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Wedges Reaffirmed

By Rob Socolow

Robert H. Socolow, Professor at Princeton University. Credit: Princeton University. 

In August 2004, Steve Pacala and I published a paper in Science about climate change mitigation. Its core messages are as valid today as seven years ago, but they have not led to action. Here, I suggest that public resistance can be partially explained by shortcomings in the way advocates of forceful action have presented their case. Addressing these shortcomings might put the world back on the course we identified.

Let’s review the messages in our 2004 paper. The paper assumes that the world wishes to act decisively and coherently to deal with climate change. It makes the case that “humanity already possesses the fundamental scientific, technical and industrial know-how to solve the carbon and climate problem for the next half-century.” This core message surprised many people, because our paper arrived at a time when the Bush administration was asserting that, unfortunately, the tools available were not suited for addressing climate change. Indeed, at a conference I attended at that time, Energy Secretary Spencer Abraham insisted that a discovery akin to the discovery of electricity was required.

Our focus on “the next half century” was novel; the favored horizon at the time was a full century — and still is. We argued that "the next fifty years is a sensible horizon from several perspectives. It is the length of a career, the lifetime of a power plant, and an interval whose technology is close enough to envision."

In a widely reproduced figure, we identified a "Stabilization Triangle," bounded by two 50-year paths. Along the upper path, the world ignores climate change for 50 years and the global emissions rate for greenhouse gases doubles. Along the lower path, with extremely hard work, the rate remains constant. We reported that starting along the flat emissions path in 2004 was consistent with “beating doubling,” i.e., capping the atmospheric carbon dioxide (CO2) concentration at below twice its “pre-industrial” concentration (the concentration a few centuries ago).  

The paper is probably best known for having introduced the “stabilization wedges,” a quantitative way to measure the level of effort associated with a mitigation strategy: a wedge of vehicle fuel efficiency, a wedge of wind power, and a wedge of avoided deforestation have the same effect on the carbon dioxide in the atmosphere. Filling the stabilization triangle required seven wedges.

The wedge concept fosters parallel discussion of alternatives and encourages the design of a portfolio of responses. Each wedge is an immense activity. In talks about this work, I like to say that we decomposed a heroic challenge into a limited set of monumental tasks.

In short, in addition to a hopeful message that humanity is not helpless, the paper contains the sobering message that the job ahead is daunting.

Today, nine wedges are required to fill the stabilization triangle, instead of seven. A two-segment global carbon-dioxide emissions trajectory that starts now instead of seven years ago — flat for 50 years, then falling nearly to zero over the following 50 years — adds another 50 parts per million to the equilibrium concentration. The delayed trajectory produces nearly half a degree Celsius (three-quarters of a degree Fahrenheit) of extra rise in the average surface temperature of the earth. (Note that there is a three-year lag in the posting of authoritative global data. We used 2001 data in our 2004 paper, and 2008 data are available now. Thus, available data do not yet reflect the recession. Between 2001 and 2008, the emissions rate climbed by more than a quarter.)

Worldwide, policymakers are scuttling away from commitments to regulations and market mechanisms that are tough enough to produce the necessary streams of investments. Given that delay brings the potential for much additional damage, what is standing in the way of action?

The 2004 "wedges" paper assumed that the objective of global mitigation would require a flat emissions rate for 50 years, followed by a falling rate. Making the same heroic assumption today would result in substantial additional emissions. 

Familiar answers include the recent recession, the political influence of the fossil fuel industries, and economic development imperatives in countries undergoing industrialization. But, I submit, advocates for prompt action, of whom I am one, also bear responsibility for the poor quality of the discussion and the lack of momentum. Over the past seven years, I wish we had been more forthcoming with three messages: We should have conceded, prominently, that the news about climate change is unwelcome, that today’s climate science is incomplete, and that every “solution” carries risk. I don’t know for sure that such candor would have produced a less polarized public discourse. But I bet it would have. Our audiences would have been reassured that we and they are on the same team – that we are not holding anything back and have the same hopes and fears.

It is not too late to bring these messages forward.

