Bryan Caplan  

Cartoon Introduction to Climate Change: Yoram's Last Word

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Frank on Phony Credentials... Partial Response to Yoram Baum...
I offered to give Yoram the last word in our exchange.  Here it is.

P.S. Yoram's non-fiction graphic novel officially releases on June 5.  That week, with his kind permission, I'll be posting a few pages from his book.


Let's focus on the major issues in my exchange with Bryan, which now cover 4 posts. (Here's post #1, which was Bryan's review of my Cartoon Introduction to Climate Change, and then here's #2 and #3. And I promise that after this post I'll stick to the comments section on this thread!)

1. Climate science basics

Here I'm delighted to report that Bryan and I agree.

I asked:

Are you comfortable saying that carbon dioxide is a greenhouse gas? That human emissions of carbon dioxide are raising atmospheric CO2 concentrations? That global temperatures have been increasing over the past century? That humans are partly responsible for those increasing global temperatures? That "it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century"?

The Cartoon Introduction to Climate Change provides my answers to these questions (Yes, Yes, Yes, Yes, and No I'm Not Comfortable Saying This But I Am Comfortable Saying That The Vast Majority Of Scientists Are Convinced), so I'd like to hear what you have to say about them, Bryan. Can you provide answers?

And he did:

My answers on all counts are the same as your answers, Yoram: Yes, Yes, Yes, Yes, and No I'm Not Comfortable Saying This But I Am Comfortable Saying That The Vast Majority Of Scientists Are Convinced.

This is great; thank you Bryan. But it would have been better for Bryan to own up to this years ago, back when he was lauding Superfreakonomics and calling global warming "instrumental-looking" and asking "What happens if you regress annual global temperature 1880-2011 on CO2 [and other stuff] like church attendance per capita, the Dow Jones, televisions per capita, etc"?

If more economists like Bryan were upfront about their agreements with basic climate science then I would feel better about not having time to respond to people like David Henderson, who goes to great linguistic lengths in an effort to argue that global temperatures have not been increasing over the past century. Plus I wouldn't have to jaw-jaw with people in the Comments section or spend my time reviewing the treatment of climate change in economics textbooks. (The books from Mitt Romney's top economic advisors, Greg Mankiw and Glenn Hubbard, both earned a top grade, so Bryan please tell your neighbors Cowen and Tabarrok that I'm hoping their forthcoming edition can improve on the C+ they earned last time.)

Bottom line: Thanks for acknowledging your comfort level with basic climate science.

2. Geoengineering

My main response is that we appear to be having a communications problem. In my cartoon book, I write that injecting sulfur particles into the atmosphere "won't stop ocean acidification." In post #2 of the current back-and-forth that Bryan and I are having, I again bring up ocean acidification and link to a RealClimate article in which scientists discuss some of their concerns about sulfur injections, including that it won't stop ocean acidification. Here's Bryan's response, in post #3: "One of the reasons I read Yoram's book, by the way, was to search out additional analysis of geoengineering. By my count, he's now missed two opportunities - his book and his response to my review - to expand my knowledge of the topic." I'm flattered that you have such high standards for my cartoon books, Bryan, but let me try again: What do you think about ocean acidification?

PS. In addition to our joint communications problem, I think that geoengineering advocates like Bryan have a communications problem of their very own, namely that they tend to oversell their position. This appears in Superfreakonomics, when Levitt and Dubner claim that "perhaps the single best objection" to their garden hose idea is that "it's too simple and too cheap." And it happens with Bryan, who starts off writing that "all things considered, geoengineering looks far superior to other policy options on the table" and that "climate activists sorely need to hear... that leading techno-fixes really do look vastly cheaper than abatement" but then ends up writing that "As best as I can gather... [after] I spent a week reading about geoengineering four years ago... [the complaints of critics] seemed weak." No wonder the general public has anti-smartest-guy-in-the-room bias when it comes to, well, just about everything.

PPS. While I'm at it, let me put a confession on the table. There are many walls that we can bang our heads against, but each of us only has one head. So we need to pick. Jeff Miron's got drug legalization, Bryan's got immigration reform, and I've got revenue-neutral carbon taxes. Bryan thinks that geoengineering is a low-cost solution to climate change, and I think that revenue-neutral carbon taxes is a low-cost solution to climate change.

Bottom line: I'm going to continue working on revenue-neutral carbon taxes, especially if geoengineering folks keep failing to address ocean acidification and all these other concerns from climate scientists.

