Washington: A team of 78 experts from 24 countries have assembled the most comprehensive study to date of temperature change of Earth`s continents over the past 1,000 to 2,000 years.
By looking regionally, the researchers found considerable complexity hidden within a global average.
"We wanted a new and ambitious effort to reconstruct past climate. One of the strongest aspects of the consortium study is that it relies on regional expertise," said Northern Arizona University Regents` professor Darrell Kaufman -the lead co-author of the study.
Members of the consortium represent eight continental-scale regions. They lent their insights about the best proxy records-such as tree-ring measurements-to use for a particular region, and how to interpret the data based on regional climatology.
While the study does not attempt to attribute temperature changes to natural or human-caused factors, Kaufman said the finding of a long-term global cooling trend that ended late in the 19th century is further evidence that increased greenhouse gasses have had an influence in later years.
"The pre-industrial trend was likely caused by natural factors that continued to operate through the 20th century, making 20th century warming more difficult to explain if not for the likely impact of increased greenhouse gasses," Kaufman said.
While that sounds like a familiar theme, the study`s findings of regional variations are less well known. Because of extensive participation by scientists working in the Southern Hemisphere, Kaufman said, data from those regions broadened what had been a view previously centered on Europe.
"We know the most about the long-term temperature history in Europe, but we find that not every region conforms to that pattern," Kaufman said. He noted that temperatures varied by region against the backdrop of the long-term cooling identified by the study.
The regional focus on the past 2,000 years is significant for two reasons, Kaufman said. First, climate change at that scale is more relevant to societies and ecosystems than global averages. And second, "regional scale differences help us to understand how the climate system works, and that information helps to improve the models used to project future climate."
Kaufman`s own research team added to the strong regional input. His research in Alaska and elsewhere formed part of the dataset.
Kaufman and his co-authors have posted the data along with frequently asked questions about the study on the PAGES project website.