Hot Enough for You?
Tree rings, ice cores, and the spectre of a maple-free Massachusetts

By Contance Villalba '93, '99G

Like a modern-day druid – English accent and all – Ray Bradley asks trees to impart to him their knowledge of Earth's ancient mysteries. Since Bradley is a climatologist – as well as a professor and head of geosciences at UMass – the mystery most intriguing him these days is the rapid heating up of Earth's atmosphere known as global warming.

     With a group of eight colleagues in UMass's Climate System Research Center, which he founded in 1998, Bradley looks not only to trees, but to corals and lake sediments, to reconstruct the climatic history of the planet. Over the past several years his group has brought forward startling evidence that global warming is not only real and the evident result of human activity, but that its progress has been swift and substantial.

     In two widely cited articles, one of them published in the journal Nature in 1998, Bradley and two other scientists charted a timeline of yearly average temperatures on Earth over the last 1,000 years. The timeline is graphic evidence of what he calls "a completely anomalous warming trend culminating in the 1990s." In fact, according to the team's calculations, 1998 was the hottest year of the millennium just past, and will likely be followed by even hotter years.

     Bradley, whose silver hair and beard normally frame a placid expression, is visibly piqued by those who minimize the significance of global warming. His own sense of that significance drives his current efforts to demonstrate how Earth's climate has changed and why. Working from international data bases as well as his own research, Bradley and his confederates have compiled information derived from "proxies," or natural archives, in Canada, Russia, Bolivia, and numerous other locations around the globe. The proxy data represent a worldwide, collaborative effort by climatologists who have x-rayed, chemically analyzed, and visually scrutinized thousands of cylindrical cores taken from trees, corals, and frozen lake sediments.

     The width and the density of tree rings, for example, vary with the interaction between species, geographic location, and the ephemeral conditions that make up climate: temperature, light, and the availability of water. In New England, for instance, a wide, dense ring on an Eastern Hemlock or a White Oak – both good proxy trees – suggests a warm, moist year. In the deserts of Arizona, where heat represents stress, a similar ring on a Foxtail Pine suggests a cooler year. And trees are affected by more than just climate, notes Bradley. They can, for example, be stifled by pest infestation or the lack of vital nutrients. So while each tree carries what Bradley calls a "climatic signal," it is "the job of the analyst to determine what part is due to climate and what part is due to other factors."

     Climatic signals are also found in the rings of corals and the laminations of lake sediments. Yearly layers of coral, which grow atop the limestone exudate of their defunct predecessors, incorporate different amounts of strontium, calcium, and magnesium depending on the water temperature in which they grow. By measuring the ratios of these minerals, climatologists can extrapolate the temperatures that correspond to each year.

     Ice cores, the annual accretions of which can remain clearly delineated for thousands of years, convey similar messages about temperature: for instance, in the relative amounts of two forms of oxygen isotope in each layer. "When water evaporates, more oxygen-16 than oxygen-18 is lost," explains one of Bradley's colleagues, Mark Abbott. "If you find a lot of oxygen-18 in a layer, the climate was colder at that time."

     "Varved," or layered, lake sediments also convey climate history. In the Canadian high arctic, where Bradley and his team do fieldwork, there are lakes that remain frozen for most of the year. Each summer thaw, depending on the heat of the season, brings a rush or a trickle of melted snow. The speed and force with which that water moves determines the size of the particles it will wash into the lake, and which will settle into the lake floor. Therefore, layers of sediment with large, coarse chunks represent hot summers. Fine grains represent cool ones.

     In essence, climatologists must decipher a manuscript which has been written in a foreign language, and of which only the last chapter has been translated. The key to unlocking the climatic signals of any natural archive lies in that last chapter: in the fact that humans have been keeping explicit records of climate for the past hundred and fifty years. Using these records, climatologists can correlate the characteristics of modern tree, coral, and sediment with the known climates in which they were generated, and draw conclusions about how climate shaped them. Then, by a process called calibration, the findings can be applied to proxy data from years when no human record of climate exists.

     The significance of Bradley's work, and the reason it received such widespread attention, is that the research which he published with colleagues Michael Mann, then at UMass, and Malcolm Hughes of the University of Arizona, was the first to apply such a comprehensive, proxy-based approach to the question of global warming.

What of those who say, in effect, "Oh, well – temperatures have always gone up and down"? Earth's current heat wave may be unprecedented by recent standards, but it has certainly been much hotter without the world coming to an end.

     Rob DeConto, assistant professor of geosciences and an expert on the Cretaceous period, might seem to be lending aid and comfort to the lackadaisical when he observes that "when dinosaurs ruled the Earth, this was a mostly ice-free planet." Antarctica, DeConto observes, was completely forested a hundred million years ago.

     "Yes, the Earth has been warmer than it is now," says DeConto. "But the point is that it was completely unrecognizable."

     DeConto's efforts to forecast how unrecognizable the Earth may become in the future involve the use of sophisticated computer climate models. Of particular interest to him is how living organisms affect climate. Until recently, he says, while climate models incorporated such environmental factors as the amount of heat emitted by the Sun and the chemical composition of the atmosphere, few accounted for biological impacts. When such models were tested against the past as revealed by proxy data, they proved inadequate.

