Friday, October 19, 2007

The Stern Review on the Economics of Climate Change: Blistering Peer Reviews

In the latest issue of the Journal of Economic Literature, two papers deliver devastating reviews on the Stern Review on the Economics of Climate Change. The reviews are by serious, mainstream economists: William Nordhaus of Yale and Martin Weitzman of Harvard. These are not individuals and articles that can or should be ignored. Of course, they will be ignored by the mainstream media – while at the same time Al Gore’s receipt of the Nobel Prize carries the media day.

The Journal of Economic Literature is the sister publication to the American Economic Review and is put out by the American Economic Association, the leading professional society for economists. Abstracts of the papers, and instructions on how to buy them, are available here.

I have always said that my objections to the prescriptions of the most vocal climate change advocates are on three levels: one, the climate models depend too much on positive feedbacks that are not understood; two, the models have not really been tested, but instead are calibrated to the historical data; and three, even if one accepts the models, one then has to move into the economics of optimal policy, and there the best analysis suggests relatively modest reductions in carbon emissions for the near term. I like to ask environmentalists to summarize their prescriptions with the appropriate tax per barrel of oil: tell me what you think the price of oil should be increased by, in order to recognize the impact of carbon.

But back to the reviews of the Stern Review. So the Stern Review made big headlines when it came out, as it was commissioned by the UK government and was ostensibly a serious analysis of the economics of climate change. Both Nordhaus and Weitzman deliver fatal blows, although they try to temper it a bit. Here is a quote from Nordhaus:

“The central methodology by which science, including economics, operates is peer review and reproducibility. By contrast, the (Stern) Review was published without an appraisal of methods and assumptions by independent outside experts. Nor can its results be easily reproduced…(this) does mean that fatal flaws in evidence and reasoning, which might have been caught in the early stages under normal ground rules, may emerge after the report has been published.”

Weitzman says,

“However, in my opinion, Stern deserves a measure of discredit for giving readers an authoritative-looking impression that seemingly objective best-available-practice professional economic analysis robustly supports its conclusions, instead of more openly disclosing the full extent to which the Review’s radical policy recommendations depend upon controversial extreme assumptions and unconventional discount rates that most mainstream economists would consider much too low.”

Now in the spirit of full disclosure, I recommend everyone look at the papers for themselves. Nordhaus and Weitzman go out of their way to point out the positive aspects of the Stern Review. But the overwhelming conclusion, especially from Nordhaus, is that the extreme policy prescriptions of the Stern Review are way overblown.

To summarize the contrast: The Stern Review calls for a carbon tax of $350 per ton of carbon in 2015. Nordhaus’ model, which has been peer-reviewed many times, calculates the optimal carbon tax in 2015 to be ONE-TENTH of that, or only $35 per ton carbon. I find it useful to put these quantities in terms of something we understand more readily: $350 per ton carbon converts to $1 per gallon of gasoline, while $35 per ton carbon converts to 10 cents per gallon of gasoline. We are talking big differences here.

So what is wrong with the Stern Review’s economics? It is real simple – they use an extremely low interest rate, close to zero. Everything follows from this, and in my opinion, the assumption is crazy.

The essence of carbon policy is that we incur costs today for benefits many years into the future. Changing our energy usage patterns will be costly for us today, and the benefits of lower temperatures come 50, 100 or even 200 years in the future. Any time you are considering investing today for benefits in the future, you have to consider the interest rate. Investments to stabilize climate should yield returns – a rate of interest – in the ballpark that other investments yield. If we can invest in human capital, for instance, and earn 10% per year, why should we invest in climate stabilization if it yields a zero rate of return?

Nordhaus has some great examples to illustrate the unreasonableness of using a zero discount rate for climate policy: “Suppose that scientists discover a wrinkle in the climate system that will cause damages equal to .1 percent of net consumption starting in (year) 2200 and continuing at that rate forever after. How large a one-time investment would be justified today to remove the wrinkle that starts only after two centuries? Using the methodology of the (Stern) Review, the answer is that we should pay up to 56 percent of one year’s world consumption today…In other words, it is worth a one-time consumption hit of approximately $30,000 billion today to fix a tiny problem that begins in 2200…the Review would justify reducing per capital consumption for one year today from $10,000 to $4,400 to prevent a reduction of consumption from $130,000 to $129,870 starting two centuries hence and continuing at that rate forever after.”

Another point in this criticism is the essential inter-generational fairness issue. Per capita income worldwide has been growing at around 1.3% over many decades – and this is the number that the Stern Review uses. At that growth rate, per capital world consumption will grow from today’s $10, 000 to about $130,000 in two centuries. Which generation is the relatively poor generation? Are we so sure that we are impoverishing our children and our children’s children? What about all the new technologies, institutions such as democracy and market economies, physical infrastructure, and knowledge that we are bequeathing them? Do we not think that people 200 years from now will enjoy more leisure, better health, better technology, and generally be better able to pursue life, liberty, and happiness?

Read the articles, they are really convincing. For most of the media, of course, that will be too difficult. Much easier to report on Al Gore winning an Academy Award – oops, I meant a Nobel Prize.


Aviad said...

A fascinating call for balanced approach considering the climate change is provided by Bjorn Lomborg, author of "The Skeptical Environmentalist"

Anonymous said...

You might want to read Brad DeLong's comments:

- Barry

David said...

I totally agree with you.

I just wish the media would place more importance on scientific views as yourself as opposed to political views. As a country we start getting into problems when politicians start trying to do the science.

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