Master thesis by Leif Skovbo – Niels Bohr Institute - University of Copenhagen

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Master thesis by Leif Skovbo

Weighting of an ensemble of climate models

Abstract:

When people talk about climate change, one question is always: 'How warm will it be
in the future?'.
To answer this question some of the most important tools are the general circulation models simulating the atmosphere and the ocean, AOGCMs. One model predicts one outcome, and the next model predicts a different outcome. Which is more correct? The average of those two will probably be better than just the one of them. In a five model ensemble four models are almost the same but one is not, so should that last model be equally weighted as the other four? IPCCs fourth assessment report was based upon a model ensemble of 23 models with equal weight leaving the future with a temperature rise in year 2100 at 2.8C (1.7 - 4.4C) in scenario SRESA1B. In this master thesis I look at a 24 model ensemble's ability to simulate the greenhouse effect, and relating it to observations from NOAAs outgoing longwave radiation (OLR) and ECMWFs ERA40 surface air temperature.
This is done in five methods, one calculating the greenhouse effect, G, and four different scatterplots, and for each method a weight is calculated for all models and averages are found for methods of global mean and zonal mean greenhouse effect. The weights are then applied to modify the model data for SRESA1B and a new temperature change for year 2100 is found.

Supervisor: Eigil Kaas, Niels Bohr Institute