A Gift that Keeps on Giving: The Contributions of Martin Weitzman to Environmental Economics

The world lost a remarkable scholar, a great economist, and a gentle soul on August 27th, when Martin Weitzman sadly passed away.

A week later, I was asked by the editors of the VOX CEPR Policy Portal (of the Centre for Economic Policy Research) – “research-based policy analysis and commentary from leading economists” ­– to write a brief intellectual biography and personal remembrance of Marty Weitzman, my colleague, friend, and long-time co-host of the Harvard Seminar on Environmental Economics and Policy.  In the essay I wrote, I sought to describe how Marty’s contributions have advanced the thinking of environmental and other economists, as well as the thoughts and actions of policymakers on many fundamental issues, including policy instrument choice, discounting, species diversity, and environmental catastrophes.  Today, I’m offering readers of this blog a slightly edited version of my Vox essay.

Photo Credit: Gernot Wagner

Martin Weitzman was a treasure – a gift that kept on giving to the research and policy worlds – for Harvard, for economists around the world, and for the global intellectual community. His work as an economic theorist who addressed a broad set of problems, and as an environmental economist who during the past decade focused on climate change, was unparalleled, and formed the basis for theoretical and empirical work carried out by legions of economists and other scholars around the world. His contributions to environmental economics in particular were unprecedented, and helped to shape the field for nearly five decades.

If economic theory is about stripping a problem down to its absolute essentials, and deriving meaningful insights from those essentials, then Weitzman was a master. Over and over again, Marty Weitzman demonstrated how careful and rigorous analysis of artfully constructed theoretical models can provide valuable and often surprising insights into difficult economic problems with real implications for the design of public policies.

Marty’s contributions have advanced the thinking of environmental economists and policymakers on policy instrument choice, discounting, species diversity, environmental catastrophes, and other fundamental issues. Across the board, the example of his rigorous and often ingenious work set high standards for theorizing in environmental economics and thereby served to elevate the entire field.

Brief intellectual biography

Born in New York City in 1942, Marty received his BA degree in mathematics and physics from Swarthmore College in 1963, and one year later, received an MS degree in statistics and operations research from Stanford University. In 1967, just three years after his Stanford degree, he was awarded a PhD degree in economics from the Massachusetts Institute of Technology (MIT), where his dissertation committee chair was Bob Solow. From 1967 to 1972, he taught in the Department of Economics at Yale University, then moved to MIT, where he taught in the Department of Economics until 1989, when he moved to Harvard University. He was a Professor of Economics at Harvard until 2018, when he became a Research Professor of Economics.

Economic planning

At the start of his research career, Weitzman studied centrally planned economies in a field that has all but disappeared from academic economics – comparative economic systems. It was during this early period of his career that Marty’s papers with titles such as ‘Soviet Postwar Economic Growth and Capital Labor Substitution’ (1970b) and ‘Iterative Multi-Level Planning with Production Targets’ (1970a) appeared.

A remarkable product of his interest in how to manage a centrally planned economy efficiently was Marty’s classic paper on ‘Prices vs. Quantities’ (1974). He began this work to address the question of whether prices or production quotas would lead to more efficient outcomes in a centrally planned economy (under conditions of uncertainty), but the paper and the subsequent literature evolved to address the question of whether a price instrument or a quantity instrument will be more efficient for environmental regulation.

Although Marty began his first forays into research and writing on environmental and natural resource problems in the 1970s (some of it developing Marxian views of common property problems), it was not until the 1990s that he turned with such passion and energy to this realm, and produced one important work after another that virtually span the field. That outpouring coincided with the beginning of my collaboration with Marty, co-hosting the Harvard Seminar on Environmental Economics and Policy (more on this below).

