On April 3, the Mauna Loa Observatory in Hawaii measured average atmospheric carbon dioxide to be 421.21 parts per million. 
This is the first time in human history that CO2 concentration has exceeded 420 ppm, which means that humans have reached the halfway point of doubling pre-industrial CO2 levels.
Doubling pre-industrial CO2 levels has a 95% chance of warming the planet by at least 2 degrees Celsius, according to a study published last year by the journal Reviews of Geophysics.
Scientists believe that beyond this threshold, climate change will lead to disastrous sea level rise and other extreme weather events.
The research station has measured atmospheric CO2 since the 1950s, when the average concentration was approx 315 ppm.
The research station has measured atmospheric CO2 since the 1950s, when the average concentration was approx 315 ppm.
Based on the past two months, scientists are expecting the average annual concentration to be over 416 ppm. CO2 accumulates in the atmosphere over time, which is why researchers are recording records despite a dramatic drop in emissions due to the pandemic.
Carbon dioxide is emitted during fossil fuel combustion, deforestation, and cement production. In 2018, the U.S. alone emitted 6,677 million metric tons of CO2 equivalent.
Citation: Dr. Pieter Tans, NOAA/GML (esrl.noaa.gov/gmd/ccgg/trend…) and Dr. Ralph Keeling, Scripps Institution of Oceanography (scrippsco2.ucsd.edu)
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