In midwinter, John Magnuson looks out his window across Wisconsin's Lake Mendota, hoping to see a swath of ice several miles long and a foot or more thick. But during several recent winters, all he saw was open water.
That's not how Wisconsin winters used to be - at least not according to 150 years of ice-cover data Dr. Magnuson, a university professor, has compiled on this seven-mile-long lake near Madison.
Lake Mendota was covered with ice four months of the year in the 1850s, compared with just 2.5 months today. Disappearing lake ice is a simple gauge widely considered among the clearest and most reliable scientific evidence that the planet is warming.
But would lake-ice data hold up, or melt, in a court of law?
An answer may come as soon as this fall as legions of scientists, possibly including Magnuson, are summoned to appear as expert witnesses in a groundbreaking case that will test the limits of environmental law and the science undergirding climate-change theory.
In a complaint filed last month in federal district court in New York, eight states (Wisconsin, Vermont, Rhode Island, New York, New Jersey, Iowa, Connecticut, and California) claimed that global warming is damaging crops, tourism, beaches, citizens' health, forests, and fish - and threatening coastal communities as sea levels rise.
Targeted in the suit are the nation's five largest public utilities. By burning coal to generate electricity - and spewing some 650 million tons of carbon-dioxide exhaust a year - the companies are major contributors to global warming, which is damaging states, the suit claims.
Unlike state lawsuits against tobacco companies, the states are not asking for damages. Instead, they want the power companies to cap their CO2 emissions. On the surface, it might seem a fairly easy case.
The standard for convicting the defendants is a "preponderance of the evidence" - a greater than 50 percent certainty that the companies are responsible for the damages. This is a far lower bar than the criminal court's "beyond a reasonable doubt" standard. Indeed, the vast majority of climate scientists now agree the planet is indeed warming and that carbon dioxide is the main villain.
"If it weren't so hard, difficult, and fraught with political controversy, the science of global warming would be a no-brainer," says Jerry Mahlman, a climate scientist with the National Center for Atmospheric Research in Boulder, Colo. "The fundamental physics of human-caused climate warming was well-known 25 years ago."
The challenge lies in the details.
For example, scientists can demonstrate that the increase in carbon dioxide and other greenhouse gases has caused a 1 degree F. rise in global temperatures since 1900. But the suit alleges that in sections of the United States, greenhouse gases have caused a jump of 2 to 4 degrees - and that future increases will cause further damage. That's tough to prove because computer models of climate change are much less conclusive when attempting to predict regional or statewide climate impacts from warming - and the rate of that warming, Mahlman and others acknowledge.
The case will be anything but a slam dunk, says Zygmunt Plater, a law professor at Boston College. "The plaintiffs [the states] have to say there has been substantial harm to the public welfare and health caused by the defendants [the companies] either negligently or knowingly."
Still, hard evidence like Magnuson's lake-ice data does show local effects of warming, say proponents of the suit.
"The facts are there and the law is there," says Thomas Dawson, assistant attorney general for the state of Wisconsin. "The science of global warming is solid."
"The science is more than conclusive enough," adds Matt Pawa, a lawyer working on a companion suit filed on behalf of the Natural Resources Defense Council. "It's well beyond the threshold that is required to establish a preponderance of the evidence. Scientists are well beyond 50 percent certain that CO2 emissions cause global warming."
Mr. Pawa acknowledges that the injuries cited in the states' complaint are not all supported by the same amount of scientific certainty. But the focus undoubtedly will be on proving those injuries where there is a high degree of scientific evidence to back up the claim.
Take, for instance, sea level rise in and around New York City, cited as a current - and future - injury in the suit. Many scientists agree that as the earth has warmed, glaciers and polar ice have melted and warming oceans have expanded. The rising sea level is measurable - up an inch per decade in New York since 1900.
"Climate change and sea-level rise, particularly with respect to New York City and the surrounding region, is fairly well understood," says William Solecki, professor of geography at City University of New York and a project coleader on a 2001 study that charted expected sea-level impacts. "We know the region is already undergoing sea-level rise."
The real question, he says, is the rate of future rise - which depends directly on just "how much warming will there be." That, in turn, depends on which computer model is used to predict the rate of future warming.
Will there be a 9-inch rise through the year 2080 - which is well within the capacity of current bulkheads on shore? Or, will it be closer to a worst-case scenario, which would see a 42-inch sea rise?
One element that could help overcome such uncertainties is the "public nuisance" claim cited in the lawsuit. Long before the Clean Air Act or Clean Water Act, environmentalists won lawsuits by basing them on public-nuisance law.
This ancient area of common law, whose precedents go back to the 17th century and earlier, requires a plaintiff to prove only that there's been a violation of the public right or that the public welfare or public health is in danger, Professor Plater says. In this case, the states only need prove that CO2, which is not regulated by the government, is heating the planet - and that it has an impact on their states.
But using the public-nuisance argument also could prove challenging, legal experts say, because the states are not only alleging current damage but also future risk. Because the case is the first of its kind, there are no clear precedents to assess damage. That will force the judge to make a nuanced reading of expert testimony, much of which may be conflicting, says Denise Antolini, associate professor at the University of Hawaii at Manoa and an expert in public-nuisance law.
Utility industry lawyers are skeptical that the strategy will work.
"Yes, we know these greenhouse-gas emissions, many of them naturally occurring, many manmade, are causing the adverse effect," says Patricia Braddock, a partner with Fulbright & Jaworski, LLP, a Houston firm that often represents the power industry in environmental litigation. "The computer models are relatively good for large areas, say, Northern or Southern Europe. But they're not so good for predicting effects in smaller areas. If [the attorneys general] say: 'This had an adverse effect on my state' - this is where there's a disconnect. The science isn't there for them; this is where they're going to have trouble."
American Electric Power - the nation's largest utility and a defendant in the suit - says filing lawsuits is not a constructive way to deal with the issue.
"Climate change is a global issue that can't be addressed by any individual company or small group of companies," says Melissa McHenry, a spokeswoman for the Columbus, Ohio, utility. "Addressing climate change requires coordinated, meaningful international action. We believe the claims are without merit."
Nevertheless, the science is not standing still. Evidence for manmade warming continues to accumulate and computer models are getting more specific all the time, says Anthony Broccoli, a climate researcher at Rutgers University's Cook College in New Brunswick, N.J. His current project is the development of a regional model for New York and New England.
"What we see are stronger and stronger conclusions that most of the warming in the latter half of the 20th century was due to human activities," he says. "I'm quite skeptical there will ever be a smoking gun on climate change. What we'll continue to see is a greater and greater preponderance of evidence ... as the climate continues to warm."
• Earth is currently experiencing a warm period between ice ages.
• Over the past century, the average surface temperature has risen about 1 degree F.
• In the next 100 years, the temperature will rise 2.2 to 10 degrees F., scientists predict.
• The 20th century's 10 warmest years occurred in its last 15 years - and 1998 was the warmest on record.
• The global sea level has risen 4 to 8 inches over the past century.
• Without emissions-control policies, today's carbon dioxide concentrations are projected to rise 30 to 150 percent by 2100.
Sources: US Environmental Protection Agency; Illinois State Museum