Climate changes recorded in tree rings

Dan Hoffman Arizona Daily Wildcat Malcolm Hughes, director of the Laboratory of Tree Ring Research, is involved in a joint project with scientists in Massachusetts to investigate global warming through tree ring data.

"This is the first time anyone has published a serious attempt to capture large-scale temperature change," Hughes said. "This is the first major attempt to do it on a global or hemispheric scale."

Hughes is director of the UA's Laboratory of Tree Ring Research. The full report by Hughes, Michael Mann and Raymond Bradley appeared in yesterday's issue of Nature.

To reconstruct the past 600 years of Earth's climate history, the team studied tree rings, ice cores and coral samples gathered from more than 100 locations around the world.

The report shows that the 15th through 19th centuries saw relatively mild temperature fluctuations connected to "climate forcing" - factors like volcanic eruptions and solar radiation levels.

Hughes and his colleagues identified two separate warming periods this century that may be tied to industrial pollutants and fossil-fuel combustion.

"It's unusual," Hughes said of this century's rises in temperature. "When we go back 1,000 years maybe it won't be."

"There's a good case that we can affect the Earth's climate, but exactly how that works we're trying to figure out," he added.

Hughes said techniques for studying climate change over hundreds and thousands of years have been in development since the early 1970s.

Application of these techniques on a global or hemispheric scale has been made possible only within the past four years, he said.

"What we've used is data collected by a large group of people over the last 20 years," Hughes said. "Perhaps half was developed right here in this lab."

The researchers supplemented the tree-ring, coral and ice core data with temperature estimates documented in regional accounts of crop yields and famines.

The team found that its reconstructed temperature pattern matched historical accounts of climate change associated with the 1791 El Ni–#241;o weather pattern and the 1816 eruption of the Indonesian volcano Tambora.

"It's the best we can do right now, but we know we'll be able to do better soon," Hughes said.

The goal is to eventually trace the record back to A.D. 1000.

That begins to pose problems, Hughes said, because there are a limited number of suitable trees available for study.

"Trees didn't start growing all at once," Hughes said. "The further back (in time) you go, the fewer trees there are."

Hughes said the results of the initial study will help scientists understand how and why global climate changes through time. As more regional studies are done, there likely will be amendments to the reconstructed weather pattern.

"If excess greenhouse warming increases over the next couple of decades we'll be better able to understand what's going on," Hughes said. "The better information we have, the more smartly the nations of the world can react."