3. Cost-benefit analysis

I think that trying to use CBA for climate change is like trying to use GPS in a cave: great idea, it just doesn't work very well.

Bryan thinks we need to try--perhaps because of a philosophical belief that CBA always passes CBA??--and he "immediately picture[s] multiple variants on the Wheat and Chessboard Problem" to convey issues regarding discount rates.

I hate to play the "because I'm the Mom" card, but look: I've now written three cartoon books, and that's three more than Bryan has written. (I do like his animated videos though.) And I feel pretty comfortable saying that the Wheat and Chessboard Problem is a lousy fit for cartoon books. (Animated video, yes. Cartoon books, no.) And I feel very comfortable saying that a Cartoon Climate Change book that tried to tackle CBA in a meaningful way would not have much room for anything else. (Remember that Cartoon Micro spent a whole chapter on the basics of discounting, and a whole chapter on the basics of expected value; the long-time-horizon and fat-tail issues with climate change are considerably more complicated, so I'd peg that at 4 chapters already, with a complete treatment taking most if not all of a 16-chapter cartoon book.)

Bottom line: Why does my book not spend much time on CBA? Because I'm the Mom. You go write a cartoon book, Bryan--my psychotherapist tells me you really really want to!--and then we can come back to this.

4. Insurance

Bryan says that "Costco.com sells a year's supply of dehydrated food for $1499.99" and ask me if I've bought it. The answer is No... because it doesn't include Cougar Jim's Freeze Dried Water!

On a serious note, I'll admit--I'm not ashamed!--that in the days before Y2K I went out and bought a few bags of ice and some extra supplies. And even now we try to keep a 3-day emergency supply of food in the house, per Three Days Three Ways. (Visit ready.gov and think about whether you should too.)

Bottom line: Low probability outcomes that are catastrophic really is a pretty good focal point for insurance. Whether you can get insurance at a reasonable price is a good second-round question--as are concerns about whether your dehydrated food would last much more than a week before somebody with a gun overcame their reverence for private property rights and came for it--but happily for climate change we've got revenue-neutral carbon taxes.

5. Comparative advantage

This is actually a new addition to our list, but I can't resist because Bryan keeps harping on it. So here's a question for you, Bryan: In Superfreakonomics, Nathan Myrvold is introduced as somebody who wants to be "every kind of scientist", as being "so polymathic as to make an everyday polymath tremble with shame." Did this set off alarm bells for you? It did for me, so I wasn't surprised to learn that Myrvold was wrong in writing that "the problem with solar cells is that they're black." You, on the other hand, didn't express any concerns about Myrvold, and neither did Levitt. Perhaps there some new kind of bias here that's worth examining?

Bottom line: You are clearly a very kind person, Bryan--for example, you post my rants on your blog, and I'm grateful for that--but in future you should be less kind to people you agree with.



COMMENTS (20 to date)
Josiah writes:

Here's what I think about ocean acidification:

1) ocean acidification is a serious problem, but it's not as serious as some of the other potential harms from warming (e.g. Sea level rise, extreme weather, etc.). Given that the expected damage from warming in the coming century is just barely more than the expected costs of mitigation, I do not see how ocean acidification only can justify those costs.

2) while some forms of geoengineering would not stop ocean acidification, there are other forms that potentially could (e.g. Iron fertilization).

3) very few people would argue for immediate implementation of geoengineering. Instead what advocates typically want is more research so we can get a better grasp of the risks and benefits involved.

4) as an added bonus, talking about geoengineering when discussing climate change tends to make conservatives more accepting of climate science, which I gather is something that is important to you.

MichaelT writes:

I don't know that I would call a revenue neutral carbon tax insurance. The link he has shows that CO2 emissions per capita are the same in BC as in the rest of Canada and it is revenue neutral, so how is a carbon tax insurance? If it doesn't raise additional revenue to invest in climate change mitigation or adaptation and does not make significant reduction in CO2 use, how is this insurance against climate change. That my biggest reservation against the carbon tax: it seems that if it is small and revenue neutral, it does nothing about the problem while creating a new bureaucracy that can be captured by special interests, or if it is high it will hurt economic growth to the point where the costs outweigh the benefits.

Eric Falkenstein writes:

'because I'm the mom'...lacks zing coming from a male, but especially when followed by 'I've now written three cartoon books'. Aren't picture books aimed at children necessarily cartoonish simplifications, useful for fantasy and inspiration, but highly tendentious? Economists typically fetishize rigor and modeling too much, but cartoons are probably even more constraining and imply substantive omissions.

