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Alan Weisman answers your questions

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(CNN) -- Environmental author Alan Weisman has fielded questions exclusively from CNN.com users on the environmental matters that concern you most. These are his answers.

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CNN reserves the right to edit submitted material for the sake of length, grammar and clarity.

QUESTION N0. 1:

I wonder whether you would revise the blame put on the Clovis people for the destruction of North America's early megafauna and instead consider the culprit a meteorite, as suggested recently by Brown University researchers and documented in this CNN story: http://www.sciencedaily.com/releases/2007/09/070924172959.htm

However, this is a minor quibble on your very moving book; a great intellectual adventure. I enjoyed it immensely and although my family and I are ardent recyclers and generally green, your chapter on plastics was horrific (in the best possible way!) and stands as a ghost behind me at every checkout counter. My deep thanks,
Dr Ken Beatty Bowen Island, Canada

Weisman:

Thanks for your kind words about the book. I've read the material on the meteorite theory, and while I tend to agree with its authors that it probably documents a genuine event that occurred during the Pleistocene extinctions, I suspect that it adds to but doesn't overturn evidence of other explanations for what happened to megafauna that once roamed the world. Obviously, during the Earth's long history, events ranging from meteor strikes to climate shifts occurred that impacted living things, at least regionally. I imagine there was certainly an impact on whatever lived in the vicinity of this meteor strike, just as hundreds of square miles of forest and forest inhabitants died when a meteor hit Siberia in the late 19th century. However, the main reason that I don't think the event discussed by the Brown researchers was the principal reason for the late Pleistocene extinctions is that the paleontologic record shows that disappearance of numerous large species on various land masses coincides suspiciously with the arrival of humans. That is as true for Australia, about 48,000 years ago, as it is for North and South America, roughly 13,000 years ago. It's also important to remember that, as described in my book, Pleistocene animals such as sloths, mammoths and moas that lived on islands hitherto undiscovered by humans survived the Pleistocene extinction for thousands of years until people finally arrived on their shores -- people who then, apparently, dispatched them rather quickly.

My book also mentions interpretations of the "black mat" phenomenon and Younger Dryas cooling that the authors of the Brown paper theorize were related to a meteor strike. Other paleontologists have cited the role that these may have played in disrupting life-as-usual in an already convulsive period on the planet. Yet many eminent ecologists remain convinced that the arrival of human hunters had a critical role in taking advantage of whatever other changes were taking place on Earth to ultimately doom many large, desirable megafaunal targets. (Besides examples noted in my book, I would refer you to the highly readable works of Australian scientist Tim Flannery.)

QUESTION NO. 2:

Dear Alan:

I enjoyed the article that appeared in Scientific American recently summarizing, "The World Without Us". I met you briefly about 5 years ago when you were visiting Humboldt State University where I work. You were here to discuss your experiences at Gaviotas in Colombia.

My question and one of the things I am working on is how to deal with the intermittency of renewable energy sources such as wind, solar, and waves as they hopefully become an increasing fraction of energy in the grid? Some of the areas I have been investigating are thermal storage (heat or cold) combined with heat pumps and storage in batteries of battery-electric and plug-in hybrid electric vehicles.
Michael Winkler Schatz Energy Research Center Humboldt State University Arcata, CA

Weisman:

Michael, if you're asking me that question, instead of the other way around, it only reinforces my concern that renewable energy technologies haven't advanced much in the nine years since I wrote Gaviotas. Wind and solar especially, because they occur over much of the Earth -- not just along coastlines like wave action -- have proven frustrating because storage mechanisms for when the wind's not blowing or the sun's not shining are complicated, chemically toxic, or require significant drains on the very energy they are hoping to store. For more than a decade I've researched hydrogen as a clean and elegant solution to energy storage (which was why I visited you at Schatz) but over the years I still see numerous obstacles to the efficient production of hydrogen -- let alone problems related to effective storage and shipping of that light gas. The technologies that can separate pure hydrogen from substances that contain harvestable amounts of it, such as water or coal, either still require more energy inputs than would be economically practical, or produce carbon wastes that we'd still have to contend with. One idea that seemed promising to me involves using wind energy during non-peak times to compress air in natural caves or artificial underground chambers, then release it when winds died down. Having not researched the progress on this for a while, I suspect that the costs in compressed air infrastructure are, at least for now, considered too expensive or untested to attract investment of necessary capital to pay for land and excavation. The storage technologies that you mention are always interesting, but we haven't gotten much better at producing batteries that are themselves cleaner or capable of storing greater quantities energy over longer periods of time. I commend you for your persistence, and please do tell if you know of promising new developments that I'm unaware of.

