Message From Climateprediction.net PI Re: April 2006 Problem

[myles]

Greetings,

As many of you will have gathered already, we found a problem with one of the input files to the BBC Climate Change Experiment, and after much heart-ache have decided the only option is to restart simulations from 1920 with a corrected file. IÂ’d like to apologise for that. IÂ’d also like to reassure you that your work so far has still been very useful, and will be used in future scientific papers. We are very grateful to you for all your help and hope you continue to take part in this important research.

Like most other people, I was just heading away for an Easter break when we finally worked out what the problem was, leaving poor Carl and Nick to handle this one on their own (and the family were none too impressed by my hugging the mobile anyway). So apologies for not having posted sooner, and thanks very much to all the moderators who have picked up on many of these points already.

1) Your crunching to date has absolutely not been wasted. This isn’t a problem with the model, just with one of the specified drivers of climate change, the “global dimming” effect. Separating out these drivers is exactly the kind of experiment we routinely perform in modelling centres, so although this particular experiment was inadvertent, we will still learn a lot from the runs you have already done by comparing them directly with the new runs you are just starting. In essence, what your models have done is show how much the world would have warmed up over the 20th century if it weren’t for the masking effect of global dimming.

2) Even with perfect 20/20 hindsight, we probably couldnÂ’t have picked up the problem more than a week earlier. It is obvious now that too many models were warming up too fast over the 20th century, but we needed a reasonable number to have got through the 1970s to be able to see this wasnÂ’t just due to chance. From the point at which it was clear that all the front-runners were crashing on the same date in 2013, I think the team tracked down the problem about as fast as was humanly possible, and we had to have a fix in place so that people would have valid runs to restart on.

3) The software was, of course, very thoroughly tested for the possibility of it interfering with other applications on your PC because our top priority is making sure it is absolutely safe for you to participate in this experiment. So while we all knew there was a risk of errors emerging on the scientific side, and there always will be (anyone who pretends otherwise has no idea what climate modelling entails), we are confident there was no risk to you, the participants. IÂ’m sure Carl would appreciate me pointing out that his software and BOINC handled a completely unexpected mode of failure of the model perfectly, without any nasty side-effects.

4) The problem, once identified, was very simple to fix, and our simulation of the 20th century has now been tested about as thoroughly as it can be, so I believe you can be confident that your new simulations will be doing what we think they are doing. Now that some of you have simulations that have reached 2000, we will use your uploaded files to start a few simulations from 2000-2080. These wonÂ’t provide valid predictions but they will mean that by the time you all reach 2000 the next time, there will be extra security against any problems like this in the second half of your simulation.

5) For those of you who are interested, the problem was a single entry in a file header, which meant that the model started reading from the wrong point in the file. Because the data and the dates in the file were OK, the problem was far from obvious. The computer scientists among you will no doubt have strong views about a file format that allow model dates and input-file dates to get out of synch in this way, and you will find that everyone who works with the Met Office climate model, including everyone at the Met Office, agrees with you. But you just have to remember that these climate models are some of the most complicated pieces of software in the world, having evolved over many years, and the Met Office model was designed to be the worldÂ’s best, not to be an easy piece of software to run on a PC.

Thank you very much for your support. IÂ’m sorry this will mean a couple of monthsÂ’ delay in announcing the result. Although this may seem like a long time to some of you, in the progress of science, believe me, it is the blink of an eye.


Yours sincerely,

Myles Allen
Principal Investigator, climateprediction.net / The BBC Climate Change Experiment

[carl]

for some further information, please see:

http://www.climateprediction.net/board/viewtopic.php?p=42756#42756