Unwelcome news

Environmental science has brought unwelcome news – that the actions of our species are capable of changing the planet at global scale. Who wouldn’t much rather live on a larger planet, where our actions mattered less? It is counterproductive for advocates of prompt action on climate change to pretend that the new knowledge has only positive consequences, such as the stimulation of green jobs and elegant new technology. Global prosperity now depends on our species’ success at a totally unfamiliar assignment: to “fit” our many billions of people on this small planet, with its finite resources and finite capacity to withstand pollution. The job will be very hard and will require sustained focus.

Credit: iStockphoto.

Confronted with unwelcome news, human beings often shoot the messenger. Consider two earlier occasions. Galileo argued that the earth wasn’t at the center of the universe. For this, he was excommunicated.  Darwin argued that human beings were part of the animal kingdom, and he was cruelly mocked.  The idea that humans can’t change our planet is as out-of-date and wrong as the earth-centered universe and the separate creation of Man, but all three ideas have such appeal that they will fade away only very slowly.

In particular, just as steadily stronger evidence for the Copernican model and for evolution only gradually won the day, we should anticipate robust resistance to the message that we are fouling our own nest with fossil fuel emissions and deforestation. Armed with insights from psychology and history, communicators of the climate change threat will more deeply understand the hostility to their message. Perhaps, communication will be more effective when shared concerns are acknowledged.

Incomplete climate science 

It would be productive for advocates of prompt action also to concede that the message from climate science is not only unwelcome but also incomplete. Feedbacks from clouds, ice, and vegetation are only partially understood – thwarting precise prediction of future climate. The best and worst future climate outcomes consistent with today’s science are very different.

Pacala calls the worst credible climate outcomes “monsters behind the door.” Among the monsters are a five-meter rise in sea level by the end of this century, major alterations of the global hydrological cycle, major changes in forest cover, and major emissions of greenhouse gases from the tundra. The monsters open their door in a world of very strong positive feedbacks, a world that spirals out of control.  Today’s science cannot predict how much atmospheric change would let these monsters in, nor how quickly they could enter.

Policymakers assessing the case for immediate forceful action and members of the general public deciding whether to endorse the policymaker’s decisions want to know the full story – both the average outcomes and the extremes (the “tails” of the distribution). In reaching a judgment about whether to act forcefully now, some will give greater weight to best guesses, others to the tails. The more risk-averse will assign greater weight to the tails.

Why, at the intersection of climate science and climate policy, is there more discussion of average outcomes than nasty ones? As I have speculated in a recent paper, one reason is that average outcomes are safer to talk about, because the science is more solid; there is less risk of being accused of alarmism. Also, acknowledging terrible outcomes of low probability requires acknowledging the other tail – a world with rising emissions but little change for quite a while. I often hear that any concession to benign outcomes (or, more accurately, outcomes that remain benign for a relatively long time) will foster complacency. I don’t understand that fear. In my experience, when I tell someone “we could be lucky,” and then I pause, the listener completes the sentence for me: “or we could be unlucky.” The listener does not hear a lullaby.

Arguments for action based on what we don’t know reinforce those based on what we do know. To build a case on what we don’t know, however, takes courage, because it requires revealing how much experts disagree. There are many contending views about sea-level rise, for example. Advocates resist calling attention to the coexistence of contending expert views – far more certain than I am that lay audiences translate such conflicts into justifications for procrastination. I think it should be possible to convey that earth systems science is an evolving human enterprise where discordant views are the norm, and then to explain why certain issues have proved hard to resolve. My working assumption is that candor creates trust.

Editor's Note: This essay continues on page 2, and includes expert commentary from high level figures in the climate policy community such as: Nicholas Stern, Ralph J. Cicerone, Rep. Rush Holt, David Hawkins, Freeman Dyson, Chris Field and others.

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« Charged


By Robert Chianese (Ventura CA 93001)
on September 27th, 2011

Excellent article. To the two scenarios Socolow wants illustrated by exhibits, one with maximum impacts and one with “lucky” minor impacts from global warming, he might add a third, based on the imperfect science of climate research (which he acknowledges), one that shows where the “unknowables” are in that science and what might be their impacts on the theory of global warming itself. I agree that we need to cut C02 emissions, stop cutting forests, etc., but as a reasonable precaution not because of a certainty about their specific climate impacts.

