Midtvejskollokvium by Leif Skovbo – Niels Bohr Institute - University of Copenhagen

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Midtvejskollokvium by Leif Skovbo

Weighting of an ensemble of climate models

When people talk about climate change one question is always 'How warm will it be in the future?'.
To answer this question we use climate models. General circulation models simulating the atmosphere and ocean, AOGCMs. One model predicts an outcome. The next 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.
Should that last model be equally weighted as the other four?
IPCCs fourth assessment report was based upon a model ensemble of 22 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 three ways, calculating G and two scatterplots. For each way a weight is calculated and the average of all three ways found. The weights are then applied to the model data for SRESA1B and a new temperature change for year 2100 is found.

Supervisor: Eigil Kaas, NBI