The SuperFreakonomics Global-Warming Fact Quiz

By the time you finish this blog post, you will understand why we differ from our critics in our conclusions.

As we write in SuperFreakonomics, there are many misconceptions about the facts surrounding global warming. Take the following true/false quiz to test your knowledge of the science, economics, and technology of global warming.

Global-warming science questions:

1. The Earth has gotten substantially warmer over the past 100 years.


2. Even if we were to immediately and permanently stabilize our carbon emissions at the current levels, or even cut these emissions substantially, climate models predict that Earth will continue to get warmer for decades.


3. When Mt. Pinatubo erupted in 1991, it spewed millions of tons of sulfur dioxide into the stratosphere. Scientists believe that the haze generated by the eruption reflected some of the Sun’s light, causing the Earth’s temperature to temporarily drop as a consequence.


4. Because the half-life of sulfur dioxide in the stratosphere is relatively short (on the order of one year), the cooling effects of the Mt. Pinatubo eruption faded within a few years.


5. Dark surfaces absorb more sunlight than light surfaces. Thus, all else equal, light surfaces cause less global warming because more of the sunlight that strikes these surfaces is reflected back into space.


6. Clouds, which are white or gray, are lighter in color than the oceans, which are blue.


The correct answer to all six of these questions, we believe, is “TRUE.” You can see our chapter on global warming (pp. 165-209) and particularly the endnotes (pp. 247-256) for citations and elaboration. It is our impression that none of the six scientific statements above is at all controversial among climate scientists. I do not believe that any of our global-warming critics would quibble with any of these facts.

And just to be perfectly clear, despite all the bluster that has surrounded our chapter on global warming, these are the six scientific facts that are critical to our analysis of geo-engineering in that chapter, a point I will expand upon below. We document many other interesting facts in the chapter, but these are the only ones that are central to our argument.

It is simply not the case that criticisms of the geo-engineering solutions that we highlight in the chapter arise because we get the scientific facts wrong, unless the critics think that any of the six statements above are false.

So let’s move on to the economic issues surrounding global warming, and let’s see if that is where we differ from the critics in our assumptions.

Global warming economics questions:

1. If the Earth’s warming leads to global catastrophe, that would be a really bad outcome.


2. Even when there is enormous uncertainty about the likelihood of future cataclysms, it makes sense to invest now in finding ways to avoid such cataclysms.


3. Economists estimate that the costs of reducing carbon emissions are likely to be upwards of $1 trillion per year.


The correct answer to all three of these economic questions is “TRUE.” These are the three key economic facts that are critical to the arguments in our chapter. The first question doesn’t require any further explanation. The answer to the second question has been hammered home by Martin Weitzman‘s work in the area, which we cite in SuperFreakonomics, as well as a newer paper that Weitzman has written. The third fact is based on the analysis of Nicholas Stern. These cost estimates are obviously highly speculative, but the true cost of reducing carbon emissions is likely to be within two orders of magnitude of this number.

As far as I know, none of our critics would disagree with any of these three economic facts about global warming. Indeed, Paul Krugman‘s attack of our chapter largely focuses on the misconception that we do not agree with fact No. 2, when clearly we do. Somehow Krugman has come to the conclusion that we are in favor of inaction, missing the main point of the chapter, which is that we think immediate and aggressive action is warranted, in the form of investment in (or implementation of) geoengineering solutions. Perhaps Krugman does not consider those steps taking action.

So if there is no disagreement on either the six key scientific facts or the three key economic facts, where is the disagreement coming from?

Perhaps it is coming from a lack of agreement over technological facts.

Global warming technological questions:

1. There exists an engineering design that provides a means of delivering enough sulfur dioxide to the stratosphere on a continuous basis to effectively cool the Earth. The estimated cost of building and implementing this technology is a few hundred million dollars.


2. There exists an engineering design that provides a means of increasing oceanic cloud cover by seeding such clouds with salt-water that is sprayed into the air by a fleet of solar powered dinghies. The estimated cost of building and implementing this technology is a few hundred million dollars.


The answer to these questions is once again “TRUE.” As we describe in SuperFreakonomics, the Seattle-based company Intellectual Ventures has designs for both a “stratoshield” (No. 1) and the cloud-seeding project (No. 2).