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By Bill Fullkerson (Lennoir City/TN/ 37771)
on September 27th, 2011

Dear Rod:  I liked your essay very much.  I don’t like your title.  Wedges haven’t been reaffirmed except to point out that now it takes more wedges because emissions have not been slowed (except by recession).  You could go further and acknowledge the criticism leveled by Marty Hoffert that you need even more wedges.  Right now the climate change contrarians are winning by counter attack as a fall out of Climate Gate.  I distributed at our last LERDWG meeting a synopsis of a Yale Project on Climate Change Communication report “Politics & Global Warming: Democrats, Republicans, Independents and the Tea Party.”  The right half of the political spectrum thinks either that climate change is not happening or if it is it is not due to human activity.  I will send it to you separately.  I become more and more convinced that the win win situation is advanced technology and the realization that if we play the RD3-policy game right we can transform the energy system of the world at low total societal cost (negative when factoring in the reduced environmental externalities) if MacKinsey and Co. and the IEA with their Blue Map Scenario are right.

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By R Rands (Hobart, TAsmania 7005)
on September 27th, 2011

Australia is currently engaged in legislating a fixed price for carbon.  The relatively uncontroversial consensus on the reality of AGW is entirely overshadowed by the lack of consensus on large-scale intervention. 

The roadblocks you mention above, “recent recession, the political influence of the fossil fuel industries, and economic development imperatives in countries undergoing industrialization”, are exploited in parliamentary debate by opponents to the legislation.  F the most part, science and economics take a distant back seat to ad hominem attacks on party leaders.  For example, in Australia, the fixed price on carbon, intended to be kept constant for a few years then given over to the market of the day to adjust, through market mechanisms, is constantly referred to as a “carbon tax”, and the Prime Minister is repeatedly labelled dihonest and untrustworthy because she promised no new taxes by her administration. 

The aim of political parties in parliamentary democracies can be reduced to three words, depending on whether they are in or out of power:  “stay in power”, or else “get into power”.

Policies that assure long-term sustainability of the earth’s ecosystems are a very remote concern of the opinion leaders and powerbrokers of most political parties.  Green parties too, are influenced by the demands and constraints of wielding effective political influence within the political ecology of a parliament or legislature. 

Ideally, governments in power act also to implement policies that are humane, equitable and sustainable in the long term, but the overriding goal of staying in power dominates the setting and implementation of policy.  One critical example of the interplay between power and policy goals is the effect of corporate funding of lobbyists and political campaigns.  Currently, staying in power and getting into power seems to mean that opposing parties make the same fundamental compromises to the corporations who put annual returns to their shareholders ahead of wider concerns.  In this case, a global ecosystem model that is sustainable in the long term because it addresses the structure and feedbacks in the global carbon cycle. And so parties in power are failing to efficaciously stem AGW, to the detriment of all. 

That said, it seems to me that as well as discussing new ways to go about improving understanding and acceptance of AGW and AGW interventions, more could be done to support current interventions.  These interventions are uncontrolled experiments at a state scale, much as AGW is an uncontrolled experiment at a global scale.  Acknowledging the experimental nature of our interventions and offering plain-language information on their relationship to other successes and failures by other states, and smaller political units, might help the wider public to understand better what is going on.

Put simply, articles in the mainstream tabloid presses and popular TV programmes, short items that offer insight into the issues currently being debated, could be immediately useful.

Currently, it seems that it is the AGW sceptics who get the most coverage in the mainstream media, and it is the people influenced by their opinions who cry most loudly for further discussion and debate.  My first reaction to your article was negative, because you too wish further discussion, although it is the nature of a regrouping rather than a retreat. 

If your discussion is to have validity and relevance, would it not benefit by being grounded in current events, and by gaining relevance in its contyribution to the unfolding of current events?

Authoritative contributions to Australian mainstream media by people such as yourself and your colleagues could influence the debate here.  Think about it, discuss it, but not for too long.

