• CO2 levels in the atmosphere were 50 percent greater than in the pre-industrial era, prompting concerns about climate change.
• Carbon dioxide levels in the atmosphere were 50% higher in May than in the pre-industrial era.
• Reaching levels not seen on Earth in about four million years, according to the leading US climate agency.

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Carbon dioxide levels in the atmosphere have above the 420 parts per million (ppm) threshold, according to the National Oceanic and Atmospheric Administration (NOAA). PPM stands for part per million and is a unit of measurement for air pollution.
Humans are responsible for global warming, which is exacerbated by the use of fossil fuels in the generation of power, transportation, cement manufacture, and even deforestation.