This tree ring section comes from Alaska's northern treeline. It was cut from a white spruce that lived from 1441 to 1931. Photo courtesy of Gordon Jacoby.
Gordon Jacoby sampling a 300- to 500-year-old Siberian pine tree in the Tarvagatay Mountains of western Mongolia. Photo courtesy of Gordon Jacoby.
“Trees are an enormous feature in the global landscape,” observes Gordon Jacoby. “They’re fascinating in that they have the ability to record environmental changes.” Tree rings generally grow wider during warm periods and narrower during cold ones, so their rate of growth provides a picture of Earth’s temperature over the past centuries.
Co-founder of the Tree Ring Laboratory at Columbia University’s Lamont-Doherty Earth Observatory, Jacoby is a dendroclimatologist. (Dendro is the Greek word for trees, and a climatologist is a scientist who studies climate.) Jacoby specializes in the study of annual growth patterns from old trees to see what they reflect about environmental conditions as they grew. “In most parts of the world, the written record of major environmental changes is short, so we don’t know the full range of variations that can occur naturally,” he notes, “and thus we can’t tell whether there have been recent, human-induced changes.” From a network of tree ring data being collected around the entire Northern Hemisphere tree line, the Tree Ring Lab has reconstructed temperature records for the Arctic and Northern Hemisphere dating back to the 1600s.
Disturbance in the normal tree growth patterns can yield information about natural phenomena, such as earthquakes and volcanic eruptions, if the tree was located close enough to record the effects of a particular event and yet survive it. But the major application of tree ring research is dendroclimatology: studying the relationship between tree growth and global climate change. The field really took off in the 1970s, when computers were able to develop and handle complex statistical models from tree ring data.
A recent expedition took Jacoby, Lamont-Doherty colleague Rosanne D’Arrigo, and Tsevegyn Davaajamts, a professor at the Institute of Botany at the Mongolian Academy of Sciences in Ulaan Bataar, to the Tarvagatay Mountains of western central Mongolia. There they took samples of 300- to 500-year old Siberian pines undisturbed by human activity and growing at the timberline more than 8,000 feet high, the northernmost limit of tree survival in that part of the world. “There the cold temperature limits trees, and growth is sensitive to temperature variations,” explains Jacoby. To obtain the samples, Jacoby used something called “a Swedish increment borer, a cylindrical steel tube that can drill into a tree and extract a core five millimeters in diameter.” He usually takes two cores per tree, each of which provides a complete cross-section of all of the annual rings from the center (the oldest rings) out to the bark. “It creates a small wound that usually heals within one or two growing seasons,” says Jacoby, comparing the process to the biopsy performed by a doctor testing for disease.
Providing data from this part of the world for the first time, the Mongolian samples help scientists to better understand the fluctuations in Earth’s recent temperature. Dating back to 1550, the record was revealing. “Three-hundred year annual temperature reconstructions for the Arctic and Northern Hemisphere, based on high-latitude tree ring data, indicate that the warming during the past century seen in instrumental data is unprecedented,” wrote the three scientists in their paper Mongolian Tree Rings and 20th-century Warming. “Specifically, these general trends are cooler conditions (more narrow rings) in the early 1700s, followed by a warming (wider rings) for the mid- to late-1700s, abrupt cooling and continued cool conditions for much of the 1800s, and a warming trend for the late 1800s and much of the 1900s.” The warming trends generally correspond with periods of increased solar brightness before the Industrial Revolution, and the cooler nineteenth century period with several major volcanic eruptions. (Volcanic particles block sunlight that would otherwise warm the atmosphere.) But sunlight alone doesn’t account for the significant warming of recent decades.
“In any proxy you’re not getting the actual temperature, you’re getting something that responds to temperature, so you’re getting a little biological noise in the temperature response,” cautions Jacoby. Yet this pattern is strikingly similar to that found in trees elsewhere in the Northern Hemisphere, including Japan, North America, Scandinavia, and Russia, all of which confirm that relative to the past three hundred years, the twentieth century is unusually warm. Climatologists are working to understand the relationship between human factors, such as the buildup of industrial greenhouse gases in the atmosphere, and of natural variations such as sunspot or volcanic activity. “An objective is to understand exactly how the climate system works so one can begin anticipating the future possibilities,” Jacoby says, “but we’re really gun-shy about the word ‘predict’.”
“On the basis of the tree ring work alone, we can’t tell which influences are caused by human activity,” he elaborates. “But our information can be used by climate modelers and others, who will be able to detect whether there’s some new element affecting the climate system. Tree rings say, ‘this is what’s happening.’ Modelers say, ‘this is unusual,’ and look for the cause.” New questions are constantly arising. For example, Jacoby is finding that in some parts of the northern tree line trees aren’t responding to temperature change the way they did in the past. “Their relationship to the environment may have changed because it’s been so warm for so long, or perhaps because of other factors we don’t yet understand,” he says. Jacoby is planning to continue using this tool in various regions where the records are sparse “to develop a better long-term record, meaning thousands of years. From those longer records we’ll be able to assess the present situation more definitively.”
As the tree ring record expands, scientists are finding out more about long-term temperature variations, extremes, and trends. For example, studies of moisture-sensitive trees are yielding information about the frequency of drought on a global scale. “We’re looking in some tropical areas, such as Thailand and Indonesia, where few long-term records of climate exist, as a means of expanding the geographical extent of the science,” says Jacoby. In tropical regions, where temperature isn’t a limiting factor, dendroclimatologists look for a moisture-stress signal—a reduction in tree ring size due to lack of water. “In areas like eastern Indonesia, an El Niño event usually causes severe droughts,” he explains, “so trees growing in that area can give you information about temperature change around the whole Pacific Ocean.”