Lonnie Thompson, Climate Scientist - NYTimes.com George Steinmetz MISSION TO BOLIVIA In a 1997 expedition, Lonnie G. Thompson, right, recovered ice with a solar-powered drill. COLUMBUS, Ohio - One day in 1991, high in the thin, crystalline air of the Peruvian Andes, Lonnie G. Thompson saw that the world's largest tropical ice cap was starting to melt. It was the moment he realized that his life's work had suddenly become a race. The discovery meant other ice caps were likely to melt, too, and the tales of past climate that they contained could disappear before scientists had a chance to learn from them. Driven by a new sense of urgency over the ensuing 20 years, he pulled off a string of achievements with few parallels in modern science. He led teams to some of the highest, most remote reaches of the earth to retrieve samples of the endangered ice. Then last October, the race against the clock became much more personal. Dr. Thompson woke up in a Columbus hospital room, a strange dream rattling in his brain. He looked down. "Wires were coming out of my chest," he said. Machinery had been implanted to keep him alive. Longer term, doctors told him, only a heart transplant would restore him to full health. Dr. Thompson, 63, is one of the most prominent of the generation of scientists who, in the latter decades of the 20th century, essentially discovered the problem of global warming. Now those scientists are beginning to age out of the field. Many of them say they grapple with the question of how hard to keep pushing themselves. Could one more finding or one more expedition help turn the tide of public awareness? Some have continued working into their 70s and 80s. One of the most vocal about the need for action, Stephen H. Schneider of Stanford University, fought off a rare form of cancer several years ago, only to die of a blood clot in 2010 after speaking in Europe about climate change. He was 65. Of this pioneering group, few were hardier than Dr. Thompson, who has taught earth sciences at Ohio State University since the 1970s. Though he routinely spent up to two months a year camped in dangerous conditions atop mountains, he despised derring-do. His enterprise was driven by a lust for hard data. Hauling six tons of equipment to South America, Africa, Asia and Europe, he and his small team raced to recover long cylinders of ice from glaciers that had built up over thousands of years. The layers in those cylinders contained dust, volcanic ash, subtle variations in water chemistry, even the occasional frozen insect - a record of climatic and geologic changes that could be retrieved, preserved and interpreted like a series of tree rings. Dr. Thompson became one of the first scientists to witness and record a broad global melting of land ice. And his ice cores proved that this sudden, coordinated melting had no parallel, at least not in the last several thousand years. To some climate scientists, the Thompson ice core record became the most convincing piece of evidence that the rapid planetary warming now going on was a result of a rise in greenhouse gases caused by human activity. "The reason Lonnie's stuff is so powerful is that it's so simple," said Daniel P. Schrag, a geochemist at Harvard and director of its Center for the Environment. His evidence dismisses the idea that this is some sort of 300-year or 500-year cycle, which is what the skeptics and the deniers want to say. You say: "No, because Lonnie's ice didn't melt then. It's melting now, but it didn't melt then." " Colleagues say Dr. Thompson neglected his own health in pursuit of his science. Now, largely confined to his office and home in Columbus, he said he has begun to appreciate the clarity afforded him by his circumstance. "I wouldn't wish it on anyone, but it's not all bad," he said. It really forces you to sit down and think about what it is you're doing and why you're doing it, and how you are using your time. Drawn to the Tropics Raised on a farm near Gassaway, W.Va., Lonnie Gene Thompson arrived at Ohio State with the idea of becoming a coal geologist, but ice soon seduced him. As a graduate student pursuing a doctorate in geology, he was put to work analyzing dust in ice cores retrieved from Antarctica, and he learned how minute chemical and physical features could be used to deduce past climate. He had recently married a fellow student, Ellen Mosley, who was drawn into Antarctic field work and became a leading Ohio State researcher in her own right. Collaborating with John Mercer, an earth scientist at the university known for his studies of glacial geology in Latin America, Lonnie Thompson was drawn to tropical ice.