MikeP writes:

Again there are disagreements about insurance and geoengineering as distinct issues.

But the key takeaway from the economics of climate change is that geoengineering is the insurance.

The conclusion I draw from reading Nordhaus and others for the last 15 years is that the optimal carbon tax policy is only modestly better than the second best policy, which is doing nothing about carbon emissions at all.

And how could it be otherwise? Wealth increases exponentially. CO2 emission increases at most linearly with wealth. And temperature increases sublinearly with CO2. So the exponential dominates, and the optimum policy can only fine tune that exponential.

This conclusion holds unless there are severe nonlinearities that lead to true catastrophes. And that is when you pull out the insurance policy of geoengineering. Geoengineering should not be used to effect modest cooling or to tune the exponential because it brings too many unknowns into play. Rather geoengineering is for use when there is no other option.

So it is not right to say (1) there is a 5% chance of catastrophic nonlinearities under business as usual, and (2) here is a 10% chance geoengineering will do more harm than good, therefore they are both bad ideas. The actual severe harm to humanity happens only when both (1) and (2) come up bad, which, using these numbers and assuming they're independent, is a 0.5% probability.

Handle writes:

Yoram Bauman can't avoid CBA or geoengineering just by throwing down 'acidification' like a trump card. The obvious response is, "Ok, so let's do a CBA on acidification too." That CBA would come out bad for his side too.

Ocean acidification is pretty overblown. The ocean is slightly basic and has become very slightly more neutral after a century of fossil fuel burning, from about 8.1 pH to 8.05 pH. Even the more extreme forecasts don't get to a point that is genuinely devastating to current species of aquatic life. That's the claim that is usually made, that the coral reefs will passively dissolve and so forth and the claim that that there is not enough biodiversity to adapt and accommodate changes of 0.01pH neutralization per decade, which is pretty doubtful.

The ocean does not have consistent temperature, salinity, or acidity from place to place or time to time in the same place, and yet species of every genus have adapted to live almost anywhere and one would expect species that live in slightly less basic environments to immediately take over any habitat made available by the very, very slow demise of competitive species.

Ice ages came and went pretty fast in geological / biological time, and had huge impacts on ocean salinity and acidity, without those concentrations having any obvious marine life impact as shown in the paleontological record aside from what is explained by the cold. Ocean salinity, for example increased from the average of 34.7 psu today all the way up to 37 in the Southern Ocean during the last glacial maximum, but which anyway isn't any higher than it typically is in the Mediterranean or Red Sea, despite the sea level falling over 120 meters.

The ocean is a big place. It doesn't make a lot of sense to say that the ocean is so large that it can absorb all the 'missing, hidden heat' that was forecast by the simulations of all the GCM's but not observed over the past 15 years, but that it is not so big that it can't absorb some extra carbonate ions as the equilibrium atmospheric concentration of CO2 increases only 1 part in 10,000 in the next half century.

David R. Henderson writes:

@MikeP,
Excellent comment. This is one of the best I’ve seen on Econlog in weeks. Next time I teach the topic, I’ll probably use this comment as a short reading.

David Friedman writes:

"But I Am Comfortable Saying That The Vast Majority Of Scientists Are Convinced"

This sort of statement bothers me not because of what I think people do or don't believe about AGW but because of its use of "scientists." Nobody knows what "the vast majority of scientists" believe about warming, since the vast majority of scientists have nothing to do with climate science and no reason to have expressed an opinion on the subject. It isn't even clear that "scientist" is well enough defined to make such a claim meaningful.

Urstoff writes:

How do you even construct a policy without even some sort of really rough CBA? Is a $100/ton carbon tax (or whatever) just pure guesswork?

Daublin writes:

From top to bottom:

1. Basic science. I'm glad there is agreement here. I would ask why it is so popular to assume that other discussants aren't familiar with information that really is well-grounded.

2. Geoengineering. At a guess, Bryan didn't think acidification was worth responding to. It's hard to be clear if that is really the issue that drives you. What about geoengineering techniques that don't affect pH--are you saying all of them are definitely not workable?

3. Cost-benefit analysis. You are dodging Bryan's claim that CBA is a good way to think about CO2 policy. You previously wrote that CBA is just plain bad for CO2 policy, but when he objected, now you are just saying that it is bad for a comic book. For what it's worth, I am still unclear why you would make even this weaker claim. If someone said, let's just use up the Earth and then all fly to Mars, you would say it's too expensive. That's CBA.