I don’t see how the critics could argue with the answers to those two questions. They might argue that the technology won’t work as Intellectual Ventures hopes it will, but there is no arguing with the fact that Intellectual Ventures has the blueprints to try to build these contraptions, and could have them up and working within a year or two.

With all of this as preamble, let’s get to the fundamental question we try to answer in the chapter:

If we need to cool the Earth in a hurry, what is the best way to do it?

Our answer to that question follows directly from the three sets of facts I presented above. Reducing carbon emissions is not a great way of cooling the Earth in a hurry for two key reasons: (1) even if we cut carbon emissions today, the Earth will continue warming for decades; and (2) reducing carbon emissions is expensive, with a price tag of at least $1 trillion per year. (There is a third problem with reducing carbon emissions, which is that it requires worldwide behavioral change, which will be hard to achieve. But even beyond that, carbon mitigation is not a great solution to the question posed above. There might be other significant benefits tor reducing carbon emissions — addressing ocean acidification, for instance.)

A much better approach, we conclude, is geoengineering. The scientific evidence suggests that either the stratoshield or increased oceanic clouds would have a large and immediate impact on cooling the Earth, unlike carbon-emission reductions. The cost of these solutions is trivial compared to the cost of lowering carbon emissions — literally thousands of times cheaper! Perhaps best of all, if something goes wrong and we decide we don’t like the results of the stratoshield or the oceanic clouds, we can stop the programs immediately and any effects will quickly disappear. These two geo-engineering solutions are completely reversible. Given the huge costs of global cataclysm and how cheap the solut
ions are, it would be crazy not to move forward with geoengineering research in order to have these solutions ready to go in case we decide we need to cool the Earth.

Why then, are our our conclusions so radically different from those of our critics? The answer:

We are answering a different question than our critics.

Our question, at noted above, is what is the cheapest, fastest way to quickly cool the Earth. Like every question we tackle in Freakonomics and SuperFreakonomics, we approach the question like economists, using data and logic to conclude that the answer to that question is geo-engineering. Not coincidentally, almost every economist who has asked the same question has come to the same conclusion, including Martin Weitzman and the economists at the Copenhagen Consensus.

But that is not the question that Al Gore and the climate scientists are trying to answer. The sorts of questions they tend to ask are “What is the ‘right’ amount of carbon to emit?” or “Is it moral for this generation to put carbon into the air when future generations will pay the price?” or “What are the responsibilities of humankind to the planet?”

Unlike the question that we are asking — How can we most efficiently cool the Earth fast? — the types of questions that environmentalists are trying to answer mix together both scientific issues and moral/ethical issues. If you have any doubts about this, watch Al Gore’s movie, in which he says explicitly that reducing carbon emissions is not a political issue, but a moral issue.

That is why someone like Ken Caldeira can agree with the facts presented in our chapter, say that the chapter is written in good faith, but still disagree with the conclusion that geoengineering is the answer. It is because the question Ken Caldeira is trying to answer is not the question we are trying to answer. The same is true of our critics. But instead of just making this simple point — that we are asking different questions — the critics have either intentionally or unintentionally confused the issues by making all sorts of extraneous arguments.

I do not mean to imply that the question we answer in the book is the most important question. It may be that the questions that environmentalists are trying to ask are more important and more interesting, but that certainly does not mean that we don’t want to know the answer to our question, a question that the environmentalists don’t bother to ask very often because they are focused on their more philosophical questions.

So for all the blogosphere shouting against our chapter, I have to be honest and say that I just don’t get it. I can’t understand why any environmentalist who really cares about the Earth’s future could say with a straight face that geoengineering doesn’t deserve a seat at the table as the global-warming debate heats up.


Environmentalists are more interested in social engineering than geo-engineering.


Two words: ocean acidification.

Dan Davies Brackett

this article's condescending tone caused me to unsubscribe. I thought you might care to know.

Larry Gritz

Here's the problem -- it may turn out that "warming" is not even the most civilization-threatening aspect of the atmospheric carbon crisis. Ocean acidification could disrupt the food chain in a way that's much more harmful to us in the long run.

So should we cut carbon emissions as rapidly as possible while ALSO investigating whether any geoengineering solutions may mitigate the damage we've already done? Absolutely.

