Global warming study suggests human causes, dating back to 1800s

The strength of global warming caused by human activity is seen from the fact that natural forces are in a cooling mode, while the earth is continuing to get warmer.

http://science.nbcnews.com/_news/2013/04/22/17864735-global-warming-study-suggests-human-causes-dating-back-to-1800s?lite

by John Roach, science.nbcnews.com
April 22nd 2013

A long-term global cooling trend ended in the late 19th century, a reversal in temperature that cannot be explained by natural variability alone, according to a new study.

The finding stems from 2,000-year-long continental-scale temperature records inferred from tree rings, ice cores, lake sediments and other so-called proxies from around the world.

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"The 18th and 19th centuries would probably have been colder than the 20th century no matter what just because there has been a bit less volcanism in this century, but the amount of warming we've seen is extremely unlikely to have happened solely due to natural processes," he told NBC News.

In fact, he and colleagues note in the study published Sunday in Nature Geoscience that the natural forces driving the long-term cooling trend are still present today.

The record is consistent other recent temperature reconstructions that show the reversal in long-term cooling coinciding with the acceleration of greenhouse gas emissions from human activity during the industrial revolution at the end of the 19th century.

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