The share economy

Along the way, Weitzman carried out important research in macroeconomics and unemployment theory. One product of this – along with dozens of journal articles (inevitably in the top periodicals) – was his best-selling 1984 book, The Share Economy – which was eventually translated into seven languages. In this brief (167-page) book, Marty laid out his proposal for how the US economy could be protected from the dual threats of unemployment and inflation with a remarkably simple idea (a hallmark of many of his contributions) – namely that instead of companies paying workers in manufacturing a fixed wage, they be paid through something akin to profit sharing, in particular by paying workers a significant share of company revenue.

In short, this would provide incentives for companies to continue adding workers as long as, through their work, they added to company revenues. This ‘novel, seemingly workable plan for equipping the economy to resist the instabilities’ that had plagued it for more than a decade (Passell 1984), was labelled in the headline of a lead New York Times 1985 editorial, ‘the best idea since Keynes’.

Policy instrument choice: prices versus quantities

For environmental economists, Marty’s most prominent contribution is probably his classic 1974 article, ‘Prices vs. Quantities’, which developed the simultaneously simple and powerful insight that – under conditions of uncertainty – the expected relative efficiency of policy instruments based on prices (such as a pollution tax) versus those based on quantities (such as a cap-and-trade system) depends on the relative slopes of the expected marginal benefit and marginal cost functions.

That work remains one of the most frequently cited articles in all of environmental economics. It stimulated a massive literature, a fact that prompted Richard Newell (Resources for the Future) to characterize the work as a ‘gift that keeps on giving’ at a symposium we held at Harvard in October 2018 to mark Marty’s retirement and celebrate his contributions, ‘Frontiers in Environmental Economics and Policy: A Symposium in Honor of Martin L Weitzman’. Even now, Marty’s 1974 paper is at the core of analysis of carbon taxes versus carbon cap-and-trade systems to address climate change (Karp and Traeger 2018; Mideksa and Weitzman 2019; Stavins 2019).

Colleagues who spoke at the celebration of Marty’s work on October 11, 2018, at the Harvard Kennedy School, “Frontiers in Environmental Economics and Policy: A Symposium in Honor of Martin L. Weitzman.” Front Row, left to right: Michael Greenstone, University of Chicago; Maureen Cropper, University of Maryland; Martin L. Weitzman, Harvard University; Robert Pindyck, Massachusetts Institute of Technology; William Nordhaus, Yale University. Back Row, left to right: James Stock, Harvard University; Charles Kolstad, Stanford University; Lawrence Goulder, Stanford University; Robert Stavins, Harvard University; Richard Newell, Resources for the Future. Photo Credit: Martha Stewart

Biodiversity

In the early 1990s, Weitzman responded to what he sensed might be the unwillingness – or the inability – of ecologists to rank ecologies in terms of their relative biodiversity, by producing a series of brilliant treatments of how these comparisons can be made quantitatively and rigorously: ‘On Diversity’ (1992); ‘What to Preserve: An Application of Diversity Theory to Crane Conservation’ (1993); ‘Patterns of Behavior in Biodiversity Preservation’ (Metrick and Weitzman 1996); and ‘The Noah’s Ark Problem’ (1998a). At the Harvard symposium, Charlie Kolstad (Stanford University) cited this body of work for its ‘significance and importance’.

Discounting

It was also in the 1990s that Marty became interested in a central issue of the economic analysis of climate change policies, namely long-term discounting. Given the long time horizons of the climate change problem, analysis of the expected net present value of alternative policies can be dominated by the choice of discount rate, which – with conventional exponential discounting – will greatly diminish the relative quantitative importance of phenomena that are decades or longer in the future.

Through careful theoretical analysis, Marty concluded that rather than a constant discount rate being employed, a rate that itself is diminishing over time is appropriate, so that benefits and costs in the near future would be subject to a typical rate, while benefits and costs further in the future would be subject to a much lower rate.

This important and influential work appeared in a series of articles, including: ‘On the Environmental Discount Rate’ (1994); ‘Why the Far-Distant Future Should be Discounted at its Lowest Possible Rate’ (1998b); and ‘Gamma Discounting’ (2001a). At the Harvard symposium, Larry Goulder (Stanford University) emphasized that this work is important ‘because it affects decisions as to how much we should invest in infrastructure, in mitigation, and in other realms.’