But it's very risky to let anybody think for even a moment that the geoengineering your describe, even if it works, implies that we should continue to transfer carbon from beneath the ground to the atmosphere at as rapid a rate as possible.


The full physics perspective of reflection is more complicated than simply light/dark. The post mentions sunlight reflected back into space, but does not mention any other radiation. It's not that simple. Sunlight that makes it to Earth and is reflected by a white surface can reflected by the atmosphere back to Earth. This radiative trapping adds heat. Also radiative absorption by dark surfaces is still reradiated in infrared which can be absorbed, reflected, or transmitted by the clouds.

The full radiative picture involves making the surface paler and the atmosphere more transparent to white light AND/OR making the surface darker and making the atmosphere more transparent to infrared light.


So all we have to do is give this company millions and millions of dollars and it'll solve all our problems with no downside? It's like those trendy diets that let you eat nothing but cookies or bacon and you lose lots of weight, renew your spouse's affection and get a $500,000 home for $250 monthly payments!

There's obviously no downside if the company doesn't disclose it. Why would they lie?

Why would our beloved Freakonomists fail to think about ocean acidification or acid rain? Why, those issues obviously are just smears launched by a frenzied anti-Chicago crowd.


@MikeM: your analogy is not quite right. It's not a smoke detector -- everybody knows the smoke is already there. It's more like some people yelling "turn down the stove" and these guys are saying, "no, let's turn on the sprinkler system, if it works then you don't have to bother turning down the stove." But the unintended consequences of the water are not yet understood, plus that there are other people in the room who get rich from operating the stove and will use any excuse to delay or avoid turning down the stove, and that might get us all in trouble.

Jose Obiols

Have you read "The Skeptical Enviromentalist", of Bjorn Lomberg. I think he has some good ideas about global warming.


It's interesting that neither of the two scientists who you rely on in the book seem to agree that we should go with geoengineering now as the cheapest option. Caldeira obviously wants drastic cuts in carbon emissions, and in his post here, Myhrvold advocated geoengineering as a "last resort" or "insurance in case we don't get global warming under control" by other means.

There are a few things that the argument here doesn't account for. One is that the damage from climate change doesn't just come from higher temperatures, it also comes from changing precipitation patterns, ocean acidification, and other consequences. If we add a sulfur pump to our carbon emissions and stabilize global mean temperature, a lot of those damaging consequences will still happen, including higher temperatures in some places and possibly droughts in China and India, and we may get new problems like damage to the ozone layer. Do we have good models & cost estimates of a world with this?

Geoengineering also isn't as easily reversible as this arguments suggests. Some of the problems that it might cause (like ozone depletion) will continue after we stop pumping sulfur dioxide into the atmosphere. Stopping would also cause a sudden spike in temperature, which could be more problematic than a more gradual rise.

It's also not clear what the long term plan is. Are we just going to keep pumping larger and larger amounts of sulfur into the atmosphere over the next century plus as carbon emissions continue to increase? How much sulfur would that require, and can the planet handle an atmosphere with that much sulfur? Or are we planning on reducing carbon emissions as well, to reach a more manageable equilibrium (with our geoengineering filling maybe a 5 degree gap rather than a 10-15 degree gap)?



Bingo, Mark ... ocean acidification is the one thing geoengineering of the type they're proposing here will not solve in the least. So the oceans will keep growing more acidic as they absorb more and more carbon dioxide, and the entire food chain will suffer (when shellfish can no longer make shells because of acidity).

Second problem with the arguments here: Once you start pumping massive amounts of sulfur dioxide into the air to cool the atmosphere, you're committed to it ... because the moment you stop, the climate goes right back to where it was. If CO2 concentrations at that point are much higher than they are today (and they will be, if we focus on geoengineering instead of on carbon emissions cuts), we are in for a heck of a lot of rapid, dramatic warming that till that point had been delayed.

Third problem: if much of the world isn't comfortable relying on the advice of thousands of the world's top climate scientists (ie, that we need to start cutting carbon emissions ASAP), how many of you will be comfortable with an international body of some kind (or maybe even a few countries acting unilaterally) agreeing to a massive geoengineering experiment? Are you willing to risk that your part of the world won't suffer severe drought (one likely side-effect) as a result? Are you willing to consign another region to that drought, with possibly deadly consequences?