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By Arno Arrak (Dix Hills, new York)
on September 28th, 2011

This paper is all about how to get politicians to start “mitigating” the effects of global warming that are leading us to certain doom if nothing is done. A “wedge” to them is an activity that will start from zero and in fifty years reduce carbon dioxide emissions by one gigaton per year.  But why even bother taking time with yet another tired re-iteration of remedies for a non-existent warming? Calling these projects wedges does not change the fact that the entire project rests on false premises about a coming climate change. I cannot explain this without using science so you will just have to take some basic science lessons from me. First, Arctic warming. We all know about the plight of the Polar bears, the Greenland glaciers retreating, the permafrost melting, the summer ice cover in the Arctic constantly decreasing. All true. But none of it is caused by the greenhouse effect you guys have memorized as the cause of global warming. Arctic warming had a sudden start at the turn of the twentieth century, paused in mid-century, then resumed about 1970 and is still going strong. This must be all new to you because I doubt that you read scientific literature. Prior to the start of this warming there was nothing but slow cooling for two thousand years. The two thousand year temperature curve for the Arctic looks like a hockey stick, only more so than that Mann-made curve does. The start of warming is extremely sudden but there is no corresponding increase in atmospheric carbon dioxide. By laws of physics it follows that this warming cannot be greenhouse warming. It turns out that its cause is warm water brought north by Atlantic Ocean currents. It began as a rearrangement of the North Atlantic current system at the turn of the twentieth century that directed warm water of the Gulf Stream into the Arctic. A recent Arctic expedition by Spielhagen et al. (Science, January 28) measured water temperatures directly. They also took a foraminiferal core near Svalbard that gave them a two thousand year temperature history for comparison. They report that temperature of the warm currents reaching the Arctic now exceeds anything seen within the last two thousand years. That is the explanation of why the Arctic is warming, and it has absolutely nothing to do with the greenhouse effect. It follows that none of the observations of Arctic warming we know of or have heard of can be used as proof of global warming. Which leaves mighty few observations that can still be used for that purpose. Next: how do you measure global warming? There are thermometers of course but for the last 31 years we also have satellite temperature measurements. They use Oxygen microwave emissions from the lower troposphere which are thermally excited as a proxy for local air temperature. Satellite data also cover both hemispheres and the oceans uniformly which cannot be said of land-based temperature curves put out by NASA, NOAA, and the Met Office. But there is a major discrepancy between satellite temperature measurements and these ground-based temperatures. In the eighties and nineties the ground-based temperature curves show steady warming but satellites simply cannot see it. What they do see is a temperature oscillation, up and down by half a degree for twenty years, but no rise until the 1998 super El Nino arrives. This is a major discrepancy because Hansen’s testimony to the Senate in 1988 used the existence of this warming as proof that greenhouse warming had arrived. Satellites also show a step warming that started in 1998, raised global temperature by a third of a degree in four years, and then stopped. That non-greenhouse warming is the only global warming within the last 31 years. It amounted to half of what is allotted to the entire twentieth century. The first part of twentieth century warming took place between 1910 and 1940. There was no warming from the end of this period to 1998, a stretch of more than fifty years. This leaves no period within the last century that can be designated as a greenhouse warming period.
And this is as short as I can make it. To summarize, Arctic warming is not greenhouse warming and observations of Arctic warming do not count as verification of greenhouse warming. Global temperature measurements that are said to record greenhouse warming likewise do not support existence of greenhouse warming. And there is absolutely no support for a prediction of dangerous global warming ahead that these wedges are supposed to fight. This makes the entire article an exercise in futility. Anyone who wants to put these wedge proposals into action is suffering from an extreme case of stupidity. To learn the full story read my book “What Warming?” available on

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By Stephen Mulkey, President, Unity College (04988)
on September 30th, 2011

During my career I have been a forest ecologist, science advisor to the Florida commission on sustainability, academic program developer, and now academic leader.  I have been faculty at 3 major universities and for about 20 years I was affiliated with the Smithsonian.  I have given uncounted talks on climate change to students, the general public, and policy makers.  In March 2007 I was silenced and dismissed by a select committee of the Florida legislature because I had begun my presentation with the Hockey Stick, which at the time had been largely validated by the NAS.  Since then, numerous similar analyses have been published in the peer reviewed literature.  To date 32 national academies of sciences have endorsed the fundamental scientific tenants of anthropogenic climate change.

