Is the rift between Nordhaus and Stern evaporating with rising temperatures?

Lead author of this joint post is Peter Howard, Economic Fellow at the Institute for Policy Integrity at New York University School of Law.

The political task of enacting carbon taxes ­― and maintaining those in place ― has proven so daunting that questions of the tax’s appropriate level have gotten short shrift. Carbon tax advocates do not often discuss: How high is the optimal carbon tax? Along what trajectory should it increase over time? What, if anything, can climate science tell us about the right carbon tax to aim for?

Prof. William D. Nordhaus, Yale University

Prof. William D. Nordhaus, Yale University

In the academic realm, the distinguished Yale economist and public intellectual William Nordhaus has taken a leading role in the discussion. Nordhaus first modeled energy-economy interactions in the 1970s, and since the early 1990s successive versions of his Dynamic Integrated model of Climate and the Economy, or DICE model, have been used to estimate costs and benefits of carbon mitigation strategies in one prestigious report after another ― most recently in the Fifth Assessment Report by the UN Intergovernmental Panel on Climate Change (IPCC).

Given Nordhaus’s concerns over global warming, reflected in his ongoing repudiations of climate change denialists as well as his impatience with cap-and-trade schemes, it has been jarring for some to see him advocate for a relatively low carbon tax. In his 2008 book, A Question of Balance, which relied on the 2007 version of DICE, Nordhaus proposed a year-2005 starting price of just $8 (U.S.) per short ton of CO2 (from his Table 5-4, adjusted to 2012 dollars and recalibrated from metric to short tons and from C to CO2), which would then take two decades to double and another 30 years to double again.

Nicholas Stern (Baron Stern of Brentford)

Nicholas Stern (Baron Stern of Brentford)

In contrast, the Carbon Tax Center and its allies at the Citizens Climate Lobby have long advocated a steeper, stepwise ramp-up, with an initial price of around $10 per ton of CO2 followed by annual increases of the same magnitude for at least a decade and perhaps much longer. This policy recommendation is more in line with the views of Nicholas Stern ― lead author of the Stern Review on the Economics of Climate Change (2006) ― who argues that strong climate policies are necessary immediately to forestall large future damages from global warming. In the past, Nordhaus (along with several other economists) disregarded these findings based on the low discount rate assumed in the report.

Recently, however, this difference in opinion between the Nordhaus and Stern camps with regards to policy (though not discount rate assumptions) has lessened. Using the latest version of the Nordhaus model, DICE-2013, Nordhaus finds an optimal initial (2015) carbon price of approximately $21 per short ton of CO2 in 2012 U.S. dollars (a near tripling from DICE-2007). Moreover, the optimal tax according to Nordhaus rises more rapidly over time as compared to DICE-2007.[1] A tax of this amount would restrict the average global temperature increase to approximately 3 degrees Celsius above pre-industrial levels.[2]

As economist and NY Times columnist Paul Krugman noted in his review of Nordhaus’s 2013 book, The Climate Casino,even Nordhaus seems surprised by his finding that both the international consensus of a 2 °C limit and the carbon tax necessary to achieve it are nearly economically rational.[3] And given that DICE-2013 fails to account for climate tipping points (as Nordhaus himself notes), an even lower temperature limit and higher carbon tax are justifiable.

Stern has now taken this recent scholarship a step further. In a June paper co-authored with economist Simon Dietz, Stern demonstrates that the DICE framework can support an even stronger mitigation effort than the latest Nordhaus specification of the model.Their paper, “Endogenous growth, convexity of damages and climate risk: how Nordhaus’ framework supports deep cuts in carbon emissions” (co-published by the Centre for Climate Change Economics and Policy as Working Paper No. 180, and by the Grantham Research Institute on Climate Change and the Environment as Working Paper No. 159), is not a rehash of the Stern-Nordhaus dispute over discounting. Rather, the paper accepts Nordhaus’s choice of discount rate for argument’s sake but modifies the 2010 edition of Nordhaus’s model in three critical ways. [Read more…]

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