Apr 22, 2013
A groundbreaking new report published Sunday by Nature Geoscience found that average worldwide temperatures over the past thirty years were "higher than any other time in nearly 1,400 years."
Researchers behind the report, "Continental-Scale Temperature Variability During the Past Two Millennia," reconstructed past temperatures for continental regions over the past one to two millennia. Over that time, the analysis shows a long term cooling trend that lasted through to the middle of last century. Then, as the climate advocacy group Tck Tck Tckwrites, the cooling "halted with a sharp reversal" in the late nineteenth century, "correlat[ing] directly with an increase in carbon emissions from human activity."
"Prior to the 20th century natural drivers were dominant, such as change in solar output and volcanic eruption, however what happened during the 20th century is that human influences and predominantly greenhouse gases become dominant," says co-author and paleoclimatologist Dr Steven Phipps of the University of New South Wales.
The reversal culminates during the period between 1971-2000 during which the researchers found the "average reconstructed temperature was higher than any other time in nearly 1,400 years."
The report also refutes the popular belief that previous warming and cooling trends spanned the globe, finding instead that they only occurred regionally.
"What we thought of in the past as being globally uniform phenomena, such as the Medieval Warm Period and the Little Ice Age, didn't happen at the same time--for example, the medieval warming happened earlier in the northern hemisphere than the southern hemisphere," Phipps says.
The implication of this is that the current widespread global warming phenomena is unique and cannot be explained by historic causes of temperature variability, such as volcanic eruptions and changes in solar irradiance.
Gathering data from corals, ice cores, tree rings, lake and marine sediments, historical records, cave deposits and climate archives to help establish the temperature trends, the study is the most comprehensive reconstruction of global temperatures to date.
The consortium of 78 authors from 24 countries are affiliated with the 2K Network of the International Geosphere Biosphere Program's Past Global Changes (PAGES) project.
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