Richard Tol's Gremlins Continue to Undermine His Work

Read time: 7 mins

This is a guest post by Brandon Shollenberger 

Global warming is, if you'll forgive the pun, possibly the most heated topic of debate this century.  While most debate over it focuses on extreme weather, sea level rise and climatic events, there is another part.  This is the part that focuses on the economics of global warming.  That is, how will global warming affect our wallets?

At the forefront of this debate is Richard Tol, professor of economics at the University of Sussex.  His work on the economics of global warming is relied upon by climate skeptics like Matt Ridley, who has used it to argue, “Climate change has done more good than harm so far and is likely to continue doing so for most of this century,” based on graphs like:

It is also used by the Intergovermental Panel on Climate Change (IPCC) which uses it in its latest report to argue “the incomplete estimates of global annual economic losses for additional temperature increases of ~2C are between 0.2 and 2.0% of income,” creating an interesting situation where both sides of the global warming debate argue from Tol's work.

Even more interesting, both sides of the debate have received criticism for their reliance on Tol's work.  After Ridley wrote an article for The Spectator with the previous quote, Bob Ward, from the Grantham Research Institute on Climate Change and the Environment, began examining a 2009 and 2012 paper by Tol which Ridley had relied upon.  Ward quickly found a number of errors in the 14 data points those papers used, including ones where Tol had plotted results which should have shown global warming as being harmful as instead being beneficial. 

Once the errors were corrected, the only results which indicated there might be any meaningful benefits from global warming were those published in a paper by Tol himself back in 2002.  At the time, Ward didn't notice the value listed for Tol's 2002 paper (a 2.3% gain in global wealth for 2.5C of warming) was only one of three estimates mentioned in the paper.  Two unreported values, a 0.2% gain and 2.7% loss in global wealth, stand in stark contrast to the one Tol chose to report in his 2009 paper.

Tol acknowledged some of the issues Ward raised while disputing some of the others.  It became a mess.  Even so, some progress was made as when Tol published a new paper in 2013, some of the previous errors were corrected.  New errors were also introduced, and things become further convoluted when the IPCC got involved.

The IPCC sends First and Second Order Drafts of all of the chapters of its reports out to anyone who signs up to be an external reviewer.  Any expert can review any part of the IPCC's work.  That is a fundamental aspect of the IPCC process which gives it credibility.  It's also an aspect that got ignored in this case.

Richard Tol was a Coordinating Lead Author for one of the chapters of the IPCC report.  His chapter had an entire section added after the the last round of external reviews was completed.  Another section received major revisions at the same time.  None of these changes were seen by any of the experts who signed up to review the IPCC report.

Even worse, the material which was added included two tables and two figures derived entirely from Tol's own work.  One table and figure directly cites his 2013 paper, reproducing errors from it.  The text of the section referred to it, saying “the initial benefits of a modest increase in temperature are probably positive, followed by losses as temperatures increase further.”

Ward discovered this, and he contacted the IPCC to complain.  The IPCC made some changes to the chapter.  It modified that quote slightly, and it changed some of the data.  Changed, not corrected.  Some of the changes introduced new errors.  Even more confusing, the IPCC continues to cite “Tol (2013)” despite the data it uses no longer matching the data used in “Tol (2013).”

On the topic of changing data, some of the journals which published Tol's work eventually forced him to publish corrections due to Ward's complaints.  The 2013 paper had a brief correction posted noting, “The author regrets that three errors crept into 'Targets for global climate policy: An overview'. In Table 1 of that paper, the author dropped the minus sign on Chris Hope׳s 2006 estimate.”

The corrections to Tol's 2009 paper were more substantial.  The notice for it began, “Gremlins intervened in the preparation of my paper “The Economic Effects of Climate Change” published in the Spring 2009 issue of this journal. In Table 1 of that paper, titled “Estimates of the Welfare Impact of Climate Change,” minus signs were dropped from the two impact estimates.”  Apparently gremlins caused Tol to flip two negative numbers into positive numbers.  There were a number of other corrections, but the most important part of the update was:

“[U]nlike the original curve (Tol 2009, Figure 1) in which there were net benefits of climate change associated with warming below about 2°C, in the corrected and updated curve (Figure 2), impacts are always negative, at least in expectation.”

Tol's new work clearly undermines the conclusions Ridley had relied upon.  But even after being forced to make such a major change to his 2009 paper, Tol was once again forced to make more changes when it was discovered there were yet more errors in his work.  This later update said:

“This correction offers a final revision and update to the figure in question. This figure is republished from the most recent report of the International Panel on Climate Change (IPCC), in Chapter 10 of the volume Climate Change 2014: Impacts, Adaptation, and Vulnerability. Richard Tol is one of the two Coordinating Lead Authors of this chapter.”

But whereas the previous versions of Tol's work included lines (curves) to try to estimate whether global warming, as it ranged from moderate to severe, would be beneficial or harmful, this new version seems to have given up:

Which may be wise given the mess of the situation.  The 2009 paper now cites the IPCC report which cites the 2013 paper which builds upon the work of the 2009 paper, and results being used in the IPCC report are being attributed to the 2013 paper despite them not being present in it.  It's all quite confusing.

To make things more confusing, a recent discussion of the situation at Retraction Watch, a web site which posts articles about scientific papers which get retracted, included a statement from Richard Tol on his view of how things stand:

    “I am grateful to the editors for their constructive attitude in a painful episode.

    Readers may also want to look at the latest assessment here: https://ideas.repec.org/p/sus/susewp/7515.html

The link he provides is to a working paper which includes a data set with many similarities to the one used in all the work discussed thus far.  There are a number of small differences, but there is one big difference.  Every previous work has included the paper Nordhaus and Yang (1996) as estimating a 1.7% loss of global wealth for a 2.5C rise in temperatures.  Tol's new working paper lists Nordhaus and Yang (1996) as estimating a 1.4% gain in global wealth for a 2.5C rise in temperatures. 

Tol kindly provides the data and calculations he uses along with his paper, so it is easy to check what happened.  Nordhaus and Yang provided the data they used.  For every paper before this one, Tol used that data.  For this one, he used it again.  Then he ignored the results he got, and he redid them with a new set of data.  You can see it right in the file he provides.  The two sets of calculations are right next to one another.

But that doesn't matter.  Because his results were still in damages.  He just forgot to include the negative sign.  The gremlins apparently came back and caused him to turn a negative number into a positive number again.

by Brandon Shollenberger


Image credit: Gremlin by Albert Ziganshin via Shutterstock.

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