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Roger:
Feedback first - I find this to be a very provocative and informative site and greatly appreciate your perspective and civility in hosting it
I do often, however, have reservations regarding views expressed and this is one instance,. While I would not disagree that both mitigation and adaptation are ultimately necessary (and I think that most of the IPCC would privately go along with that view), I believe that their stronger focus on mitigation stems from a few factors that you don’t really allude to, partially because you focus on the GDP aspects of the problem. One of these is that too often adaptation/ development perspectives take on purely anthropocentric values which largely discount ecosystem factors that may not heavily weigh in GDP calculations (at least not in a way that can be accurately calculated even within the limited time spans that we are discussing), yet they may have a value nevertheless that cannot be expressed in economic terms. The impact on biodiversity of climate change, for example, is a factor of indeterminate relevance to GDP, yet the extinction of large numbers of species as a potential consequence will not be abated by measures that constrain coastal development (which is not to say such measures should not be considered).

Another consideration is that both the ecological and economic impacts of GW are very likely to be not only unevenly distributed, but indeed to be maldistributed in the sense that early beneficiaries of carbon and GHG generating practices (who hold responsibility for getting the planet to the point where GHG accumulation is a global problem), will likely be less impacted than others who got to the game late. The extent to which ecosystem changes may lead to population displacement in less developed areas is a vulnerability that will inevitably receive less attention than in it will in wealthy (and significantly larger) states, and based on current attitudes (and general track records) on things like immigration policy and development aid, the prognosis for a focus on adaptation/development might seem to be at best to increase dependency and at worse to increase global instability and/or regional destitution. Again, this is not to say that adaptation is something to be ignored, but to suggest that there may well be valid reasons for focusing on mitigation.

Best,
MB

I broadly agree with you that to tackle climate change only by looking at mitigation and more or less rule out adaptation is misguided.

Nevertheless, there are two points in this post that are a bit off, in my opinion.

First, the way you combine the Stern damage estimates with per capita GDP from the SRES scenarios is wrong. Stern uses the very strange concept of balanced growth equivalence (BGE) and express impacts in terms of changes of that. Here is how it roughly works: You calculate a consumption path for the future that has no impacts from climate change. In order to calculate the BGE for that you proceed as follows: You look for a consumption path that has a constant growth rate to which you are indifferent in terms of an ordering of a given social welfare function. That is, you come up with a scenario of economic growth that has constant economic growth that is equally desirable in terms of a social welfare function (note that you have two parameters that you can tweek to come up with such BGE scenario: The initial GDP and the constant growth rate). Next, you calculate a consumption path with climate impacts, and again calculate a BGE for that path, i.e. again a consumption path that is equally desirable from the point of view of your welfare function to this consumption path with climate impacts. Now you got two BGE: One for the scenario without and one for the scenario with climate impacts. The difference in those two BGE is what Stern calculated. But this does not at all tell you anything about the actual damages in a particular year calculated by Stern (like you assumed for the year 2100). Stern's numbers are a welfare measure in commodity units, not calculations of future impacts.

If you find that confusing, you are not alone ;) I am not sure why they did it that way. I guess the theoretical justification is that this way you can quantify a change in terms of percentage change even under an ordinal welfare ordering. On the other hand you will inevitably get people to misunderstand your numbers (and I guess this post is just one example).

Second point: They way you present the SRES scenarios, it almost appears as if they represent choices society can make, i.e. as if policy makers can choose whether they want to tweek some decision variable that effects which of the SRES scenarios will happen or they can tweek another set of variables that effect which climate scenario will happen. To some extend that might be true, but certainly the SRES authors have not presented their scenario families like that. They are not presented as a deck of policy choices, neither are they presented as a probabalistic forcast. All they are is possible futures, no more, no less. Whether societies can choose between them, or whether they represent scientific uncertainty about what will happen or some other kind of uncertainty is precisly not defined for them.

Alright, two pedantic points, I believe the general spirit of your post is spot on.

MB- Thanks for your comments. I pretty much agree with everything that you write.

The focus in this post on GDP is simply because that is the main quantified metric presented by both IPCC and Stern. This post is about following through the logic of these reports. IPCC and Stern could have (and maybe should have) decided, as you did, to emphasize equity, biodiversity, and past policy successes in development policy as the basis for arguing for action on climate change. But they did not, instead choosing to focus attention on the quantified impacts of climate change on GDP. This post shows where that thinking ultimately leads.

Thanks!

David-

Thanks much for these specific comments. Your first point is a fair one.

