Global cooling?

Recent trend reversal

field scientists
Scientists use acoustic imaging to select a coring site at a lake in southern Alaska. Image: Darrell S. Kaufmann, Northern Arizona University

field scientist
A field scientist examines a lake sediment core with intricate layering indicating environmental and climatic changes over centuries. Image: Darrell S. Kaufmann, Northern Arizona University

May 1, 2013 — The most comprehensive evaluation of temperature change on Earth's continents over the past 1,000 to 2,000 years indicates that a long-term cooling trend--caused by factors including fluctuations in the amount and distribution of heat from the sun, and increases in volcanic activity--ended late in the 19th century.

The study also finds 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, say the researchers.

A consortium of 78 authors from 24 countries, some of them supported by the National Science Foundation (NSF), also note in research published in the May 2013 issue of the journal Nature Geoscience that there were regional differences in temperature evolution.

"This is an example of a large international team effort, collaborating to synthesize new scientific results from a very large, publicly available dataset," said Paul E. Filmer, program director for the Paleoclimate, Sedimentary Geology and Paleobiology and ArcSEES programs in NSF's Geosciences Directorate.

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.

"Global warming that has occurred since the end of the 19th century reversed a persistent long-term global cooling trend," the researchers write in the 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.

However, the researchers note, 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 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," Filmer said.

"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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Based on materials obtained from AAAS