Friday, November 1, 2002
A Variable Nature?
I enjoyed your article "The New Ice Age" [September]. However, there is one point that needs clarification. It is common knowledge that Earth has had variations in climate, ocean temperature, ocean salinity, and ocean currents throughout its history. The article states that the most recent little ice age, from 1300 to 1850, was not caused by human-related greenhouse gases. Why then is the current increase in Earth's overall temperature assumed to be caused by, or at least aggravated by, human activity? After all, these variations have always occurred, even before human beings existed. It seems silly to think that we caused it this time but that all the previous climate variations were natural phenomena.

Daniel L. Heitman
—Alexandria, Louisiana

Brad Lemley responds: Teasing out the impact of human activity from the natural variations in global climate is extremely difficult. Nonetheless, the Environmental Protection Agency notes that since preindustrial times, atmospheric concentrations of so-called greenhouse gases have increased dramatically: carbon dioxide by more than 30 percent, methane by 145 percent, and nitrous oxide by 15 percent. The Intergovernmental Panel on Climate Change, formed jointly by the United Nations and the World Meteorological Organization, has concluded there is a "discernable human influence" on climate. Just how much that human influence has contributed to rising temperatures is an open question.

Race to the Finish
Regarding the very interesting article "The Race to Find Out How the Universe Will End" [September]: It seems to me that the validity of extrapolating what occurred billions of years ago to what is presently occurring—and unseen—at the outer reaches of the universe is somewhat questionable. If there is antigravity energy [dark energy] in the universe, shouldn't it have some effect on the movement of the planets around the sun and be detectable? Could it be that this antigravity energy has dissipated and that the outer galaxies have already turned inward, not to be observed from Earth until a few billion years have passed?

James Hendry
—Florissant, Missouri

Corey S. Powell responds: Obviously, astronomers can study only the universe's past, not its future. When they look at a mix of extremely remote, medium-distance, and nearby galaxies, they see evidence that the rate of cosmic expansion slowed for the first few billion years, then turned around and began to accelerate. That pattern is consistent with what theorists would expect in an evolving tug-of-war between gravity and dark energy. It is possible that dark energy will dissipate in the future and that gravity will again take over, but energy seems to dominate for now. Unfortunately, there is no known way to study dark energy within our solar system. The small, cumulative effect of dark energy can be detected only over millions of light-years of distance and millions of years of time.

Garbage Disposal
I have just finished reading the excellent article on Yucca Mountain [September]. As in all the other articles I have read concerning the storage of nuclear waste, there seems to be an unstated assumption that technology and knowledge will not advance in the future. Thus, we must design a 10,000-year solution today. Would it not be simpler and more logical to design a 100-year solution? In those 100 years, scientists and engineers would continue to look for ways to store the material for a longer period. Assuming they succeed, and I believe they will, a new method of storage would then be used, which might be good for, say, 250 years. And so it goes until, eventually, a safe, permanent solution is discovered. The storage of nuclear waste is not a technological problem; it is a political problem. Unfortunately, technology cannot solve political problems in the short term. In the long term, it will.

Eric A. Dorbeck
—Jackson, Michigan

Kudos to Discover and Jeff Wheelwright for a superbly accurate and balanced article on Yucca Mountain. Such attributes are rare in information concerning this technically and politically charged effort. Way back in the 1970s, when we were inventing performance assessment methods, mathematical models had as few as eight performance factors. Now there are thousands of factors included and characterized on the basis of actual and estimated data. The uncertainties in the performance assessments allow opposing parties to question the validity of both the results and the methods, however. During the past decade, there have been major changes in the Department of Energy's approach to disposal at Yucca Mountain and thus major challenges to the regulatory framework. Still, I think that safety for the real mountain will be better than that estimated for the virtual mountain.

John W. Bartlett
—Arlington, Virginia
Former director,
Civilian Radioactive Waste Management,
U.S. Department of Energy

In "Antigravity Plumbing" (Works in Progress, September), we stated that the 20 atmospheres of negative pressure in the xylem of trees is comparable to the amount of positive pressure in an automobile tire. The pressure in most car tires is in fact slightly more than two atmospheres. In September's NeuroQuest, we stated that the golden number (1.618 . . .) is a transcendental number. It is actually an irrational number. The correct Web site address for the Mercer Museum, reviewed in September, is www.mercermuseum.org.

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