I have used the wedges numerous times in white papers and presentations to the public and policy makers.  I have no issue with their usefulness, and I think it tragic that we continue to debate when we could be acting.  That said, I disagree with the implication that climate science has failed to show that the more extreme outcomes are increasingly likely.  From my reading of the literature, the more extreme projections of the general circulation models are entirely consistent with the observed stochastic development of more extreme weather and other empirical patterns of global change.  The observed biological responses are especially sobering.  My best understanding of the science indicates that we must begin to reduce emissions no later than 2020, and that these reductions should be steep.  I am concerned that this article inappropriately emphasizes the uncertainty of the science.  Indeed, emphasizing the uncertainty is partly why the players cited in the Merchants of Doubt have been so successful in delaying action.

I also take strong issue with the remarks of Freeman Dyson.  His scholarship is highly questionable and his political assertions are unfounded.  I won’t spend time refuting his remarks.  With all due respect to his great accomplishments over a long career, I gently suggest that he is not a legitimate expert in climate science, climate change, or any aspect of climate policy.

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By John McCormick (Alexandria VA)
on October 1st, 2011

Sokolow and Pacala continue to point the way forward and since 2004 have had the advantage of seeing how their initial offering was received.  Very discouraging, indeed.  However, these eleven years have more clearly defined the cost of no action via the weather chaos we 7 billion inhabitants are experiencing. 

That said, their paper reads as a blueprint for America to follow.  We all know CO2 mitigation cannot be successful if only one of the economic superpowers embraces that aggressively.  Americans opt to use less oil.  Good strategy for us, great opportunities for the BRICs because there will be more oil for their economic expansion .  Thus, I take slight issue with their reiteration because it does not put enough emphasis on the very fact that either the world’s energy consumers unite in a common struggle to reduce CO2 emitting energy consumption as a desperate survival decision or they (we) unite on our common destruction.

I suggest to Secretaries Clinton and Paneta and to CIA Director David Petraeus they convene a discussion with the President to petition the United Nations for a special meeting of UN members to discuss the Sokolow and Pacala paper.

If we cannot bring a topic of this urgency to the pinnacle of the international dialogue, what value has it really?  Climate chaos is a time-related challenge that will either doom the planet’s living organisms (save for those organism that survive at the base of vents at the bottom of the oceans) in this century or be the reason for all nations to unite, for once, in a common purpose.  Can there be any greater cause than acting on our instinct to survive?

Finally, I agree with Dr. Mulkey’s comment regarding Freeman Dyson.  Dyson’s comments make me think of a once great mind wasting its time.

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By Roger Shamel (Lexington, Massachusetts 02420)
on October 2nd, 2011

Great article!  It’s perfectly logical that waiting to act will ramp up the magnitudes of the required responses.

Einstein and others have hinted at the benefits of thinking outside the box when problem solving.  Let’s take this one step further.  Although unprecedented in US history, as is AGW, it seems that the quickest way to move the ball forward would be for the president (BO) to give a nationally-televised, prime-time, Oval Office, “State of the Climate” address.

Properly worded, including consideration of some of the key points in this paper, and backed up by the proper experts (including Rob, John Holdren, Steve Chu, etc.), such an address could provide enough information, from credible enough sources, to motivate millions of misinformed Americans to support the types of actions that are proposed.

Right now the biggest enemy of action is the lack of appreciation that most Americans have for the magnitudes of the opportunities and risks presented by global warming.  Too much money is being spent by those who would prefer to preserve the status quo for a clear picture to appear.  Global warming is a stealth black swan, visible to only a few.

As a small step in the right direction, please “like” this idea:, and spread it widely.

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By Susan Anderson (Boston MA)
on October 15th, 2011

I’d suggest that “experts” and the chattering classes severely uncerestimate the perception of the general public.  They know and can see that things have changed.  While I know there are places where it is politically incorrect the notice patterns in weather change over time and the conclusions of scientists, most of the non-academic people I talk with are quite open, interested, and fearful.

We’ve to some extent allowed ourselves to be spooked by appearances.  OTOH, the Roman Circus that is infotainment these days is busy throwing as much golden dust into our eyes as they can manage.  Not sure why they don’t take a look around and think of their own futures, but there you are.

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By Michael Reisner (54481)
on October 21st, 2011

I have always found the wedges exercise extremely useful as an organizing tool for examining the concept of mitigation for my energy and natural resources policy courses.  I wish this commentary would have taken the opportunity to, as many of my students over the years have observed, explain the monumental undertaking necessary to achieve just one wedge when one considers the political inertia encountered.  Many of the wedges may be within or technolgical and economic grasp, but we remain essentially at the starting line because of the lack of strategic public policies and political will.

