Global Warming Balance Sheet: What We Really Know

It is important for Americans to address the questions: So what if the climate changes? How much would disruption of the expected climate disrupt nature or our economy?

We've already heard from climate researchers that the earth's surface temperature has risen about 1 degree F. since the 19th century. We've been told about the potential for further warming of several degrees that may cause ecological disruption. But the tough economic - political really - question is, how much is our stewardship of nature worth? Scientists have published hundreds of papers on the potential impacts of projected climactic changes. Their estimates range from small benefits to catastrophic losses.

To try to sort this out, Yale economist William Nordhaus asked 19 economists, technologists, and natural scientists familiar with the scores of studies of "climate damages" to estimate such damage as a percentage of lost gross domestic product (GDP) for the world. Using a hypothetical scenario of 3 degrees C. warming by AD 2100, the best estimate of the group Nordhaus referred to as mainstream economists was about a 1 percent GDP loss - with fairly large uncertainty. The natural scientists estimated 10 times greater damage - but with even greater uncertainty.

Part of the difference in these professionals' world views is the higher value that natural scientists put on nature's unpriced services, like waste recycling, flood control, or biotic diversity. Economists are more optimistic that humans can invent substitutes for such ecological services. Let's suppose the economists are right. Even 1 percent of world GDP lost annually is, in today's terms, some $200 billion each year! Where do such dollar values come from?

First, consider sea level. Hurricane Andrew caused unprecedented losses, about $40 billion. Before 1987 no weather event had caused damages in excess of $1 billion. Since then, several have caused tens of billions of dollars in damages. The insurance industry is understandably very alarmed.

Although there is some theoretical reason to expect that warmer ocean temperatures could produce stronger storms, this analysis is controversial, and no one can credibly attribute some percentage of Andrew's damage to global warming. But we do know that as the earth warmed, sea levels became 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 one-half foot to three 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 - and even to the existence of some island states.

Now, consider 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 nature, or, rather, are we "loading the climate dice?"

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.

So now to some data. Tom Karl and his colleagues at the National Climate Center in Asheville, N.C., have analyzed reports from thousands of weather stations in the US over the past century and found about a 10 percent increase in precipitation since 1910. More significantly, most of this increase occurred in the top 10 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 "nonlinear" 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, nonlinear 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."

NOBEL LAUREATES ON 21ST CENTURY CLIMATE

* Professor Schneider, Stanford University climatologist and author of the adjoining column, recently joined six other natural scientists at a public briefing for President Clinton. Their message: that global warming is a real phenomenon.

Dr. Schneider adapted this article from his remarks at the briefing. His colleagues were Nobel Laureates Mario Molina of MIT and Sherwood Rowland of the University of California at Irvine, discoverers of the ozone hole; Nobel Laureate Henry Kendall of MIT, head of the Union for Concerned Scientists; ecologist Jane Lubchenco of Oregon State University; infectious disease expert Robert Shope of the University of Texas; and energy expert John Holdren of Harvard.