Green national accounting

A topic that has pervaded decades of analysis and commentary in the environmental sphere is the reality that conventional measures of economic growth, such as gross domestic product, are not measures of welfare, since they do not account for externalities (among other non-market economic phenomena). In 1999, the National Research Council published Nature’s Numbers: Expanding the National Economic Accounts to Include the Environment, produced by a committee chaired by Bill Nordhaus and including Marty Weitzman (Nordhaus and Kokkelenberg 1999). That was linked with several contributions that Weitzman subsequently made to the scholarly literature, including: ‘Does NNP Growth Indicate Welfare Improvement’ (Asheim and Weitzman 2001); and ‘A Contribution to the Theory of Welfare Accounting’ (2001b).

At the Harvard symposium, Bill Nordhaus emphasized Marty’s contributions in this realm, and launched his keynote presentation, ‘The Intellectual Footprint of Martin Weitzman in Environmental Economics’, by stating that Marty ‘has changed the way we think about economics and the environment.’ He concluded that ‘those who claim that environmental regulations hurt growth are completely wrong, because they are using the wrong yardstick. Pollution should be in our measures of national output, but with a negative sign, and if we use green national output as our standard, then environmental and safety regulations have increased true economic growth substantially in recent years… For this important insight we applaud Martin Weitzman, a radically innovative spirit in economics.’

Fisheries

Some will be surprised to learn that a theorist such as Marty Weitzman was as immersed as he was in concerns about the real world of natural resource management and environmental protection. One example comes from his research and outreach in the realm of fisheries management. His modelling of Icelandic commercial fisheries affected thinking and discussion around the world regarding the use of taxes and quotas to regulate open-access fisheries.

As Maureen Cropper (University of Maryland) said at the Harvard symposium, ‘this is another example of the use of a simple model and treatment of uncertainty that really did start a conversation among fisheries economists’. This application of Weitzman’s previously developed theory of instrument choice was documented in his 2002 paper ‘Landing Fees vs Harvest Quotas with Uncertain Fish Stocks’.

Fat tails

In recent years, Marty made prominent and important contributions to thinking about long-term climate change policy with his development of a theory of how positive biophysical feedback loops could lead to uncertainty about the damages of climate change that is best characterized by a probability distribution of damages with fat tails, such as a Pareto distribution, rather than a conventional Gaussian (normal) distribution. The result is greater weight being given to catastrophic (but relatively small probability) outcomes.

Speaking at the Harvard symposium, Bob Pindyck of MIT pointed to Weitzman’s prescient 2007 paper, ‘Subjective Expectations and Asset-Return Puzzles’ as having had a profound influence on Marty’s subsequent modelling of catastrophic climate change. A small subset of the papers Marty published on this topic include: ‘On Modeling and Interpreting the Economics of Catastrophic Climate Change’ (2009); ‘Fat-Tailed Uncertainty in the Economics of Climate Change’ (2011); and ‘Fat Tails and the Social Cost of Carbon’ (2014b).

Domestic and international climate change policy

Marty Weitzman always searched for topics for his research that were not only interesting, but also relevant and important for real-world applications. His recent work exploring alternative policy instruments to address climate change and his critical examinations of the form of international climate agreements provide telling examples of this. It was in this regard that Jim Stock (Harvard University) credited Weitzman for the ‘tremendous influence’ his ideas have had on the formulation of public policy around the world.

Just a few of the many papers that could be cited in this context are: ‘Can Negotiating a Uniform Carbon Price help to Internalize the Global Warming Externality’ (2014a); ‘A Voting Architecture for the Governance of Free-Driver Externalities, with Application to Geoengineering’ (2015); and ‘On a World Climate Assembly and the Social Cost of Carbon’ (2017). Also, of course, Marty and his former student, Gernot Wagner, wrote a lucid and compelling book, Climate Shock: The Economic Consequences of a Hotter Planet (2015).

