Harte is a thoughtful, serious man, and his words come soft and measured; in conversation, his eyes do most of the work. Depending on the subject, they hold a curious look, or one that is stern or playful or, as is the case right now, intense. He leans forward in his chair, uncrosses his arms, and begins talking about ice.

The Antarctic ice sheet comprises layers and layers of compressed snow and ice, to a depth of more than two miles. The layers, like the rings of a tree, represent different epochs of time; near the bottom of the sheet is ice that formed hundreds of thousands of years ago. Throughout the 1980s and 1990s, a group of scientists at an Antarctic research station called Vostok drilled deep into this surface, extracting an ice core more than 11,000 feet long. The core contains oxygen isotopes that can be analyzed to determine the earth's temperature at various points in time; it also contains trapped air bubbles, which can be used to quantify levels of greenhouse gases, carbon dioxide (CO2) in particular. Together, the temperature and CO2 data from the Vostok core offer the best picture we have of the earth's climate history over the past half-million years. The core confirmed much of what we already knew from glacial records: that over great stretches of time the earth's climate has swung between eras of relative cold and hot. No fewer than four glacial periods were recorded in the Vostok core.

From his chair, Harte reaches for a book on a nearby shelf. He opens it to a page on which two graphs are printed side by side. One graph plots the Vostok temperature data over a period of hundreds of thousands of years. The other plots the CO2 data over the same time. "Do you see any similarities?" he asks. In fact, the two graphs look almost exactly alike-two jagged lines that rise and fall in near-perfect synchronicity, like outlines of the same serrated knife.

The graphs illustrate a widely accepted view that climate change is driven by increasing amounts of CO2 in the atmosphere. But what interests Harte is less the changing temperature than the regular surges in CO2."What is the mechanism? Where is the carbon dioxide coming from?" Harte asks. "That is the mystery of Vostok." In an age before humans, before carbon could be burned as fuel and released into the atmosphere, what would cause CO2 to rise so precipitously? Where was all that carbon coming from? The answer, at least in part, lies in Harte's meadow in Colorado.

A number of things happened to the meadow as it warmed, but the most important thing was this: The flowers began to die.

Alpine meadows are full of leafy perennials such as the purple-flowered larkspur, yellow glacier lilies, and sunflowers. But in the changed climate of Harte's heated plots, which were warmed by only 3.5 degrees Fahrenheit (a 1988 approximation of the warming that will occur by 2040), these flowers suffered. In time a new plant began to take over: sagebrush, a woody shrub more often associated with the arid plateaus of Colorado than with its peaks.

The heated meadow began to look different-more desertlike than pastoral-but the changes weren't just aesthetic. Sagebrush is much less reflective than the flowers; where the light-colored flowers once reflected much of the sun's energy back into space, sagebrush, with its dark bark, absorbed it. So as the sagebrush spread, the meadow began to grow even warmer.

The flowers had also served as an excellent carbon sink. They took CO2 out of the atmosphere and converted it through photosynthesis into leaves and petals and roots. During the short alpine summer, a succession of flowers would grow and die, and through decomposition that plant material would enter the soil, where microbes would eventually convert it back into carbon dioxide. In this way, the meadow maintained a carefully balanced carbon cycle, taking carbon out of the atmosphere and releasing it in equal measure.

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