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The following response to Frank Ackerman’s recent post is authored by Dr. Richard Tol.

Over the last 20 years, I have developed (and later co-developed) an integrated assessment model of climate change called FUND.  Model code and documentation used to be available to anyone on request. It can now be freely downloaded. Reproducibility and transparency are cornerstones of scientific inquiry.

Some modelers prefer to keep their code private. There are a number of reasons for this. One reason is the potential for abuse. Someone may borrow your model, do something inappropriate or silly with it, and use the results to embarrass you.

I have borrowed other people’s models. I have found bugs in their codes, or what seemed to be bugs. I always discuss this with the modeler in question. If there really was an error – more often it is a misunderstanding on the part of the outsider – I left it to the modeler to correct this and whatever results that were affected. Colleagues have treated me, my code and my data in the same way. I have had to publish two errata.

Frank Ackerman borrowed our model. He discovered that in one equation there may be a division by zero. He contacted my co-developer, David  Anthoff, on 16 December 2010. In the ensuing email exchange, David introduced Mr Ackerman to our standard diagnostic procedures and explained to him that our results are not affected. David added bespoke diagnostic tests on Mr Ackerman’s request and again found immaterial effects.

Much to our surprise, Mr Ackerman claimed in a blog post on 15 March 2011 that there is an error in FUND. He recently repeated this claim (and again). In an email dated 17 March 2011, Mr Ackerman admits that he used different methods than we do to work around the division by zero; and that if he uses these methods, there is a substantial error in the estimates of the social cost of carbon.

In a comment to yet another one of his postings, Mr Ackerman admits that he “made a one-line correction to the code”. A single change to its code can affect the integrity of a model. Mr Ackerman is wrong to present the changed code as work by David Anthoff and me. In his email of 17 March 2011 mentions more extensive changes to the code. Unfortunately, I cannot release that email without violating Mr Ackerman’s legal right to privacy. I have asked Mr Ackerman for permission to make his email public.

In sum, the computational error identified by Mr Ackerman is his own. He falsely puts the blame on us.

I am reminded of Phil Jones’ “Why should I give information to you when all you want to do is find something wrong with it?” although in this case it is more like “Why should I give my code to you when all you want to do is introduce a bug and blame it on us?” Nonetheless, FUND will remain in the public domain.

Dr. Richard S.J. Tol, Research Professor, Economic and Social Research Institute, Dublin Professor of the Economics of Climate Change, Vrije Universiteit Amsterdam Adjunct Professor, Department of Economics, Trinity College, Dublin.