THE STERN REVIEW OF THE ECONOMICS OF CLIMATE CHANGE: A
COMMENT
Richard S.J. Tol
Economic and Social Research InstituteHamburg, Vrije and Carnegie Mellon Universities
November 2, 2006

Introduction
The Stern Review of the Economics of Climate Change (Stern et al., 2006) is a report to
the Prime Minister and the Chancellor of the Exchequer of the United Kingdom. A teamof 23 people, led by Sir Nicholas Stern and supported by many consultants, worked for alittle over a year to produce a report of some 700 pages on the economics of climatechange. The report says many things, some better supported than others. I agree with theStern Review on a number of things. Firstly, climate change is real, and climate change is
a problem. Secondly, climate policy should be guided (but not dictated) by an assessmentof the costs and benefits of greenhouse gas emission abatement. Thirdly, climate policy isbest implemented with market-based instruments such as taxes and tradable permits.
In this comment, I focus on two conclusions. Firstly, the Stern Review argues that “theoverall costs and risks of climate change will be equivalent to losing at least 5% of global
GDP1 each year, now and forever.” These are “risks of major disruption to economic andsocial activity, on a scale similar to those associated with the great wars and the economic
depression of the first half of the 20th century”. Secondly, the Stern Review argues that“the benefits of strong early action outweigh the costs”. This action would keep
concentrations of greenhouse gases below 550 ppm CO2 equivalent.
Intriguingly, the 550 ppm CO2eq target coincides with climate change target adopted
earlier by the UK government (RCEP, 2000). The Stern Review should therefore not be
understood as a revision. Earlier, HM Treasury had released a report (Clarkson andDeyes, 2002) that also justified the 550 ppm CO2eq target. The earlier report has been
criticized for being out of step with the peer-reviewed literature (Pearce, 2003; Tol,2005). For anyone familiar with the literature on the economic impacts of climate change(Smith et al., 2001) or the literature on cost-benefit analysis on climate change(Nordhaus, 1991), the headline conclusions of the Stern Review come as a surprise too:
The Stern Review estimates are well outside the usual range. The Select Committee for
Economic Affairs of the House of Lords (2005) had warned the UK government forbeing out of step with the economic literature on climate change – Pearce (2006) adds
1

On page 163, 5% of GDP is in fact the mean for one particular scenario. The five-percentile may be as
low as 0.3% of GDP. The 95%ile may be as high as 33%.

more detail. The Stern Review missed an opportunity to help align UK climate policy tothis literature.
In this commentary, I review the impact estimates in the Stern Review and assess thecost-benefit analysis in that report before reaching a conclusion.
Economic impacts of climate change
Let us first examine the Stern Review conclusion that climate change will cause
economic disruption now and forever. The “now and forever” is preposterous.2 The world
economy is growing briskly; immediate threats to economic growth are imbalances in theUS, overheating in China, and lack of reform in the EU. But the “forever” part is also
problematic. It assumes that society will never get used to higher but stable temperatures,changed rainfall patterns, or higher sea levels. This is a rather dim view of humaningenuity. It contradicts what we know about technological progress, adaptation, andevolution.
The Stern Review highlights several impacts of climate change. One is water. The work
here is based on Arnell (2004). The Stern Review correctly that Arnell (2004) does “notinclude adaptation” and is therefore severely biased. Food is another highlighted impact.
Climate change would hamper agricultural productivity in some parts of the world,
particularly Africa. This would be a problem in today’s world. However, in all of thesocio-economic scenarios used by the Stern Review, African economies would growrapidly. This is inconsistent with famine. Middle-income countries would import food(global food production is not threatened by climate change) rather than starve.
Furthermore, it is hard to imagine rapid economic growth without substantial
improvements in agriculture productivity; at present, African agriculture is particularly
inefficient. For health, the Stern Review makes the same mistake: It worries about people
dieing of diarrhea and malaria, diseases that can be controlled at little expense. The Stern
Review extrapolates the increase of damage due to weather-related natural disasters. It
uses the estimates of Muir-Wood et al. (2006), ignoring the opposite (and peer-reviewed)
conclusions by Pielke et al. (2005) and Pielke (2005).3 For water, agriculture, health andinsurance, the Stern Review consistently selects the most pessimistic study in theliterature. For refugees, the Myers and Kent (1995) are the highest, and the Stern Review
duly highlights that “some estimates suggest that 150-200 million people may become
permanently displaced”. Myers and Kent (1995) was not peer-reviewed.4 Norman Myersis a known alarmist. For sea level rise, the Stern Review only quotes the “millions at risk”
from Nicholls and Tol (2005) – this metric ignores adaptation, which is very effective

2

It is clear from page 162 that this is a certainty-equivalent annuity. Note that the used discount rate isparticularly low, and at odds with the discount rate recommended by HM Treasury (2003). See Guo et al.
(2006) for a discussion of discount rates and marginal damage costs of CO2 emissions.
3
It is surprising that the Stern Review overlooked Pielke’s work on hurricane damages, as it was presented
at the same meeting as Muir-Wood’s work, and Pielke alerted Stern to this (Pielke, personalcommunication, 2006). Pielke’s work on hurricanes is referred to in Chapter 1 of the Stern Review.
4
The current author was on the advisory board of the project that led to the Myers and Kent report. The
board was very critical of its findings.

