TCS Daily

The Adaptable Animal

By Charles Murtaugh - August 23, 2002 12:00 AM

CAMBRIDGE - The heat wave of the past week-and-a-half has finally ended here in the Northeast, giving city health officials the chance to tally up the heat-related deaths. As Slate and The New Yorker recently pointed out, excessive heat kills more Americans every year than all other natural disasters combined. So as global warming turns up the thermostat, there must be more such deaths every summer, right?

Not necessarily, if the example of Philadelphia gives any guidance. After 118 deaths in the summer of 1993, the city set up a heat emergency plan, mobilizing local block captains and health care workers to treat and monitor the elderly citizens who are at special risk during heat waves. So far this summer, which has been at least as hot as that of 1993, there have been 27 deaths - 27 too many, but still a marked improvement on the past record. The bottom line: when people put effort into solving a problem, an apparently inexorable trend can be reversed.

This simple fact - perhaps a defining fact of human history - ought to be kept in mind when considering the ecological disaster warnings flying to and fro in advance of the World Summit on Sustainable Development. This week The New York Times devoted their Science section to stories about the environment, some admirably balanced, others less so. Consider, for instance, Denise Grady's article on the threat of emerging diseases "on an altered planet."

Grady points to a number of diseases that are either entirely new to humans or that have only recently spread out of their previously restricted ranges. The premier example is West Nile virus, which she describes as, "just one example of an infectious disease whose incidence and geographic range have expanded because of human activities affecting the mosquitoes, birds, rodents and other animals that help spread the infection." Last week I suggested that West Nile isn't much of a threat to public health, but other diseases mentioned by Grady, such as HIV (recently emerged) and malaria (staging a tragic comeback), have already wreaked havoc. Still others, such as Nipah virus or mad cow disease, have yet to make major global impacts but are dangerous enough to keep a close watch on.

Nipah virus, which appeared several years ago in Malaysian pig farms, is one of several diseases whose emergence is related to human farming practices. This should be no surprise: food gathering brings humans into close contact with other animals, both livestock and wild, which may harbor infectious diseases capable of spreading to humans (referred to as zoonoses). It is now thought that HIV originated in this way, as a variant of a simian virus inadvertently passed to humans during the slaughter of chimpanzees for meat. Grady intimates that this is a new and growing problem, but as Jared Diamond points out in Guns, Germs and Steel, zoonoses have around since the dawn of agriculture.

What may be new, in fact, is our ability to catch, classify and contain such outbreaks - fifty or a hundred years ago, would the death of a hundred Malaysians in that country's pig farming industry have been noticed around the world? A new and deadly epidemic may yet emerge from our interaction with animals, but that fact shouldn't be considered an indictment of humanity's "footprint" on nature, or an argument that this footprint is somehow unsustainable.

The other culprit in Grady's analysis of emerging diseases is climate change; unsurprisingly for the Times, her article swallows all the arguments of global warming hypochondriacs, and ignores any skepticism. (I write this as someone more than a little worried about global warming - certainly more than the norm here at TCS - but unwilling to make bad arguments for fighting it.) She cites a champion of the theory that global warming promotes insect-borne disease:

Dr. Paul R. Epstein, associate director of the Center for Health and the Global Environment at the Harvard Medical School, said an important consequence of warming was an increase in "extreme weather events" - droughts punctuated by torrential rains. Drought, he said, helps the mosquito species Culex pipiens, which plays a major role in spreading West Nile.

He added that drought might also wipe out darning needles, dragonflies and amphibians, which destroy mosquitoes. Drought may also aid the spread of infection by drawing thirsty birds to the pools and puddles where mosquitoes breed. "Hot weather plays a role, too," Dr. Epstein said. "Warmth increases the rate at which pathogens mature inside mosquitoes."

. . .In addition, he said, "warming has contributed to the northern movement of ticks, and warm winters allow for overwintering."

Globally, warming is widely thought to be contributing to the spread of malaria and dengue, each carried by mosquitoes, to high altitudes in Africa and Central and South America where the diseases had not occurred before.

Epstein presumably didn't mention to Grady that his theory has met widespread skepticism among epidemiologists, as described in a 1997 Science article, "Apocalypse Not". Like too many pro-environmental arguments, his underplays the positive impact of human ingenuity in the face of changing conditions.

The example of malaria particularly undercuts his argument. On the one hand, epidemiologists who have modeled its spread in response to global warming find no cause for alarm: by 2050, assuming that the pessimists are right about the extent of future warming, there should be a less than 1% increase in malaria cases. On the other hand, when the actual spread of high altitude malaria was examined in East Africa, no correlation was found between disease and climate. According to the authors of the latter study, published this past February in Nature, "Economic, social and political factors can therefore explain recent resurgences in malaria and other mosquito-borne diseases with no need to invoke climate change."

Economic, social and political factors can endanger human health, but they are also our best defense against future threats. Consider the past history of malaria, which Grady does not mention but which is wonderfully explored in Mark Honigsbaum's new book, The Fever Trail. Until the past century, the disease was endemic across the Northern hemisphere. During the American Civil War, some ten thousand Union soldiers were killed by malaria, and similar calamities routinely befell armies across Europe. The parasite was beaten back not by a cooling climate, but by human ingenuity, in draining swamps (i.e. "destroying wetlands") and developing insecticides. That is, the same human environmental impacts that supposedly endanger our future health helped preserve it in decades past.

It's ironic that one of the most commonly cited indicators of climate change is the shift of migratory birds and butterflies into warming northern latitudes. If birds do it, and butterflies do it, can't we adapt as well?

We don't want to press the limits of our adaptability, and we can't ignore global warming or other negative human impacts on the environment (if indeed global warming is a human impact), but those of us who do worry about such things shouldn't exaggerate their immediate threat to human health. It is in the interest even of global warming pessimists to avoid alarmism about disease spread: what if Kyoto - or an even more radical anti-carbon treaty - were ratified tomorrow, and malaria continued to spread because of political mismanagement? Would this help the Green case, or hurt it?



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