Northern Climate, Ecosystems Driven By Cycles Of Changing Sunlight

Researchers at six institutions report the findings in the Sept. 26 issue of the journal Science. The data, they say, help to explain past changes on land and in freshwater ecosystems in northern latitudes and may provide information to help project the future.

The scientists identified cycles lasting 200, 435, 590 and 950 years during the Holocene Epoch, said principal investigator Feng Sheng Hu of the University of Illinois at Urbana-Champaign. The pattern of environmental variations they found also matches nicely with cyclic changes in solar irradiance and the extent of sea ice in the North Atlantic.

"We found natural cycles involving climate and ecosystems that seem to be related to weak solar cycles, which, if verified, could be an important factor to help us understand potential future changes of Earth's climate," Hu said.

"Will changes in solar irradiation in the future mitigate or exacerbate global warming in the future? They may do both," said Hu, a professor in the plant biology and geology departments at Illinois. "A period of high solar irradiance on top of high levels of greenhouse gases could result in unprecedented warming."

The new data come from Arolik Lake sediment in the tundra region near the Ahklun Mountains, along the southwestern coast of Alaska. Hu and co-author Darrell Kaufman of Northern Arizona University in Flagstaff have conducted climate-change research in that region for more than a decade.

"To our knowledge, this is the first data set from the North Pacific high latitudes that has enough details to evaluate the effects of centennial scale solar cycles on climate and ecosystems," Hu said.

Sediment samples were tested for a variety of biological and chemical components related to environmental qualities, including their composition of biogenic silica, pollen and isotopes. The new data combined with recent findings of North Atlantic ice cover and production records of the cosmogenic nuclides beryllium-10 and carbon-14 strongly suggest that variations of Holocene climate on multi-centennial timescales reflect changes in solar intensity, the researchers wrote. Sun-ocean-climate linkages may account for similarities in the North Atlantic and North Pacific, Hu said.

In a Science paper published in 2001, one of Hu's colleagues, Gerard Bond of Columbia University in New York, and nine other authors documented a close connection between North Atlantic drift ice and changes in the cosmogenic nuclides beryllium-10 and carbon-14. "Now, Hu's findings in the North Pacific not only strongly corroborate the sun-climate connection we proposed, but they also imply that the response to solar variations may have involved much if not all of the Northern Hemisphere," Bond said.

Hu and colleagues linked the solar cycles to changes in lake productivity and plant densities, as well as variations in temperatures and moisture in the Alaskan tundra. The abundance of pollen from shrubs varied up to 25 percent between cycle peaks.

"When there have been high aquatic production and abundant shrubs, then warmer, more moist weather conditions are found at our site, and these conditions coincide with the presence of less drift ice in the North Atlantic and of higher solar irradiance," Hu said.

Data from biogenic silica (single-celled algae that reflect lake productivity), North Atlantic sea ice, and baryllium-10 and radiocarbon measures were "strikingly consistent" during the cycles, with the exception of conflicting correlations that occurred in a less-defined cycle that occurred between 5,000 and 6,000 years ago, the researchers wrote.

The presence of predictable cycles dating back thousands of years provides data that are not detectable in instrumental records, which are largely restricted to just the last 100 years, Hu said.

Illinois co-authors on the paper were microbiologist Sumiko Yoneji and graduate students David Nelson, Jian Tian and Benjamin Clegg. Other co-authors were Bond of Columbia University, Aldo Shemesh of the Weizmann Institute of Science in Israel, Yongsong Huang of Brown University and Thomas Brown of the Lawrence Livermore National Laboratory in California.

The David and Lucile Packard Foundation, National Science Foundation and Israeli Science Foundation funded the research through individual grants to the participants.