It's funny how so many people who label climate change science with the epithet "religion" are willing to play God themselves with a giant geoengineering experiment of many unknown consequences.



I think you've one fairly major mistake when you say the six facts you present are without dispute. In fact the first is subject to both interpretation and dispute. You say the earth has warmed "substantially" in the last 100 years. In fact the warming is only about 0.75 degrees C over that period, this is a link to the Hadcrut data:

If 2% or 0.75 degrees C is "significant" then you might be right. Some might not call it "significant" considering the daily variablity of temperature and the location variation.

More importantly, there are substantial problems with the data underlying the various temperature reconstructions of the last 100 years. Those include urban heat islands, station drop out, over reliance on North American observations, inconsistent techniques, time of day problems and many others.
While it seems likely the temperature is up over the last 100 years, I doubt the rise is greater than the uncertainty in the data caused by its many problems.

I look forward to reading you new book and enjoyed the last.



Is the Earth the warmest it has ever bee? Absolutely NOT. My geology training included the FACT backed up by the geologic record that glaciers extended almost all the way to the equator and have virtually disappeared over the past few million years.. Since the last major glacier (Ice Age) the Earth has been warming for tens of thousands of years. Looking at the last 100 years and trying to predict the future? You might as well read tea leaves. Wall Street, and the politicians that like to tax Wall Street, see gold the new carbon economy, and the Environemntalist get to see and end to oil exploration and desecration of the Earth by man. A win-win, right? Let nature decide if man is welcome on the earth and lets stay the course!


LG@27: You're close. People are saying, "Quick, rip the stove out and cook only with a large magnifying glass!"

And that's not an acceptable solution, either.

Jens F!

What I dont understand is, does it have to be one or the other, cant it be both?

It is unrealistic to cut down carbon dioxides significantly in the near future. But do we really just want to build more and more ships and more and more rockets to mitigate the screw-ups we make?

And considering all the other negative consequences of fossil fuels. Dependence on foreign fuel for the US. Ocean acidification. Conflicts. And remember, oil is running out, maybe not in the near future, but eventually.

Denmark is the worlds 41st largest exporter of oil, okay, 41st is not great, but it is still an export [1]. But Denmark is still very focused on renewable energy. According to the UN 17% of the Danes energy consumption is renewable. And climbing. 20% by 2011, and 30% by 2020. There are 5,300 wind turbines, one per 1,000 Danes.

I think in 50 years time, 100% of the energy will be renewable. But only if we decide to go that way. And we need to go that way. In the short run, let us consider geo-engineering. Medium term, use renewables.One does not exclude the other. The battle starts today. The prospects are dim, but fortunately, we have two strong horses to bet on. Lowering emissions and geo-(re)engineering.

If we do not act on both fronts now, our children will be embarrassed about our actions. Climate change is the single biggest threat to man kind in recent history, maybe with the exception of total nuclear war. Nothing has the potential for killing more people, forcing more people from their home and loved ones, and causing more damage to buildings and other assets, than - global warming.

Let us hope, no, let us pray for a strong reduction in emissions from the Cop15 summit. No, better yet, send a letter to your local congressman or senator, requiring action now. Letting our children pick up the bill, is not how I was raised.




I have not read the book this article is referring to, but strictly from an economics/investment perspective, it seems to me a highly speculative conclusion that geoengineering is the cheapest long term solution.

If the goal is to prevent the potential cataclysm global warming presents, all long term costs of every solution must be examined. For example, I find it unlikely that continuously spraying the atmosphere would not affect ecosystems and human health, both of which would be costs directly associated with this solution, and there are many more similar examples.

Additionally, by "fixing" global warming with a bandaid in the short term, there is no incentive to treat the growing wound beneath it. If the bandaid fails for whatever reason, the cataclysm we face today could pale in comparison to that future tragedy, thus making this a speculative solution.

The prudent investor would minimize risk and choose short term loss for long term stability and gain.


Max Kaehn

Your reasoning runs aground on the rocks of reality when you use "all else equal". It's not all equal. You are neglecting many things-- off the top of my head, that list includes the costs of ocean acidification, the costs of acid rain that will result from all that sulfur dioxide coming down, the effects on crop production due to reduced sunlight. This kind of narrow analysis, where you magically get to hold all other factors constant than the ones you are varying, does not belong to the domain of real-world problem-solving.