against sea level rise. Note that Nicholls and Tol (2005) do report impact measures withadaptation too.5
In the chapter on the impact of climate change on development, the Stern Review quotes
the works of Nordhaus (2006) and Sachs (2001) – who find that a tropical climate
negatively affects economic development. The Stern Review ignores the work of
Acemoglu et al. (2001, 2005) and Easterly and Levine (2003), who argue that climate has
at most a minor, indirect effect in the (distant) past – and the climate-change-specificstudies of Fankhauser and Tol (2005) and Tol (forthcoming), who show that climatechange will have a limited effect on development. In their poverty projections, the Stern
Review interprets the income-loss-equivalent-welfare-losses (market impacts only) withactual income losses – this is a very crude approximation.
The economic impact estimates of the Stern Review are in fact all based on a single
integrated assessment model, PAGE2002 by Hope (2006). Although a single model
makes for easy presentation, it also implies a lack of robustness. Integrated assessment
models differ considerably in their representation of impacts (cf. Tol and Fankhauser,
1998). The PAGE2002 model stands out for two reasons. Firstly, there is less than a 5%
probability that the climate change impacts are negative, even in the short run (cf.
Mendelsohn et al., 2000). Secondly, the model assumes that vulnerability to climatechange is independent of development (cf. Yohe and Tol, 2002). Both assumptions are atodds with the state of the art – and both assumptions imply that the impact estimates areoverly pessimistic.
The Stern Review provides too little information to understand what is in thecalculations. There appear to be no equity weights in the Stern Review. The report adds
otherwise unspecified and unquantified “market impacts” (annuitized at 2.1% of GDP),“non-market impacts” (5.9%) and “catastrophic risk” (2.9%). The reported “mean” isapparently a certainty equivalent, although the “mean” is presented together with aconfidence interval. The catastrophic risk apart, the Stern Review also assumes that there
is a 0.1% annual probability (10% in a century) of extinction of the human race. The
welfare loss of that is added too, apparently in addition to the “catastrophic risk”.
Together with the certainty equivalent, risk seems to be counted three times.
Cost-benefit analysis and emission reduction targets
The Stern Review overestimates the impacts of climate change, and therefore the benefitsof emission reduction. Its estimates of the costs of emission reduction are largely inspiredby the Innovation Modeling Comparison Project (Edenhofer et al., 2006; Grubb et al.,2006; Koehler et al., 2006), a group of models that make overly optimistic assumptions
on technological progress and the costs of emission abatement (see Weyant, 2004, andvan Vuuren et al., 2006, for more mainstream estimates). High benefits and low costs
together imply that the Stern Review recommends more stringent emission reduction thanstandard cost-benefit analysis (Azar and Lindgren, 2003; Keller et al., 2004, 2005;
Maddison, 1995; Manne et al., 1995; Nordhaus, 1991, 1993, 1994; Nordhaus and Boyer,
5

The Stern Review does discuss adaptation. There are 260 pages of text between the discussion of impacts
and the discussion of adaptation.

2000; Nordhaus and Yang, 1996; Peck and Teisberg, 1992, 1994; Tol, 1997, 1999, 2001,
2002).
The Stern Review does not include a cost-benefit analysis, apart from a ranking of two
projects. It compares the magnitudes of the costs of abatement (around 1% of GDP) tothe costs of climate change (5-20% of GDP) and concludes that the latter justifies theformer. There are two mistakes here. Firstly, the costs of climate change do not equal thebenefits of emission reduction – any abatement will only slow climate change rather than
avoid it altogether – therefore, the benefits of emission reduction are smaller than thecosts of climate change (Tol and Yohe, 2006). Secondly, marginal costs should becompared to marginal benefits, rather than total costs to total benefits. The Stern Review
is silent on marginal abatement costs. It does report marginal damage costs though. For
instance, it says “the mean value of the estimates in the study by Tol [2005] was about$29/tCO2” but omits that Tol (2005) concludes that “it is unlikely that the marginal
damage costs of carbon dioxide emissions exceed $50/tC [$14/tCO2] and are likely to be
substantially smaller than that.” The Stern Review does report that “the current social
cost of carbon […] might be around $85/tCO2”, but it does not provide any more detail –except that this number is preliminary and results from PAGE2002 (Hope, 2006).6$85/tCO2 equals $314/tC, and is therefore an outlier in the marginal damage cost
literature (Tol, 2005).
Surprisingly, the Stern Review supports a marginal damage cost estimate that is threetimes as high as the previous estimate of HM Treasury (Clarkson and Deyes, 2002) – but
it supports the same target for stabilizing greenhouse gas concentrations. The total
climate change impacts of the Stern Review are about 8 times those of the CEC (2005),
while Stern’s abatement costs estimates are only about 4 times as high – yet, the Stern
Review advocates a climate target that is less stringent than does CEC (2005). The Stern
Review does not elucidate these discrepancies.
Conclusion
In sum, the Stern Review is very selective in the studies it quotes on the impacts ofclimate change. The selection bias is not random, but emphasizes the most pessimistic
studies. In this sense, the Stern Review reminds one of Lomborg (2001). The discountrate used is lower than the official recommendations by HM Treasury. Results areoccasionally misinterpreted. The report claims that a cost-benefit analysis was done, but
none was carried out. The Stern Review can therefore be dismissed as alarmist andincompetent.
This is not to say that climate change is not a problem, nor that greenhouse gas emissions
should not be reduced. There are sound arguments for emission reduction. However,unsound analyses like the Stern Review only provide fodder for those skeptical of climatechange and climate policy – and may well further polarize the debate.

6

According to Hope (personal communication, 2006), the marginal damage cost was calculated using a
0.1% rate of pure time preference.

Climate policy is for the long-term. It will only be successful if a broad coalition – ofcountries and of stakeholders within countries – supports climate policy and continues tosupport climate policy. To my mind, this calls for a sober analysis, rather than hyperbole.
Acknowledgements
Frans Berkhout, David Henderson, Chris Hope, Sue Scott, Joel Smith and Nick Stern had
useful comments on earlier versions of this commentary.
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