The SRES projects world economic per capita growth rates of between 1.3% and 2.7. If these growth rates are reduced by 5% and 20% per year starting this year then the resulting impact on 2100 global per capita GDP is to reduce 2100 GDP by between 7% and 46% across the extremes of the SRES scenarios for the year 2100.

This way of calculating future growth would reduce importance of storylines from 19-74 times more important that greenhouse gas emissions to 8-53 times more important. (Which I suppose is why you agree that this point as pedantic!;-) if I write this up for broader discussion I will note this distinction.

On the second point, I agree with you that the IPCC presented the storylines as if they are exogenous to policymaking. The reality is that the storylines are the result of policymaking! The IPCC frames development as outside human control and carbon-reduction policy as being within human control. Is it no wonder that their focus is narrowly on greenhouse gases over adaptation? This post is meant to illustrate the consequences of this kind of wrongheaded thinking. A forthcoming paper of mine illustrates this in the specific case of future hurricane losses. I am waiting for the editor to give me the go-ahead on pre-pub posting . . .

Thanks much for your comments!

Roger,
I don't understand. How is 1% to 5% consistent with 5% to 20%?

D. F. Linton- Thanks for your question, if I understand it correctly the 5% and 20% values that I chose are the high ends of the IPCC WGII and Stern Report respectively.

Choosing the lower ends of each (i.e., 1% for IPCC, 5% for Stern) would result in even more dramatic results!

Ask again if this doesn't get what you were after, Thanks.

Roger,

Just a note on measures other than GDP to determine the impact of AGW.
A Times article
http://www.timesonline.co.uk/tol/news/world/article637004.ece
notes a study in the Proceedings of the National Academy of Sciences on changes in forest density worldwide.

A quote from the article is interesting:

"Wealth is one of the clearest indicators of a country’s success in reversing deforestation. Of the countries surveyed, all of those with a GDP per capita greater than $4,600 (£2,400) — roughly equivalent to that of Chile — had increased their forest cover since 1990. "

This may suggest that increased wealth could have a positive impact on biodiversity. Therefore policies that increase global GDP may be better for the environemnt.
Obviously I am not saying from this one study we can fix all environmental problems by policies that increase global GDP, but I was intrigued by the correlation between GDP and forest density and it did seem to tie in with the comment by MB above.

"The unavoidable conclusion: we should be discussing development pathways, and not simply carbon dioxide."

The IPCC is not allowed to do this. Its role is very narrowly defined as provoding scientific advice to political deciders and the public. They are not the ones who get to make political decisions, and they would be vilified even more in the science-averse parts of the political discourse if they tried to suggest otherwise.

Roger, piglet is right about the prescribed role of the IPCC, which is a creature of the parties to the UNCCC.

While I agree completely with your conclusion that we should be discussing development pathways and not simply carbon dioxide, since the US, EU and others set the IPCC's agenda it would be clearer if you targetted them and not the IPCC, as you also do here: "The IPCC frames development as outside human control and carbon-reduction policy as being within human control. Is it no wonder that their focus is narrowly on greenhouse gases over adaptation?"

The fact of the matter is that UNCCC and the Kyoto Protocol focus primarily on mitigation. I wholly agree that this is rather narrow and gives short shrift to the importance of adaptation, especially in the developing world. However, this is not the fault of the IPCC.

Issues of development are extremely difficult, so it is no surprise that countries declined to integrate them comprehensively into the ambit of the IPCC, the UNCCC and Kyoto. It is indeed unfortunate the the discussion of adaptation and development have been further ignored over the past six years while we debate further whether climate change is occurring and the contributing role of human activities, but of course discussions of development have continued on many tracks - even as they too have been put on the back burner due to wars in the Middle East, North Korea issues, etc.

I think you are focussed on the right issues, but perhaps are flogging the wrong horse.

piglet and Tom- Thanks for your comments. I am not sure where you get your ideas on the scope of the IPCC mandate, but no where is it limited to mitigation or the FCCC. In fact, the IPCC goes to great length to explain that it is not wedded to a single policy approach.

It is of course the IPCC that raises the issues of development pathways as a starting point for everything that it does. The idea that the IPCC must focus on the FCCC or mitigation because governments demand that it does so is simply wrong.

Thanks!

Thanks, Roger.

Clearly I need to do a bit more reading in the IPCC and its relationship to the UNFCCC.

Thanks for the great article you wrote.It was very detailed and interesting information inside.I really liked "The A1 storyline and scenario". It is the most probable scenario I think will happen.Continue in the same way with your work.Have a nice day!