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The rings tell the story: global warming ahead

By Sarah Spivack
Arizona Daily Wildcat
October 5, 1998
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Randy Metcalf
Arizona Daily Wildcat

Malcolm Hughes, director of the UA Tree Ring Laboratory, gives a speech entitled "Is the Twentieth Century Unusually Hot?" Friday in the Harvill Building. His speech focused on world global warming over the past hundred years, which has resulted in the 20th century being the warmest the Earth has experienced in 1,000 years.

Malcolm Hughes knows it's getting hotter, and he has the tree rings to prove it.

Hughes, director of the UA Tree Ring Laboratory, has read the history of climatology on tree trunks and interpreted the data to explain the increased global temperatures.

Rex Adams, a research specialist in Hughes' lab, explained that temperature has an effect on the density of the wood in trees.

Scientists can use tree rings to read exactly when certain geological events took place, because each ring corresponds to a specific year in history.

"Because we can pinpoint things to the year and even within the year through looking at rings, we can relate other factors to ring growth," Adams said.

The 20th century has been the warmest Earth has experienced in the past millennium - and this decade is the hottest yet, according to Hughes.

"The evidence points to artificial greenhouse warming and our need to do something about it," he said. "I see nothing else that would be consistent with what we're seeing."

While researchers cannot pin down a cause-and-effect relationship between carbon dioxide gas and global warming, there is overwhelming evidence that such a relationship exists, Hughes said in a lecture Friday to about 50 UA students and faculty.

The "greenhouse effect" occurs when carbon dioxide in the atmosphere traps heat that is radiated by the earth, contributing to overall temperature increases.

Car exhaust and burning coal are among sources that contribute to the warming.

Hughes said people should take global warming as seriously as a person would signs of poor physical health.

"I don't know that obesity or high blood pressure or high cholesterol is going to kill me - but there is a risk," he said.

In the past there have been periods of unusual warmth or cold, but these were always focused in one spot on Earth. Never before has there been a significant change in global temperature like we are experiencing in the 20th century, Hughes said.

After tracking temperature patterns and how they have varied over several centuries, the 20th century stands out as unique in its temperature leap, he said.

He said the greenhouse effect has caused the average global temperature to go up about 1 degree Fahrenheit over the past 100 years.

To find out about global climate conditions stretching back to the 15th century, Hughes' team looks at "natural archives."

Researchers look at coral reef growth layers, tree rings and glacial ice layers to discover what the weather has been like historically all over the globe.

"The record I'm most concerned with is that which is most widespread - that is annual tree rings," Hughes said.

By studying natural archives, Hughes and his team have determined average global temperature for the last several hundred years and have matched temperature changes with geological causes such as volcanic eruptions.

For example, a volcanic eruption in Indonesia in 1815 caused a "year without a summer" the following year, Hughes said.

Dust and ash from the eruption blocked out enough sunlight to drop temperatures around the globe. A tree ring that dates back to 1816 would record the temperature change by marking the year with wood of lower density.

Hughes' research team looked at a variety of data to determine global temperature patterns.

During the last 150 years or so, humans have kept records of temperature - but data from weather stations are insufficient for making judgments about global changes, Hughes said.

There are almost no instrumental records from Eastern Europe or Africa, for example, and data that exists does not always meet modern scientific standards for accuracy. Also, the best instrumental data has come in the last century - after the burning of fossil fuels had already begun to influence climate.

Hughes and his research team examined global temperature records from the 13th century to the present. While the average global temperature has fluctuated over the last 600 years, in the 20th century it has skyrocketed far beyond the normal range of variability, Hughes said.

Since the Industrial Revolution began near the end of the 19th century and people began burning massive amounts of fossil fuel, increased levels of carbon dioxide gas in the Earth's atmosphere are probably responsible for the rise in global temperature, he said.

The increase in temperature already has observable effects, Hughes said. In the Arctic, for example, the ground never completely thaws. Since temperatures have risen, the soil has thawed out to a deeper level than is usual, causing buildings to tip.

He said more extreme results are yet to come.

Island nations in the Pacific ocean have considerable reason to worry about the heating of the oceans, which will expand with the heat and may cause destructive flooding.

If the warming trend continues, "we should, maybe already, or in the next decade or so, see an increase in extreme droughts, and an increase in hurricanes," Hughes said.

While he said it is fairly obvious that the accumulation of carbon dioxide gas is responsible for global warming, the task of scientists now is to figure out how exactly the greenhouse effect works - whether it takes a year, a decade or a century for us to see the results of production of this gas.

Hughes' colleague, UA professor of geography Andrew Comrie, said while the average increase in temperature is only 1 degree, some areas experience an increase of 5 or 10 degrees.

These areas could suffer most from heat-related problems like drought.

Comrie said logistically, it wouldn't be too difficult for people to burn less fossil fuel and slow down warming trends, but there is another obstacle.

"We're addicted to using fossil fuels and no one wants to change their lifestyle," he said.

Sarah Spivack can be reached via e-mail at Sarah.Spivack@wildcat.arizona.edu.