4. Insurance. You did not respond to Bryan's economic analysis. You told two good stories and then said "bottom line" and repeated your claim.

5. Comparitive advantage. I'm not completely sure what you are getting at, but I would raise Michael Mann and most of the IPCC authors as examples. Their expertise is in atmospheric science, but they are happy to weigh in on economic issues such as insurance.

Mark Bahner writes:
And how could it be otherwise? Wealth increases exponentially. CO2 emission increases at most linearly with wealth. And temperature increases sublinearly with CO2. So the exponential dominates, and the optimum policy can only fine tune that exponential.

Indeed. I made a comment on David Friedman's blog a while ago to the effect that it only takes a few minutes of analysis to see why it's unlikely that global warming will be a big deal. That's the sort of analysis to which I was referring when I made that comment.

So one key question is: does economic growth really increase exponentially? I maintain that it is likely to not merely increase exponentially in the 21st century...it's likely to increase super-exponentially. That is, the percentage annual growth is likely to increase.

So when I meet advocates for the people of the present sacrificing for the people of the distant future, I wonder if they think that economic growth is *not* likely to increase exponentially. That's why I asked Yoram for his opinions on what the world would be like in 2050 and 2100. But like everyone else who advocates for the people of the present sacrificing for the people of the distant future, he blew me off.

Sooo...desperate times call for desperate measures. I'll make this public wager: I bet Yoram $10 that he will never provide his answers to fill in the table on my blog:

What is the Morality of the Less-Well-Off Sacrificing for the Better-Off?

MikeP writes:

So when I meet advocates for the people of the present sacrificing for the people of the distant future, I wonder if they think that economic growth is *not* likely to increase exponentially.

I think there is no popular understanding of just what 100 years of economic growth will do for humanity's wealth. That's probably unsurprising. But I have watched with astonishment as climate scientists utterly ignore the opportunity cost represented in their own projections of what a climate-conscious world would be missing.

As I commented back in 2007 after AR4 came out:

And consider that the difference in year 2100 per capita world GDP between the high-growth A1 course and the environmentally conscious B1 course of the IPCC SRES is 30,000 1990 dollars.

You know, it actually belies a subtle bias for anti-growth policies or a blatant failure to understand economics that the fantastic difference between the SRES per capita GDPs doesn't raise alarm bells from more IPCC scientists.

If we could ask our great-grandchildren whether they would rather have 60% greater income or have the global temperature between 2 and 4 degrees Fahrenheit cooler, what would they answer? That it occurs to no one in the policy push today to even pose the question speaks volumes.

Todd Kreider writes:

As a guy with a physics degree,(sadly, my PhD is in economics), I don't take someone like Yoram seriously at all. It isn't an insult, but he has no idea what he is talking about with respect to the science of global warming. None at all, and he reveals it in his posts.

Those writing here have been very kind to him -- after all, he seems like a great guy. But if he had to present his arguments in front of physicists, he wouldn't last 3 minutes before they (politely) ripped him to shreds.

Mark Bahner writes:
And consider that the difference in year 2100 per capita world GDP between the high-growth A1 course and the environmentally conscious B1 course of the IPCC SRES is 30,000 1990 dollars.

I think the IPCC "solved" this problem by drastically cutting the GDP projection for their highest-emissions scenario, RCP 8.5, versus A1FI.

Note how RCP 2.6 (the lowest-emissions) scenario is now the highest-GDP scenario:

The IPCC, shakin' and bakin' in AR5

Mark Bahner writes:
I think there is no popular understanding of just what 100 years of economic growth will do for humanity's wealth.

Yes, another important point is that wealth actually accumulates, as GDP/income grow exponentially. For example, most of the largest buildings in the world built in the last decade or two will probably still be around in 2100...much as the Empire State Building and Chrysler Building are around now.

MikeP writes:

I think the IPCC "solved" this problem by drastically cutting the GDP projection for their highest-emissions scenario, RCP 8.5, versus A1FI.

Interesting link. But from that article:

In contrast, the RCP8.5 was based on a revised version of the SRES A2 scenario; here, the storyline emphasizes high population growth and lower incomes in developing countries”.

RCP8.5 is based on A2, not A1, thus sweeping the high growth, high wealth potential future under the rug.