Wondering in CO

A nice piece of marketing. But not an altogether honest accounting of the problem. As an atmospheric scientist who teaches courses with the title "Climate Science and Policy", I find it easy to agree with their scientific propositions, and am willing to grant their economic ones for the purpose of discussion. However, it seems to me that Levitt and Dubner have foisted off a sloppy bit of reasoning on fans of snark eager to avoid carbon diets. I am amused that they are 'shocked' at being called on it.

It is likely that the climate on the NewEarth that Levitt and Dubner are engineering will be quite different from the climate we enjoy on Earth. Temperature is not the only climate variable of interest. For example, rain may well fall in different places at different rates and at different times. Robock et al. (GRL 2009?) argue that NewEarth will have more African and Asian droughts than Earth does in the case of stratospheric albedo enhancement. Will NewEarth be as congenial for agriculture as Earth is now? This is one of many important questions not addressed by our geoengineering enthusiasts.

Nor do they address the end of the experiment. What happens when the geoengineering does have unexpected consequences that convince us to stop? Soon after (the stratospheric aerosol and artificial clouds go away quickly, while the accumulated greenhouse gases are here for centuries), the forestalled warming will resume and quickly catch up with the levels that would have occurred in the absence of the particles. Then what?

So, while their particular fixes (pun intended) may pay off for some of us, they are likely just pushing the problem of accumulated greenhouse gases onto future generations, and compounding the accumulation and problems. The kindest way to describe this is as a failure to embrace intergenerational justice.

Additional reviewer's comments:
Dubner and Levitt tell us that sea level rise results from the warming of the seas and not only from glacier melt and that airborne particles have a significant impact in the earth's energy balance. The uninitiated denier will thrill at these revelations, thinking that they somehow damage the case that CO2 warms the atmosphere and poses a threat. Dubner's and Levitt's tone and blurbs suggest that they are offering revelations. Let me assure the public that the climate community has appreciated these facts for a long, long time and that their quantitative representations are given due weight in the IPCC reports. They do not change their conclusions: CO2 warms and more CO2 warms more. If we do not stop soon, our posterity will not like it.

Levitt claims that he just wants a place at the table for geoengineering. He need not worry, research continues. The real worry is that geoengineering will gain a gloss as being a cheap and easy solution. Actual research done to date suggests that it will not be a solution . But selling a lot of snarky books may well contribute to the contrary illusion. The guys who style themselves as revealing the real inner workings of an obscure world have generated a fog in Chapter 5. Thanks a lot.

Chuck Wilson
Golden Colorado



Mr Levitt, any rational person would agree with your argument, but the people that are criticizing you are pseudo-religious fanatics. In their eyes you have committed blasphemy, and thus all your arguments will fall on deaf ears. It's like trying to argue with a brick. Just accept that some things cannot be changed and save your energy for something else.


It is interesting that the first economic question is:

1. If the Earth's warming leads to global catastrophe, that would be a really bad outcome.

Who can argue with that?

The real question that has not been adequately answered is if the earth's warming will lead to a global catastrophe. There is no evidence to suggest it will, and in fact, there is evidence to suggest the exact opposite. The greatest advances in human history have been during periods of global warming.

The other real issue is that the whole global warming issue is not about the environment at all, but about controlling human activity, particularly economic activity. It's about massively constraining the free market through regulation and central planning. The whole movement is nothing more than a fascist movement to increase government control over the economy.

Joel K.

None of the above speaks to one of the major points of criticism - the "false consensus" in the 70s that there was a global cooling trend and the parallel to the real consensus of the global warming trend now.

Why would you choose to include this in your chapter if for no other reason but to (dishonestly) discredit the current consensus on global warming?

Frankly, it appears there is a lot of cherry-picking going on here in order to support the case for geo-engineering. There are certainly valid concerns coming from your critics. Instead of addressing these concerns analytically, you have painted your critics as having political motives to challenge your assumptions.

By definition, SuperFreakonomics seeks to challenge conventional wisdom. Thus, such a challenge should be able to withstand counter-challenges. So far, I don't think you've passed this test.