I must also take exception with the long-term policy goals.  They either ignore or do not fully appreciate the policy implications of much of the climate change science since 2004.  As I constantly remind my students, you need to separate the policy goal from the policy means of achieving that goal.  Given the daunting challenge that humanity faces, the goal needs to be visionary, a call to arms to stir the best of humanity, and initiate amplifying policy breakthroughs, technological innovation and deployment, etc. that push against the many powerful feedbacks in the carbon cycle and climate system that are currently working against us.

Endorsing a 9.7 GtC annual emissions budget for the year 2061 is a recipie for disaster and gives us almost no chance of staying below 2 degrees of warming and probably commits us to closer to 3 degrees warming in this century (and this makes a lot of relatively optimistic assumptions regarding the climate system).  The more science has progressed over the last decade, the more dire the situation in which we find ourselves-impacts are having faster and are more severe for a given temperature increase than we predicted just a decade ago.  The risk of crossing one or more tipping points is simply too great with 2-3 degrees of temperature increase.  The long-term policy goal recommended in this commentary underscores the urgency and magnitude of the problem to the American public: 9 wedges does not seem to difficult, we don’t need major new policies. 

The students in my Introduction to Energy Policy course just debated many of the same isssues covered in this commentary and responses. As a class, they remarkably reached a consensus that they wanted a 80% certainty of staying at or below a 2 degree temperature increase in the year 2050. Considering just CO2 emissions, this requires that we stabilize atmospheric CO@ levels somewhere around 366 ppm.  In terms of a cumulative carbon budget, this temperature target and degree of certainty translates into about a 650 Gt of carbon cumulative emissions budget, of which only about 132 Gt of Carbon remains to be used by humanity (Anderson and Bows 2008 and subsequent papers). This means the annual carbon budget between now and 2061 is about 2.64 GtC per year (round it up to 3 GtC). 

BEING OVERLY SIMPLISTIC, WE NEED 9 WEDGES TO STABLIZE EMISSIONS AND AT LEAST 6 MORE (DEPENDING ON HOW FAST EMISSIONS PEAK AND HOW FAST USE UP 132 GT OF REMAINING LOAD) TO HAVE AN 805 CHANCE OF STAYING BELOW 2 DEGREES. Given the political inertia and fact that my daughters will have to live on this Earth in 2061, I would add another 3 wedges bringing the total to 18 wedges.

Now lets start a discussion of the public policies needs at the International, national, state, and local levels to tackle 18 wedges and at the same time use it as an opportunity to make the world a better place for ALL of humanity.




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By Tim Toledo (Toledo Oh 43623)
on October 24th, 2011

Every comment noted on this blog other than the one by Arno Arrak assumes that the question at hand is what to do about a problem that is potentially so catastrophic that we cannot afford to not deal with it.  So while Mr Arrak presents intriging science that suggests doubt about the basic premise of AGW, his comments are convenienty ingored here.

My take is this: Once you start looking at an issue in terms of “Risk Management” and “Wedges” as solutions,  you are no longer open to any questions about whether there is a problem in the first place.  Since this is the current state of acedemia based “Climate Science”,  it appears that humanity has uncharted territory to travese before true enlightment is possible here.

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By Amory Lovins (Snowmass CO 81654)
on December 8th, 2011

Thanks to Rob for this latest excellent contribution. Readers would probably find useful our business book *Reinventing Fire: Bold Business Solutions for the New Energy Era*, published 27 Oct 2011 by Chelsea Green (, It details how to run a 2.6x-bigger 2050 US economy with no oil, no coal, no nuclear energy, and one-third less natural gas, at a net-present-value cost $5 trillion less than business-as-usual (valuing all externalities at zero). Its fossil carbon emissions by 2050 are 82-86% below 2000 levels. The transition it describes requires no new inventions and no Act of Congress, and is led by business for profit. This independent “grand synthesis” is extensively peer-reviewed and documented, and its analytic basis is fully scrutable and transparent. Whether you believe climate science or not, if you care about either the economy (profits, jobs, competitive advantage) or national security, you’ll find the proposed actions attractive and their appeal strongly trans-ideological. The initial response, especially by its business-leader audience, is encouraging.

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