Humans have been changing Earth's climate for 180 years, earlier than scientists had previously believed, a new study reports.
The research, published in the journal Nature, involved 25 scientists from Australia, the United States, Europe and Asia, working together as part of the Past Global Changes 2000 Year (PAGES 2K) Consortium. Lead author Nerilie Abram, an associate professor at Australian National University, and colleagues examined detailed climate reconstructions spanning the past 500 years to determine when the current warming trend truly began, and analyzed thousands of years of climate models to confirm its cause.
After poring through climate histories preserved in the likes of corals, cave decorations, tree rings and ice cores, Abram and her team established that the first signals of climate change could be observed as early as the 1830s.
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That industrial age is generally considered to have begun in Britain in the mid-to-late 1700s; the development of mass production transformed the country and ultimately the world, but the use of coal to power the steam engines and factories that made it possible released carbon dioxide, a greenhouse gas, levels of which have been increasing in the atmosphere ever since. During ice ages, CO2 levels were around 200 parts per million (ppm), and prior to the Industrial Revolution, they were around 280 ppm; in 2013, CO2 levels surpassed 400 ppm.
Interestingly, the study found that warming's footprints spread gradually across the globe. The Arctic began warming first, and continues to show the greatest extent of warming on the planet. Tropical oceans show signs of climate change at around the same time, followed in short order by Europe, Asia and North America, with sustained warming beginning about 50 years later in Australasia and South America. There are localized variations within such regional changes, however: Sea surface temperature in the eastern tropical Pacific may have undergone sustained warming "markedly later" than other tropical ocean regions.
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The authors note that the point at which they say warming began was immediately preceded by some cooling in the tropical oceans and over Northern Hemisphere landmasses, largely caused by increased volcanic activity, such as the 1815 Tambora eruption. However, their reconstructions found that, even after accounting for a recovery from such conditions, the evidence was unambiguous: even the relatively small amount of greenhouse gases that had been released to that point were enough to stimulate warming.
"It was an extraordinary finding," said Abram. "It was one of those moments where science really surprised us. But the results were clear. The climate warming we are witnessing today started about 180 years ago."
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