Current Warming Period Is Longest in 1,200 Years, Study Says

Sara Goudarzi
for National Geographic News
February 9, 2006
It's not normal, a new study says of the current global warming period.

Researchers analyzed tree rings, ice cores, fossils, and other "proxy climate records" and found that the present warming phase has lasted longer and affected a broader area than any other such period in the last 1,200 years.

The two English researchers behind the study reached their conclusion after studying proxy records from 14 sites around the globe. Each of these records shows how its local environment changed over time.

The researchers set out to identify extended periods of warming and cooling that occurred during the past several centuries and affected different regions of the planet at roughly the same time.

The study, conducted by Timothy Osborn and Keith Briffa from the University of East Anglia in England, will be reported in tomorrow's issue of the journal Science.

Looking to the Past

The use of proxy climate records is an established method for studying historical climate change.

These records provide clues to growing conditions, the chemical composition of surrounding material during a particular season, and other factors. This information can then be checked against known temperature changes in that region.

For example, the width of tree rings is a measure of how well a tree grew in a particular year. A tree ring also indicates approximately how warm the summer was while the ring was being formed.

"The field has come a long way, growing increasingly more rigorous in recent decades," said Michael E. Mann, director of the Earth System Science Center at the Pennsylvania State University in the town of University Park.

"There is enough information now to draw reasonably robust conclusions."

First the scientists had to make sure that the tree rings, ice cores, and other natural clues accurately reflected their time periods' temperatures.

To do so, the researchers compared 20th-century proxy climate records from 14 locations with thermometer-based measurements from those same areas—a comparison that isn't possible for proxy records from past centuries, for which thermometer records are scarce or nonexistent.

Once it was clear that the proxy records matched the thermometer records, the researchers assumed that the earlier portions of the records were in fact accurate.

The team limited its study to the Northern Hemisphere during the last 1,200 years, for which there are relatively rich proxy climate data.

"We found that between [A.D.] 890 and 1170, there was statistically significant widespread warmth corresponding approximately to the so-called Medieval Warm Period," Osborn said.

However, the most widespread warmth was found not in the Middle Ages but during the 20th century.

Heating Up

Proxy records indicate warm conditions in the mid- and late 20th century. But the thermometer measurements clearly show that the expanding area of warmer-than-normal conditions continued through to the present day.

By now almost the entire Northern Hemisphere is warmer than normal, Osborn said.

Other studies have shown similar results, but they typically focused on average temperatures of vast regions. Average temperatures can become skewed if a few spots in a large region are very hot.

Osborn and Briffa's study, by contrast, didn't rely on average temperatures. It looked at temperature readings from many locations throughout the Northern Hemisphere.

"The 20th century stands out as the only period in the past 1,200 years when the records all indicate warmth at the same time," Osborn said.

Such studies have been conducted in the past. But because this approach is independent from the others, the study's findings add weight to those previous findings, says Mann, the Penn State scientist.

"This latest paper might be the nail in the coffin for the small minority of very vocal climate change denialists who continue to challenge the conclusion that the recent warming of the Earth's surface is out of the ordinary," Mann said.

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