image: Researchers are examining the sediment from lakes such as Cascade Lake in southwestern Alaska to determine changes in climate through the amount of glacier melting as reflected in the amount of sediment accumulated in the lake each year. view more
Credit: Darrell Kaufman, Northern Arizona University
(Flagstaff, Ariz. Sept. 4, 2009) —A clearer picture of climate change is emerging from the sediment drawn from the bottom of Arctic lakes.
An international team of scientists, led by Darrell Kaufman, a Northern Arizona University professor of geology and environmental science, recently completed a five-year study that places the recent warming in the context of long-term climate change.
Because this warming occurred abruptly during the 20th century while atmospheric greenhouses gases were accumulating, these findings provide additional evidence that humans are influencing climate.
The evidence was found by generating a 2,000-year-long reconstruction of Arctic summer temperature using natural archives of climate change from tree rings, glacier ice and mostly from lake sediments from across the Arctic, a region that responds sensitively to global changes.
"Our reconstruction shows that the last half-century was the warmest of the last 2,000 years," Kaufman said. "Not only was it the warmest, but it reversed the long-term, millennial-scale trend toward cooler temperatures. The cooling coincided with the slow and well-known cycle in Earth's orbit around the sun, and it should have continued through the 20th century."
He said during the past few decades, the Arctic has warmed at two or three times the rate of the rest of the world, and this enhanced warming is expected to continue.
"The Arctic amplifies climate change as reflective snow and ice are replaced by dark, heat-absorbing water and vegetated surfaces," Kaufman explained. "This has consequences globally because, as the Arctic warms, glacier ice will melt, contributing to sea-level rise and impacting coastal communities around the globe, and thawing permafrost will release methane adding to the global greenhouse effect."
Supported by a $2 million grant from the Arctic System Science Program of the National Science Foundation, results from the study, "Recent Warming Reverses Long-Term Arctic Cooling," will be featured in Science magazine's Sept. 4, 2009 issue. Over the past five years, Kaufman, along with NAU students and researchers from more than a dozen universities, including the University of Arizona, collected sediment cores from Arctic lakes to decipher how the climate has changed over the last 2,000 years.
The period includes the Little Ice Age from about A.D. 1500-1800, when the Earth's climate experienced significant changes, and extends back to relatively warm conditions during the first few centuries of the first millennium.
The springboard to the synthesis was the team's climate records from 14 lakes, which were publish individually in January as a special issue of the Journal of Paleolimnology. The volume was edited by Kaufman and included papers based on master's theses of two NAU geology students.
One of the former students, Caleb Schiff, who now manages NAU's Sedimentary Records of Environmental Change Laboratory, has participated in several of the lake-coring expeditions to Alaska. He specializes in using isotopes contained in sedimentary algae to investigate how storm tracks have changed with climate over thousands of years.
"The lake cores contain the 'story' of the environmental changes that have occurred in and around the lake over thousands of years. Each sediment layer is like a page of a book," Schiff noted. "It is relatively easy to analyze the lake cores, but it is not always easy to interpret the climatic significance from those data."
To develop a robust reconstruction of temperature change, the "proxy" temperature records from lake sediments were combined with existing data from tree rings and glacier ice.
"We focus on lakes as archives of information about past environmental and climate changes because lakes are more widely distributed across the landscape than are ice caps, and their climate records generally extend further back in time than tree rings," Kaufman explained.
He said one of the most important and challenging aspects of the study was the comparison between his group's field-based temperature reconstruction and the output of a computer model of climate change by the study's co-authors at the National Center for Atmospheric Research.
"We don't have a crystal ball; therefore, we need retrospective studies to provide boundaries for climate scenarios," said R. Scott Anderson, an NAU professor of environmental sciences who uses pollen from lake sediment to reconstruct past vegetation changes, including those related to climate. "As with any kind of model, the inputs need to be grounded in what actually happened. So looking at the past tells us what actually happened and provides a reality check for models of the future."
Kaufman, who has been studying past climate changes for more than 20 years, focuses on Alaska because the U.S. is an Arctic nation, and because climate changes in Alaska involve weather systems that affect both the Arctic and the North Pacific. He said similar studies in Canada, Greenland, Iceland and Eurasia added to the regional synthesis.
Kaufman also teaches a new NAU Liberal Studies Climate Change course. He said he wrote the Science article last spring as his class was learning about the causes of climate change, including volcanic and solar activity, and changes in ocean currents.
"Students in the class were the first to hear the results of the study. I was thrilled for them to share in my discovery," he said. "I was pleased that they had gained the understanding about the climate system to realize that none of those natural factors that caused climate to change in the past is powerful enough to account for the recent warming trend, which occurred along with the build-up of greenhouse gases."
For more information, go to http://www.arcus.org/synthesis2k/index.php
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