A global long-term cooling trend ended late in the 19th century and was followed decades later by the warmest temperatures in nearly 1,400 years, a sweeping study of temperature change showed.
The study, by a consortium of 78 authors in 24 countries, said its 2,000 years of data made it harder to discount the impact on higher temperatures of increased greenhouse gases due to human activity.
"Global warming that has occurred since the end of the 19th century reversed a persistent long-term global cooling trend," the National Science Foundation, one of the study's sponsors, quoted the report as saying.
Researchers found that various factors, including fluctuations in the amount and distribution of heat from the Sun and increases in volcanic activity, fed an overall change in temperature patterns.
The researchers were part of 2K Network of the International Geosphere Biosphere Program's Past Global Changes (PAGES) project. The research was published online on Sunday by the Nature Geoscience journal.
The National Science Foundation and the Swiss National Science Foundation jointly support the PAGES office. The U.S. agency called the study the most comprehensive evaluation of temperature change on the Earth's continents over the last 1,000 to 2,000 years.
The PAGES study relied mainly on analysis of tree growth rings, pollen, skeletons of coral that register sea surface temperatures, polar and glacier ice samples and lake sediments, the National Science Foundation said.
The 20th century ranked as the warmest or nearly the warmest century on all the continents except Antartica. Africa lacked enough data to be included in the analysis.
An abstract of the report on the Nature Geoscience website said that reconstructions of temperature showed generally cold conditions between 1580 and 1880. The trend was punctuated in some areas by warm decades in the 18th century.
From 1971 to 2000, the weighted average temperature was higher than any other time in nearly 1,400 years, it said.
