Answering Climate Modelling Questions

Two climate science blogs Real Climate and Climate Science, run by different groups of well known climate scientists, recently answered a set of questions about climate modelling.

The answers are interesting, in terms of learning more about climate modelling and seeing how different climate models are from ecological models.

Below is an example of their answers to a question:

5. How do errors in estimating the forcing functions, or in simulating the internal variability impact the results?

From Real Climate:

A. Good question. Uncertainties in the forcing functions can be tested and that leads directly to uncertainties in simulations of past climate. Sometimes the uncertainties can be constrained (but not eliminated) by comparison of the modelled climate change to the observations but often many different scenarios could be consistent with the observations given known uncertainties in (for instance) the model’s climate sensitivity. Errors in the simulation of internal variability have more subtle impacts on the results. Obsviously, if a certain mode of variability is not very well simulated, changes in that mode through time are not likely to be of much use. Sometimes results are robust over a wide range of simulated variability and in such cases the phenomena can be considered robust (see Santer et al, 2005 for an example in the tropical atmosphere). Thus the answer will depend on the circumstances, and it will affect some parts of the model more than others.

From Climate Science:

I agree with Gavin that this is a good question. However, until we include all of the first-order climate forcings and feedbacks, as well as successfully model sudden climate transitions, we have large remaining errors of an unknown magnitude. We also have to show prediction skill in the quasi-linear global and regional long term trends of important climate metrics (regional precipitation, regional layer-averaged tropospheric temperatures, etc). Whether or not we agree the models have shown skill in reproducing global temperature averages or not, they certainly have not demonstrated regional skill for the spectrum of important climate metrics.

These first-order climate forcings are identified in the National Research Council report Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties, while the first-order climate feedbacks are identified in Understanding Climate Change Feedbacks. Sudden climate transitions are discussed in the National Research Council report Abrupt Climate Change: Inevitable Surprises.

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