"A Sunshade for Planet Earth," by Robert Kunzig, describes various geoengineering proposals to slow or reverse global warming. As an engineer, I appreciated the proposed technologies, but as an economist, I was appalled at what they would mean. First, it has been established that regulating a risky behavior only encourages more of that behavior. This tendency would mean that a technical fix to global warming would only encourage more carbon emissions. A related problem is that curbing emissions also lowers the current costs of dealing with the problem, making the development of other solutions less likely. Finally, big engineering fixes would require global support, yet there is a "free rider problem" in that many countries or coalitions will be unwilling to pay their share, and countries will have the incentive to cheat on the agreement. A neat engineering approach could lead to harsh economic and environmental conditions.

Michael Ollinger
Washington, D.C.

I find the fact that geoengineering solutions to global warming are now being given serious scientific consideration disturbing because there is no way to be certain that all the effects of these schemes can be accounted for. Many problems may be so indirectly caused as to be fundamentally unforeseeable yet so dangerous as to render the cure far more threatening than the disease. I am reminded of the scheme of introducing cane toads to Australia to combat pests: not only did the toads fail to control the pests, but they also became a major pest themselves. Action is needed against global warming, but geoengineering should be regarded as a last resort.

Jesse Hall

In "Overshadowing Difficulties" [Perspectives], the editors discuss the costs and potential dangers of geoengineering our way out of the global-warming crisis and conclude that we must focus on emissions reductions instead. But emissions reductions and increased use of renewable energy alone cannot avert the crisis. We must reduce the amount of carbon dioxide in the atmosphere, not simply the amount we are currently adding to it!

Any solution will require large-scale carbon removal from the atmosphere, and we must find a way to pay for that. Fortunately, there is a simple way: a carbon tax, which would simultaneously raise revenues for carbon removal and by increasing the cost of carbon-based fuels drive innovation in energy efficiency, emissions reduction and renewable energy.

Quinton Y. Zondervan
Cambridge, Mass.

In discussing brain-machine interfaces, "Jacking into the Brain," by Gary Stix, refers to efforts to create a method to "download" information into the brain. This concept is based on a complete failure to understand what it means to comprehend something. Comprehension is not a passive information transfer; it involves the active construction of meaning: the contents to be learned have to be assimilated by the learner, reflected on, and linked to that person's personal store of knowledge and experiences. Having the text of an F-15 aircraft manual in one's brain would provide no advantage at all over reading it the old-fashioned way. There are several good ideas in this article that should not get mixed up with a truly bad one.

Walter Kintsch Professor Emeritus
University of Colorado at Boulder

I agree with Michael Shermer's main thesis in "Stage Fright" [Skeptic] that theories of predictable life stages in psychology (such as Elisabeth K bler-Ross's five stages of grief) are, for the most part, "toast." But the column seems to equate stage theories in developmental psychology with stories (narrative accounts of one's life thought to shape one's identity) in narrative psychotherapy. As both a developmental psychologist and a narrative psychotherapist, I cannot find anything they have in common.

Stages are invariant across individuals; narratives are unique to each individual. Stages are sequential; narratives are not. Stages are presumed to be largely innate; narratives are defined as creations of personal interaction and cultural influences.

David L. Ransen
Nova Southeastern University

Stages in developmental psychology, particularly those linked to the physical development of the brain (such as the maturation of the prefrontal cortex) and body (such as the timed release of hormones), are different from the type of stages that appear to have no basis in biology that I am skeptical about, such as the stages of grief, personality and moral development. If there is a direct connection between a psychological state and a biological development, then the case can be made that there will be a chronological sequence to development that could be described in stages (although even here the labels given to the stages are subjective, and the timing can vary).

But research by those who study grief shows unequivocally that stage sequence is quite variant across individuals, and I cited two sources for such research in my column: Russell P. Friedman and John W. James's The Grief Recovery Handbook (HarperCollins, 1998) and Robert A. Neimeyer's Meaning Reconstruction and the Experience of Loss (American Psychological Association, 2001). I also recommend the introductory psychology textbook Psychology, by Carole Wade and Carol Tavris, published by Prentice Hall, which gives an excellent overview of the research.

ERRATUM A source for the proton-related images on page 94 of "The Incredible Shrinking Scanner," by Bernhard Bl mich, went uncredited. It was chapter 2 of the textbook All You Really Need to Know about MRI Physics, by Moriel NessAiver, available at www.simplyphysics.com

CLARIFICATION "Dinner and a Show," by Mark Fischetti [Working Knowledge], states that as water molecules absorb microwave energy they create friction that produces heat. Moving molecules do not create heat; they are heat.