Theoretical foundations for empirical analyses

It should be emphasized that Marty Weitzman’s theoretical work was not only important for other theorists, but also for empirical economists. In many of the realms described above, his insights were fundamental as the foundation for sound empirical analysis. As Michael Greenstone (University of Chicago) noted at the Harvard symposium, Marty’s work ‘takes something you are kind of confused about, and then after you read it, you can’t understand how in the world you were confused beforehand. It just clarifies things in a way that is really beautiful.’

A remarkable scholar

Marty Weitzman was thus a real treasure – a ‘gift that kept on giving’ – for both the research and policy worlds. His work as a theorist on environment broadly and on climate change in particular was unparalleled, and formed the basis of much theoretical and empirical research carried out by others over several decades. His work – from examining price versus quantity instruments in the early 1970s through his examinations in the last few years of the implications of fat tails in the probability distribution of possible climate damages – have changed the way economists and others think about the environment and policies to protect it.

His contributions were well recognized. He was elected a Fellow of the Econometric Society in 1976; a Fellow of the American Academy of Arts and Sciences in 1986; three times won the annual award for ‘Publication of Enduring Quality’ from the Association of Environmental and Resource Economists; received the 20th Anniversary Prize from Fondazione Eni Enrico Mattei, the Leontief Prize, and the Eric Kempe Prize in 2011; and the John Kenneth Galbraith Award in 2013.

Memories

My greatest personal remembrance will be that I learned an immense amount from Marty by co-hosting with him for 26 years the Harvard Seminar on Environmental Economics and Policy. That’s 52 semesters involving more than 400 seminars, each with a distinct paper and presentation by a leading scholar from across the United States and around the world. I found that Marty’s questions and comments were often as insightful as the speaker’s presentation.

Of course, we did not always agree. I remember our spirited discussions contrasting Marty’s strong view of the superiority of carbon taxes and my view of the relative symmetry of price and quantity instruments for climate change. Also we had some long discussions about the 2015 Paris Agreement on climate change, which Marty saw (accurately) for what it lacks, and I saw for its improvements over the international policy architecture that had preceded it. We disagreed, but were never disagreeable (and I never succeeded in changing his mind!). All in all, for three decades, I consistently learned from this remarkable scholar. He truly was a gift that kept on giving.

Marty receiving from the author a book of testimonials by 100 scholars around the world at the celebration of Marty’s work on October 11, 2018, at the Harvard Kennedy School, “Frontiers in Environmental Economics and Policy: A Symposium in Honor of Martin L. Weitzman.” Photo Credit: Martha Stewart

REFERENCES

Asheim, Geir B, and Martin L Weitzman (2001), ‘Does NNP Growth Indicate Welfare Improvement?’ Economic Letters 73(2): 233-39.

Karp, Larry, and Christian Traeger (2018), ‘Prices versus Quantities Reassessed’, CESifo Working Paper No. 7331.

Metrick, Andrew, and Martin L Weitzman (1996), ‘Patterns of Behavior in Endangered Species Preservation’, Land Economics 72(1): 1-16.

Mideksa Torben, and Martin L Weitzman (2019), ‘Prices versus Quantities across Jurisdictions’, Journal of the Association of Environmental and Resource Economists 6(5): 883-891.

New York Times (1985), ‘Best Idea Since Keynes’, Editorial, March 28, Section A, page 30.

Nordhaus, William D, and Edward C Kokkelenberg, editors (1999), Nature’s Numbers: Expanding the National Economic Accounts to Include the Environment, Panel on Integrated Environmental and Economic Accounting, National Academy Press.

Nordhaus, William D (2019), ‘The Intellectual Footprint of Martin Weitzman in Environmental Economics’, presentation at ‘Frontiers in Environmental Economics and Policy: A Symposium in Honor of Martin L. Weitzman’, Harvard Kennedy School, October 11.

