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If the industrialized and industrializing nations of the world do not commit to reducing fossil fuel emissions in the coming years, Earth will cross a climate change threshold by 2036, figures Michael E. Mann, one of the original authors of the famous "hockey stick graph" which showed the exponential rise of global carbon emissions. "In 2013 atmospheric CO2 briefly reached 400 ppm for the first time in recorded history—and perhaps for the first time in millions of years, according to geologic evidence. To avoid breaching the 405-ppm threshold, fossil-fuel burning would essentially have to cease immediately."

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Mann believes that the Earth's climate can reasonably adjust to a temperature change of approximately two degrees Celsius. In order to avoid the kinds of agricultural disasters that could result from a broad and consistent change in temperature, carbon emissions would need to be reduced to levels found during the mid-20th century. "That would require widespread deployment of expensive 'air capture' technology that actively removes CO2 from the atmosphere." While the United Nations now says a change of 2.5 degrees Celsius is tolerable, Mann doesn't want to risk having a false sense of security. 

Read more at Scientific American