QUESTION NO. 3:

Hi Alan,

Are positive feedback loops added into climate models, and if so do you believe they are accurate? I fear events will occur much faster then current models predict due to positive feedbacks.
Paul Rocheleau, Canada

Weisman:

They certainly are included in many models I've seen, but the accuracy of those models is limited by how much we actually can know, because there are so many variables in the global ecosystem. Only recently, a new one was to the mix, as scientists realized that the ocean's capacity to absorb carbon dioxide has been hampered by the higher winds that are blowing across its surface as climate change generates more powerful storms. That, in turn, will cause global warming to accelerate, because more CO2 will remain airborne.

Melting permafrost exhaling methane to the atmosphere will in turn raise polar temperatures, causing more permafrost to thaw and release more methane -- but we have only a rough idea of how much and how deeply permafrost will be affected. This is due to there being so many factors to consider, including changes in surrounding ocean temperature, due on the one hand to diminishing reflective ice cover, and on the other, to significant additions of cold, fresh, glacial meltwater to Arctic seas. Where this all is headed is anyone's guess. But before we start dreading imminent catastrophe, we should recall that ice cores show that in the past our planet's atmosphere has had elevated amounts of CO2 equal to today's, which surely contributed to increased methane releases. Yet while certainly more tropical than today, the Earth managed to avoid runaway positive feedback loops that might have turned it into Venus.

QUESTION NO. 4:

What is the carbon footprint of the current biggest off shore wind turbines? Given the best efficiency of 25% over the forecast lifetime of about 25 years, will the "free" electricity generated ever pay back the efforts into bringing this system into use?
Kind Regards
Richard Henderson, Wales, UK.

Weisman:

I don't know the answer to this one, because I've never had cause to research it and don't discuss it (or anything about renewable energy) in my book The World Without Us, the writing of which pretty much monopolized my attention over the past four years. However, I can tell you how I'd go about researching it. First, I'd check to see what the wind turbine companies and their proponents are claiming, and what their competitors are saying, just to get a sense of the current range of opinions. Then I'd try to come up with my own figure by dissecting every component of wind turbine manufacture and operation I could imagine, such as:

How much energy was expended in mining and shipping the raw materials that make up a wind turbine's various parts; how much energy was required in their manufacture, how much is expended in maintenance (e.g., how often do trucks drive to each wind turbine to either repair or periodically service them); how much and what kind of lubrication is required, and is the lubricant recyclable; what is the lifespan of the turbine's parts; what is the cost of transmission lines, and is the energy loss through them different than from a conventional power plant's because wind energy is intermittent; what land use was sacrificed to the turbine's presence; what potential damage might it be causing (ranging from decreased real-estate values to hazards to birds and bats, etc.) Finally, after all that, I'd consult the literature and experts to see what other carbon costs I might have overlooked.

QUESTION NO. 5:

Dear Alan,

How did you come up with the premise for "The World Without Us"? What message do you want people to take away from your book?
Lloyd Greenberg, Singapore

Weisman:

As I describe in the book's acknowledgments, an editor at Discover Magazine asked me to write an article about what the world might be like if people suddenly disappeared. She had been inspired by an article I wrote in 1994 for Harper's at Chernobyl, in which I described how villages abandoned around the destroyed nuclear reactor were being overtaken by their own landscaping and surrounding natural foliage. Over the years, she said, what at first had seemed a depressing account of an irradiated landscape had segued for her into a hopeful story, because it showed that, despite one of our worst environmental disasters, nature will seek a way to recover and renew itself.

What, she asked me, would the world be like if that happened everywhere? I realized immediately that the topic was too vast to fit gracefully into a mere magazine article: a book-length treatment would be required to deal with all the scenarios I would have to consider worldwide. However, I accepted the assignment, knowing that this article could be the launch pad for exactly the kind of book I'd wanted to write for years. It would look at our impacts on the planet, not by tabulating one human-caused devastation after another -- each of which would only add to guilt and fright levels that would inevitably drive all but the most environmentally committed readers away -- but one that instead would intrigue a much wider readership by giving them an irresistible, educated glimpse of a possible future, even as it avoided raising fear over whether damages we've wreaked might kill us all off, because according to my scenario, we're all already dead.

I'm grateful to say that this seems to have worked: The book is appearing in nearly thirty languages, and has become a bestseller in many countries. My purpose, however, is not to argue that we humans should disappear. Rather, by theoretically clearing us off the planet, we can more easily see what else lives here, how it might thrive without the incessant pressures we heap on it, and how it would contend with all the stuff we'd leave behind, ranging from our infrastructure and manmade materials to all the carbon dioxide we've pumped up our chimneys and out our tailpipes. My hope is that once readers see how beautifully nature could recover from losses it has sustained from our excesses, that we'll want to find a way to let nature flourish anew but keep ourselves as part of it -- in balance, not in mortal combat, with nature itself. E-mail to a friend E-mail to a friend

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