     For instance, according to the old, biology-free models, the interiors of Earth's continents should have been very cold during the Cretaceous. Yet paleontologists working far inland have found fossil remains of crocodilians - crocodile ancestors - who could never have survived such temperatures.

     DeConto says that seeing Earth's surface and biology as interactive improves such models. For example, the forests that shrouded Earth's poles during the Cretaceous would not have reflected the Sun's rays as the polar ice caps do today: foliage traps heat in much the same way as a dark T-shirt on a hot summer day. Likewise, it may have been the vegetation of the ancient continental interiors that kept them hospitable to crocodile-types.

     Among Earth's living constituents, human beings have been among the most interactive. According to Bradley and his colleagues, the planet's heating trend can be traced back to the industrial revolution, when human use of fossil fuels became commonplace. Burning coal, oil, or natural gas releases carbon dioxide and other compounds into the air; carbon dioxide is one of several atmospheric gases which literally blanket the Earth and prevent heat from dissipating. CO2, along with methane, and nitrous oxide, has become increasingly abundant in the atmosphere in the last two centuries, and Bradley, DeConto, Abbott, and virtually all scientists in this field agree that increasing levels of CO2, combined with deforestation, are responsible for the recent monumental changes in temperature.

     A transparency that Bradley uses in his talks on global warming makes graphic the differences between modern and pre-industrial amounts of atmospheric CO2. Prior to 1850, he says, "carbon dioxide levels were at about 280 parts per million. Today they are about 360 parts per million." And if CO2 in the atmosphere continues to increase at its current rate, it will double its pre-industrial level in the coming century.

One of the textbooks used by Karen Searcy '84G, curator of the UMass herbarium, to teach Introductory Ecology supports DeConto's notion of an unrecognizable future world. If carbon dioxide levels were to double, it says, "beech trees presently distributed throughout all of the eastern United States and southeastern Canada would die back in all areas except northern Maine, northern New Brunswick, and Quebec." All of the plants and animals that populate Massachusetts, Searcy adds, could change to suit the climate. Species that need colder temperatures could migrate north, while others that have never grown here may flourish.

     "One of the species that is predicted to migrate north is the sugar maple," says Searcy, "which would take the intensity out of our autumn foliage. You would see more russets and browns, less orange and red." And muted autumn colors, of course, would be among the more benign consequences of global warming. DeConto points to the melting of glaciers that could raise sea levels and devastate flat and low-lying coastal regions. (Boston's Back Bay, not to mention much of Bangladesh, would be at risk.) Fresh water from glaciers could also disrupt the churning flow of sea water that normally blunts extreme temperature changes. Perhaps most importantly, the planet is "getting closer and closer" to the limits of its food-producing capacity, DeConto says. "If the climate changes enough to impact an area that supports corn - " he shrugs, leaving his sentence uncompleted.

     Such predictions for the future of the planetary environment are, of course, speculative. But in this case, they also come from a working understanding of climate, an understanding which continues to be deepened by studies at UMass's Climate System Research Center.

     Housed in an octagonal, multi-windowed room on the fourth floor of Hasbrouk – the former home of the physics library – the center supports a multidirectional approach to climate. It is supported in turn by funding from the National Science Foundation, the National Oceanographic and Atmospheric Administration, and the Department of Energy.

     Meteorological sensors with automatic data loggers, and tools for geologic coring and analysis, are among the resources gathered there for use by the university's climatologists.

     These resources are used to study not only global warming, but how climate varies naturally. Abbott, especially, is interested in "differentiating between a normal climate with natural rhythms and what's man-made." Clearly, human use of fossil fuel is not the only factor driving changes in climate. Consider the 1991 eruption of Mount Pinatubo in the Philippines. Dust and ash particles from the eruption shrouded – literally shaded – the islands for months, producing an exceptionally cold summer.

     More generally, says Abbott, the energy emitted by the Sun has been increasing since the early 19th century, and the variable distance and angle between the Sun and the wobbly Earth gradually changes over time. His research shows that water levels in Lake Titicaca, his study site on the border between Bolivia and Peru, have risen and fallen with the wobbling of the Earth on its axis. "The intertropical convergence zone - the low pressure belt that brings rain - follows the hottest spot on the continent," he says. When changes in the angle of Earth to Sun shift the warm zone to another part of the globe, the rains go with it. Notably, Abbott has found elevations of water level in Venezuelan lakes that precisely coincide with the drops in Bolivia and Peru.

     Regardless of the impact of natural factors on Earth's climate, the fact remains that humans are at least partially to blame for potentially devastating increases in temperature. What can we do? Sign the Kyoto protocol, says Bradley. Nations that sign this international agreement promise to reduce greenhouse gas emissions to 1990 levels by 2010. "It's not perfect," he says, "but it's a step in the right direction."

     Bradley is well-acquainted with the argument that the protocol is a prohibitively costly approach to a threat that's at best remote. Not surprisingly, he disagrees. "We've made huge investments in the past to protect ourselves from fairly unlikely events like the Russian launch," he says. "Why not guard against an event that is slowly but undeniably creeping up on us?

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