurs because BMA tends to select a few high-skilled models and down-weight the others. Additionally, Bayesian
results exhibit larger means and broader PDFs in the global mean predictions than the unweighted AEM. Multi-modality is more pronounced in the continental analysis using 30-year mean (2070–2099) SATs while there is only a little effect of Bayesian weighting
on the 5–95% range. These results indicate that this approach to observationally constrained probabilistic predictions can be highly sensitive to the method of training, particularly for the later half of the twenty-first century, and that a more comprehensive approach combining different regions and/or variables is required.

Keywords: global climate change; Bayesian model averaging; probabilistic prediction; surface air temperature.

Corresponding author:

E-mail: seung-ki.min@ec.gc.ca

 
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