The last long-term cooling trend, caused by a number of factors including fluctuations in the amount and distribution of heat from the sun, ended in the late 19th century, an analysis of over 2,000 years of climate records reveals.
The study also found that the 20th century ranks as the warmest or nearly the warmest century on all of the continents, except Antarctica. Africa had insufficient data to be included in the analysis.
Global warming that has occurred since the end of the 19th century reversed a persistent long-term global cooling trend, said the researchers.
The researchers are members of the "2K Network" of the International Geosphere Biosphere Program (IGBP) Past Global Changes (PAGES) project. The Swiss National Science Foundation and the US NSF jointly support the PAGES International Project Office.
Because long-range cooling was caused by natural factors that continued to exist in the 20th century, the researchers argued, the warming of the 20th century makes it more difficult to discount the effects of the increase of greenhouse gases in the global increase of temperatures measured in recent decades.
However, the researchers noted, their study was not specifically designed to assess the extent to which temperature changes can be attributed to various natural and human-caused factors.
The research will appear in the May 2013 issue of the journal Nature Geoscience.