This isn't the only time they've been so blatantly disingenuous. I commented on a particularly egregious case last year:

Alerted by the mention of the SRES scenarios, we can find in Box 1 of the Executive Summary the following astonishing fraud:
In this report, the two SRES emissions scenarios recommended for use in impact studies are a higher emissions scenario (the A2 scenario from SRES) and a lower emissions scenario (the B1 scenario from SRES). These two scenarios do not encompass the full range of possible futures: emissions can change less than those scenarios imply, or they can change even more. Recent carbon dioxide emissions are, in fact, above the A2 scenario. Whether this will continue is unknown.

Their two recommended scenarios are A2 -- the dysfunctional, globally divided high carbon future -- and B1 -- the utopian, globally harmonized low carbon future. Only this way could they get the future GDP of the low carbon projection to exceed that of the high carbon projection and take out the argument that, other things equal, greater carbon consumption correlates with greater wealth.

Yoram Bauman writes:

I am on holiday and therefore overcome with an insatiable desire to give people a chance to win my money... so let's start with @Todd Kreider, who writes:


As a guy with a physics degree,(sadly, my PhD is in economics), I don't take someone like Yoram seriously at all. It isn't an insult, but he has no idea what he is talking about with respect to the science of global warming. None at all, and he reveals it in his posts... Those writing here have been very kind to him -- after all, he seems like a great guy. But if he had to present his arguments in front of physicists, he wouldn't last 3 minutes before they (politely) ripped him to shreds.

Okay, @Todd, how about we both send a check for $500 to Bryan, who can keep $50 and then pay $150 to ask a physicist of his choice to review my Cartoon Introduction to Climate Change. If the physicist agrees that I have "no idea what [I am] talking about with respect to the science of global warming", then you get the remaining $800, and otherwise I get the remaining $800. Please Mister let's do this!

Next: @MikeP, who writes

The conclusion I draw from reading Nordhaus and others for the last 15 years is that the optimal carbon tax policy is only modestly better than the second best policy, which is doing nothing about carbon emissions at all... And how could it be otherwise? Wealth increases exponentially. CO2 emission increases at most linearly with wealth. And temperature increases sublinearly with CO2. So the exponential dominates, and the optimum policy can only fine tune that exponential... This conclusion holds unless there are severe nonlinearities that lead to true catastrophes.

@David R Henderson is so impressed that he writes "@MikeP, Excellent comment. This is one of the best I’ve seen on Econlog in weeks." I would have to agree---most EconLog comments are much worse!---and in particular I am most impressed by the fact that your "exponential dominates" argument is so powerful that you don't need either time frames or units: "x grows exponentially, and y grows sublinearly, so the exponential dominates!" This is so brilliant that I am overcome by a desire to trade with you, @MikeP. Here is what I propose: each year beginning with this year (x=0) I will give you 10^(.001x) grains of sand in exchange for 100 bricks of gold. Clearly the exponential dominates so how can you say no? There are no nonlinearities, and no true catastrophes that I can see. (And, heck, the bricks of gold are not just growing sublinearly, they're not growing at all!.) Please Mister let's do this! (PS. Try googling "Weitzman reactive damage function." Or for that matter go read Nordhaus again; if all he had to say was "the exponential dominates" then why would he have to do DICE modeling or write giant books? And PS both Nordhaus and Weitzman had nice things to say about my book :)


Beyond that:

@Josiah: I spent about 1% of the Cartoon Climate book talking about geoengineering, and that seems reasonable. (Carbon pricing got about 6%.)

@MichaelT: Please re-read my post on the BC carbon tax, which has reduced emissions without hurting the economy.

@Eric Falkenstein: Cartoon books are not "picture books aimed at children"... at least mine aren't :)

@Handle: Go read NOAA: "Since the Industrial Revolution, the global average pH of the surface ocean has decreased by 0.11, which corresponds to approximately a 30% increase in the hydrogen ion concentration." As for whether a 0.11 decrease in pH might be a big deal, just for comparison sake: "A drop of 0.1 pH units in human blood pH can result in rather profound health consequences, including seizures, heart arrhythmia, or even coma."

@David Friedman: I use my (admittedly unscientific) encounters with scientists at UW and lots of other schools. I ask them what they think about climate science and I listen to what they say.

@Urstoff: Do you use CBA for everything you do in life? If so... good luck with that.

