Cascade Lake

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.

Darrell Kaufman

Darrell Kaufman points out landscape features during a field trip with his geomorphology class.

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 greenhouse 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.”

Arctic climate change research
gets further funding

Darrell Kaufman’s climate-science research recently got an impressive boost from the National Science Foundation, which will fund $2.5 million, plus field logistical expenses, to support his collaborative research into the sensitivities of the Arctic climate system.

The study builds on Kaufman’s previous research (see story at left), and combines evidence of past climate change from lake sediments with the output of climate models to help understand the tipping points in the Arctic that have led to abrupt climate changes dating back 10,000 years.

“This project will contribute to understanding climatic variability,” said Kaufman, a professor in NAU’s new School of Earth Sciences and Environmental Sustainability.“It will provide insights into why the Arctic is changing faster than any region on Earth, and where it is heading in the future.”

Kaufman is the lead principal investigator on the “Nonlinearties in the Arctic climate system during the Holocene” project that includes 12 other NSF scientists and a host of international collaborators committed to using lakes as archives of information about climate change. They will team up with climate modelers at the National Center for Atmospheric Research to improve the ability of climate models to accurately simulate thresholds in the climate system.

NAU’s portion of the funding is $316,000.

The three-year research project will focus on lakes in Alaska and in and the northwest Atlantic region, and will provide training experience for nine graduate students and many undergraduates in global-change and system-science research. It also will support two postdoctoral students and several junior faculty members.

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 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 published 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.”

Greyling Lake

At Upper Greyling Lake in Alaska, researchers combine the geomorphic evidence for glacier fluctuation with lake-core evidence to determine and interpret past climate change.

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 www.arcus.org/synthesis2k/index.php

Artic
New research shows that the Arctic reversed a long-term cooling trend and began warming rapidly in recent decades. The blue line shows estimates of Arctic temperatures over the last 2,000 years, based on proxy records from lake sediments, ice cores and tree rings. The green line shows the long-term cooling trend. The red line shows the recent warming based on actual observations. A 2000-year transient climate simulation with the National Center for Atmospheric Research’s Community Climate System Model shows the same overall temperature decrease as does the proxy temperature reconstruction, which gives scientists confidence that their estimates are accurate. (Courtesy Science, modified by the University Corporation for Atmospheric Research.)