Passell, Peter (1984), ‘The Editorial Notebook: The Hidden Boon in Profit-Sharing’, New York Times, November 15, Section A, Page 30.

Stavins, Robert N (2019), ‘The Future of U.S. Carbon-Pricing Policy’, NBER Working Paper No. 25912 (http://www.nber.org/papers/w25912).

Wagner, Gernot, and Martin L Weitzman (2015), Climate Shock: The Economic Consequences of a Hotter Planet, Princeton University Press.

Weitzman, Martin L (1970a), ‘Iterative Multi-Level Planning with Production Targets’, Econometrica 38(1): 50-65.

Weitzman, Martin L (1970b), ‘Soviet Postwar Economic Growth and Capital Labor Substitution’, American Economic Review 60 (4): 676-92.

Weitzman, Martin L (1974), ‘Prices vs. Quantities’, Review of Economic Studies 41(4): 477-91.

Weitzman, Martin L (1984), The Share Economy, Harvard University Press.

Weitzman, Martin L (1992), ‘On Diversity’, Quarterly Journal of Economics 107(2): 363-405.

Weitzman, Martin L (1993), ‘What to Preserve: An Application of Diversity Theory to Crane Conservation’, Quarterly Journal of Economics 108(1): 157-83.

Weitzman, Martin L (1994), ‘On the ‘Environmental’ Discount Rate’, Journal of Environmental Economics and Management 26: 200-9.

Weitzman, Martin L (1998a), ‘The Noah’s Ark Problem’, Econometrica 66(6): 1279-98.

Weitzman, Martin L (1998b), ‘Why the Far-Distant Future Should be Discounted at its Lowest Possible Rate’, Journal of Environmental Economics and Management 36(3): 201-8.

Weitzman, Martin L (2001a), ‘Gamma Discounting’, American Economic Review 91(1): 260-71.

Weitzman, Martin L (2001b), ‘A Contribution to the Theory of Welfare Accounting’, Scandinavian Journal of Economics 103(1): 1-23.

Weitzman, Martin L (2002), ‘Landing Fees vs Harvest Quotas with Uncertain Fish Stocks’, Journal of Environmental Economics and Management 43: 325-38.

Weitzman, Martin L (2007), ‘Subjective Expectations and Asset-Return Puzzles’, American Economic Review 97(4): 1102-30.

Weitzman, Martin L (2009), ‘On Modeling and Interpreting the Economics of Catastrophic Climate Change’, Review of Economics and Statistics 91(1): 1-19.

Weitzman, Martin L (2011), ‘Fat-Tailed Uncertainty in the Economics of Catastrophic Climate Change’, Review of Environmental Economics and Policy 5(2): 275-92.

Weitzman, Martin L (2014a), ‘Can Negotiating a Uniform Carbon Price Help to Internalize the Global Warming Externality?’, Journal of the Association of Environmental and Resource Economists 1(1/2): 29-49.

Weitzman, Martin L (2014b), ‘Fat Tails and the Social Cost of Carbon’, American Economic Review Papers and Proceedings 104(5): 544-6.

Weitzman, Martin L (2015), ‘A Voting Architecture for the Governance of Free-Driver Externalities, with Application to Geoengineering’, Scandinavian Journal of Economics 117(4): 1049-68.

Weitzman, Martin L (2017), ‘On a World Climate Assembly and the Social Cost of Carbon’, Economica 84(336): 559-86.

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Does economic analysis shortchange the future?

Decisions made today usually have impacts both now and in the future. In the environmental realm, many of the future impacts are benefits, and such future benefits — as well as costs — are typically discounted by economists in their analyses.  Why do economists do this, and does it give insufficient weight to future benefits and thus to the well-being of future generations?

