The last long-term global cooling trend ended late in the 19th century, according to a new international study that is published in the May 2013 issue of the journal Nature Geoscience.
The scientific team evaluated the temperature change on the Earth's continents over the past 1,000 to 2,000 years using tree rings data, tree pollen, corals, ice cores and lake sediments.
This particular long-term cooling trend was caused by factors such as fluctuations in the amount and distribution of heat from the sun and increases in volcanic activity, according to the National Science Foundation report.
The researchers also found that the 20th century ranked as the warmest or nearly warmest century on almost all of the continents. The exception was Antarctica. There was insufficient data to rank Africa.
Key excerpts from the NSF report...
Because long-range cooling was caused by natural factors that continued to exist in the 20th century, the authors argue, 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.
"The new results show that climate change is, as usual, more complicated than we expected: long, millennial natural cooling trends were punctuated by warming episodes that turned out to be more local than we thought," said Paul E. Filmer, program director for the Paleoclimate, Sedimentary Geology and Paleobiology and ArcSEES programs in NSF's Geosciences Directorate.
"The natural forces driving the cooling are still present today, but since the nineteenth century an additional, stronger, warming driver has been added: human activity. We cannot match the temperature records since then without factoring in this new driver."
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