@Mark Bahner: Thanks for the kind offer of $10, but I looked at your survey and it's too long for $10 to cover it at my standard hourly rate. Make it $20 with payment in advance (you can contribute to CarbonWA.org, or send me an email and I'll get you my address) and I'll fill out your survey, I promise!

@Daublin: Most of your questions can be answered by going back and carefully reading my previous posts and comments on this thread, e.g. "I am no expert on geoengineering. I certainly have nothing against considering it."

MikeP writes:

I am most impressed by the fact that your "exponential dominates" argument is so powerful that you don't need either time frames or units: "x grows exponentially, and y grows sublinearly, so the exponential dominates!"

The difference between global warming and your gold example is that for global warming the exponential is an input that drives the sublinear output. People don't burn carbon fuels for fun. CO2 is a collateral effect of economic production. As humanity gets wealthier -- exponentially -- it burns more carbon, raising the temperature less than the exponential.

Yes, one can respond with predictions of superlinear damages with temperature increase. I could have googled "wild guess" just as well as "Weitzman reactive damage function". I'll also note that even the high estimates of those functions kick in at temperatures no one predicts for this century.

Or for that matter go read Nordhaus again; if all he had to say was "the exponential dominates" then why would he have to do DICE modeling or write giant books?

I assume that Nordhaus does DICE modeling and writes the gold standard texts on the economics of climate change because he has a comparative advantage in it. Also, the models do indeed show that the exponential can be tuned by the optimal carbon tax. But, most critically, they show that aggressively limiting CO2 emission is amazingly costly to us and to later generations. This result is likely not at all obvious to proponents of hard temperature or CO2 concentration limits.

I have to admit, I haven't read Nordhaus's latest. I noted that his 2008 results hadn't changed materially from his 2000 results, which I commented on in 2007:

According to his models, the net present value of the cost of unrestrained global warming is 3.9 trillion dollars. He tests several policies for their cost and their benefit. Of all the policies tested, only the optimal policy -- taxing gasoline according to its environmental shadow price -- finds the benefit exceeding the cost. Yet the benefit is only 283 billion dollars while the cost is 92 billion. That is a very small dent in the 3.9 trillion dollar damage presumed due to global warming, and the net result is a decrease in warming for the next century from 2.5 degrees to 2.4 degrees.

Looking at this table, Nordhaus's optimal strategy may be the best strategy, but the second best strategy is to do absolutely nothing at all. Furthermore, considering how very close the optimal strategy is in its effects to doing nothing at all, any strategy that is much more aggressive is sure to have negative economic impact. Finally, given the improbability that governments can be moved to implement something close to the optimal policy without incurring vast deadweight costs to the economy, doing absolutely nothing about global warming looks like the best deal possible.

MikeP writes:

My apologies... The above quoted text is from a 2006 comment.

Yoram Bauman writes:

After being repeatedly rebuffed, I am giving up in my efforts to make trades or bets with @MikeP, @Todd, etc. As Bryan Caplan discovered seven years ago in "Why I Believe in Moderate Global Warming", it seems like commenters here are not interested in putting their money where their mouths are. Or, to put it in @MikeP's language, the quantity of baloney in the comments section grows exponentially while the willingness to put money down grows sublinearly (and in fact is constant at zero). The exponential dominates!

MikeP writes:

After being repeatedly rebuffed, I am giving up in my efforts to make trades or bets with @MikeP...

Well, if bars of gold are supposed to be analogous to damages from climate change, you should change the trade to start with 0 bars today and maybe give me some bars along with the sand for the next few decades. Then maybe the balance will shift and I'll start owing you bars of gold. But then I'm not too familiar with 60-year betting contracts.

Do you believe the SRES projections of global per capita GDP in 2100 of $80,000 for A1 and $50,000 for B1? Do you believe that that difference is worth the greater warming from B1's 1.8°C to A1's 2.8°C (if low-carbon energy becomes as cheap as high-carbon) or 4.0°C (if not)?

Do you believe that the only way to address climate change is to get ourselves off of A1-type scenarios and onto B1, thus giving up $30,000 per capita in 2100? Or do instead believe that revenue neutral carbon taxes can be applied to A1 to cut the projected warming at a lesser cost in lost wealth?

If you believe the latter, you believe the that the greater growth of the unrestricted energy A1 yields higher returns over a century -- i.e., you believe that the exponential dominates. If you don't believe the exponential dominates, then you should drop your advocacy of something as modest as carbon taxes and work to get the world on a B1-type scenario through more heavy handed means.

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