This is a question my colleague, Lawrence Goulder, a professor of economics at Stanford University, and I addressed in an article in Nature.  We noted that as economists, we often encounter skepticism about discounting, especially from non-economists. Some of the skepticism seems quite valid, yet some reflects misconceptions about the nature and purposes of discounting.  In this post, I hope to clarify the concept and the practice.

It helps to begin with the use of discounting in private investments, where the rationale stems from the fact that capital is productive ­– money earns interest.  Consider a company trying to decide whether to invest $1 million in the purchase of a copper mine, and suppose that the most profitable strategy involves extracting the available copper 3 years from now, yielding revenues (net of extraction costs) of $1,150,000. Would investing in this mine make sense?  Assume the company has the alternative of putting the $1 million in the bank at 5 per cent annual interest. Then, on a purely financial basis, the company would do better by putting the money in the bank, as it will have $1,000,000 x (1.05)3, or $1,157,625, that is, $7,625 more than it would earn from the copper mine investment.

I compared the alternatives by compounding to the future the up-front cost of the project. It is mathematically equivalent to compare the options by discounting to the present the future revenues or benefits from the copper mine. The discounted revenue is $1,150,000 divided by (1.05)3, or $993,413, which is less than the cost of the investment ($1 million).  So the project would not earn as much as the alternative of putting the money in the bank.

Discounting translates future dollars into equivalent current dollars; it undoes the effects of compound interest. It is not aimed at accounting for inflation, as even if there were no inflation, it would still be necessary to discount future revenues to account for the fact that a dollar today translates (via compound interest) into more dollars in the future.

Can this same kind of thinking be applied to investments made by the public sector?  Since my purpose is to clarify a few key issues in the starkest terms, I will use a highly stylized example that abstracts from many of the subtleties.  Suppose that a policy, if introduced today and maintained, would avoid significant damage to the environment and human welfare 100 years from now. The ‘return on investment’ is avoided future damages to the environment and people’s well-being. Suppose that this policy costs $4 billion to implement, and that this cost is completely borne today.  It is anticipated that the benefits – avoided damages to the environment – will be worth $800 billion to people alive 100 years from now.  Should the policy be implemented?

If we adopt the economic efficiency criterion I have described in previous posts, the question becomes whether the future benefits are large enough so that the winners could potentially compensate the losers and still be no worse off?  Here discounting is helpful. If, over the next 100 years, the average rate of interest on ordinary investments is 5 per cent, the gains of $800 billion to people 100 years from now are equivalent to $6.08 billion today.  Equivalently, $6.08 billion today, compounded at an annual interest rate of 5 per cent, will become $800 billion in 100 years. The project satisfies the principle of efficiency if it costs current generations less than $6.08 billion, otherwise not.

Since the $4 billion of up-front costs are less than $6.08 billion, the benefits to future generations are more than enough to offset the costs to current generations. Discounting serves the purpose of converting costs and benefits from various periods into equivalent dollars of some given period.  Applying a discount rate is not giving less weight to future generations’ welfare.  Rather, it is simply converting the (full) impacts that occur at different points of time into common units.

Much skepticism about discounting and, more broadly, the use of benefit-cost analysis, is connected to uncertainties in estimating future impacts. Consider the difficulties of ascertaining, for example, the benefits that future generations would enjoy from a regulation that protects certain endangered species. Some of the gain to future generations might come in the form of pharmaceutical products derived from the protected species. Such benefits are impossible to predict. Benefits also depend on the values future generations would attach to the protected species – the enjoyment of observing them in the wild or just knowing of their existence. But how can we predict future generations’ values?  Economists and other social scientists try to infer them through surveys and by inferring preferences from individuals’ behavior.  But these approaches are far from perfect, and at best they indicate only the values or tastes of people alive today.

The uncertainties are substantial and unavoidable, but they do not invalidate the use of discounting (or benefit-cost analysis).  They do oblige analysts, however, to assess and acknowledge those uncertainties in their policy assessments, a topic I discussed in my last post (“What Baseball Can Teach Policymakers”), and a topic to which I will return in the future.

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