The findings come from a comprehensive geochemical analysis of sediment samples taken from Farewell Lake in a remote, environmentally sensitive area of Alaska. The work provides the first continuous record of temperature change spanning the last two millennia from that region, they write in the Aug. 21 issue of the Proceedings of the National Academy of Sciences.
"Naturally, the big question is whether human activity is causing the current warming," said principal investigator Feng Sheng Hu, a professor of plant biology and geology at the University of Illinois. "This study, however, doesn’t provide us with the analytical confidence to answer that directly. We can say that two apparently naturally occurring warm periods existed previously.
"This type of studies offer baseline information on natural climatic variability that will allow us to pursue a variety of climate-related questions,"he said. The study provides a snapshot of 2,000 years of growing seasons. Researchers analyzed lime deposits in the lake samples for their oxygen and carbon isotopic composition as well as trace-element contents. Such material is ideal for geochemical analysis and environmental reconstruction, Hu said.
The researchers concluded that warm climatic conditions occurred in A.D. 0-300 and 850-1200. During these periods, overall conditions were drier than the colder periods, they found. The initial warm period matches documented conditions in Northern Europe and wet weather in the American Southwest. The second warm period corresponds to a period known as the Medieval Climatic Anomaly.
A period of cold, reaching a peak in about A.D. 600, possibly contributed to the demise of the Kachemak culture in the northwestern Gulf of Alaska. A prolonged period of dry weather occurred in the American Southwest at this time.
In a follow-up study, Hu and postdoctoral associate Willy Tinner have found, based on preliminary data, a counter-intuitive discovery. They found that forest fires were more abundant during the colder conditions of the Little Ice Age (1400 to 1700). Such a finding is contradictory to many global warming predictions. Tinner presented the finding at the annual meeting of the Ecological Society of America in August.
The co-authors of the PNAS study are Hu, Emi Ito (University of Minnesota), Thomas Brown (Lawrence Livermore National Laboratory), Brandon Curry (Illinois State Geological Survey) and Daniel Engstrom (Minnesota Science Museum). The National Science Foundation funded the research.
Journal
Proceedings of the National Academy of Sciences