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Global warming is real, seven scientists warned President Clinton and Vice President Gore in a public briefing on July 24.
The scientists were Stanford climatologist Steve Schneider; Nobel Laureates Sherwood Rowland of the University of California-Irvine and Mario Molina of MIT, discoverers of the atmospheric ozone hole; Nobel laureate Henry Kendall of MIT, head of the Union for Concerned Scientists; ecologist Jane Lubchenco of Oregon State University, past president of the American Association for the Advancement of Science; infectious disease expert Robert Shope of the University of Texas; and energy expert John Holdren of Harvard.
They warned that global warming and the greenhouse effect are real phenomena, and that the potential impacts could include dramatic changes in natural systems ranging from the inundation of the Everglades by rising sea level to the introduction of disease-carrying insects to northern climates. Kendall argued that even if the United States has the resources to protect its croplands from the effects of climate change, the potential loss of food crops in developing countries makes this a national security issue as well as an environmental one.
Following are Schneider's remarks in the briefing:
It is important for us to address the question: So what if the climate changes? How much would disruption of the expected climate disrupt our economy?
We've already heard [from previous speakers] that the Earth's surface temperature has risen about 1 F since the 19th century. We've been told about the potential for further warming of several degrees to cause ecological disruption. But the tough economic political, really question is, How much is our stewardship for nature worth? Scientists have published hundreds of papers on the potential impacts of projected climatic changes, estimating damages that typically range from nothing to catastrophic.
To try to sort out some of this confusion, Yale economist William Nordhaus asked 19 economists, technologists and natural scientists who were familiar with the scores of studies of so-called "climate damages" to estimate this as a percentage of lost gross domestic product (GDP) for the world. For a hypothetical scenario of 3 C warming by [the year] 2100, the group he referred to as mainstream economists' best guess was about a 1 percent GDP loss with fairly large uncertainty. The natural scientists estimated 10 times greater damage but with even greater uncertainty. Nordhaus quipped that those who know the most about the economy were only modestly worried, whereas I countered that those who know the most about nature are more seriously concerned.
Part of that difference in these professionals' world views is the higher value that natural scientists put on nature's unpriced services, like waste recycling or flood control or biotic diversity. Economists are more optimistic that humans can invent substitutes for such ecological services. But suppose the economists are right: even 1 percent of world GDP lost is, in today's terms, some $200 billion!
Where do such dollar values come from? I'll give two examples: sea level rise and hydrological extremes.
First, to sea level. Hurricane Andrew caused unprecedented losses, about $40 billion. Before 1987 no weather casualty was greater than $1 billion, but since then several have caused tens of billions [in losses] and the insurance industry is understandably very alarmed. Although there is some theoretical reasoning to expect that warmer ocean temperatures could produce stronger storms, this is controversial and no one can credibly attribute some percentage of Andrew's damage to global warming. But we do know that sea levels are about 4 to 10 inches higher than a century ago, which means that any storm, natural or enhanced by global warming, will have an accompanying storm surge that penetrates farther inland and creates greater damages.
Typical global warming scenarios include projections for a 1/2-foot to 3-foot greater sea level rise over the next 100 years. Those rises clearly will pose costs and risks to hundreds of millions of coastal dwellers around the world and even to the existence of some island states.
Now, to hydrological extremes. By hydrological extremes I mean droughts and floods. Are the many costly floods that occurred across the United States over the past five years, or the droughts in 1988, or the heat wave in 1995 that killed hundreds of vulnerable elderly people in Chicago, merely two "snake eyes" in a row from a perverse nature, or, rather, are we "loading the climate dice"?
How could humans be involved in the weather act? First, some theory. Since we add heat-trapping gases like carbon dioxide to the atmosphere, which, in turn, add energy to the Earth's surface, some of that energy will be used to evaporate water no one who is knowledgeable disputes that. More evaporation globally means more rainfall globally. Thus, when the atmosphere configures itself for locally heavy rains, more evaporation means heavier rains, on average. Likewise, as farmers or lawn waterers know, when the atmosphere is in a drier mode, higher temperatures suck more water from the soils. Taken together, these physical arguments provide the rationale for forecasting increased droughts and floods from global warming. But this is physical reasoning, not proof, which reminds me of the old scientific cliché: In God we trust, but for the rest of us, please show some data!
Now to some data. Tom Karl and his colleagues at the National Climate Center in Asheville, N.C. have analyzed thousands of weather stations in the U.S. over the past century and found about a 10 percent increase in precipitation in the United States since 1910. More significantly, most of this increase occurred in the top 10th percentile of extreme daily rainfall events that is, the "gully washers" that insurance companies fear. While these observations are consistent both with theory and climate model predictions, and thus are strong circumstantial evidence for a global warming impact, certain proof will take a few more decades of performing this unplanned experiment on "Laboratory Earth."
Finally, in a system as complex as the earth-atmosphere-ocean-ice-biosphere system what scientists call a "non-linear" system we cannot have precise forecasts that are credible. But, as the Intergovernmental Panel on Climate Change noted in concluding its 1995 assessment report, "When rapidly forced, non-linear systems are especially subject to unexpected behavior." My free translation of this concern is that reducing the pressure that humans put on nature is an insurance policy against "nasty surprises."
In summary, my view of the current balance of evidence is metaphorically that the canary in the cage is starting to quiver. Whether that shiver is from breathing dangerous air will become increasingly clear if we continue to accelerate our use of the atmosphere as an unpriced sewer.