for National Geographic News
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.
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