The concentration of atmospheric carbon dioxide surged past 420 parts per million for the first time in recorded history this past weekend, according to a measurement taken at the Mauna Loa Observatory on the Big Island of Hawaii.

When the National Oceanic and Atmospheric Administration (NOAA) research station “began collecting CO2 measurements in the late 1950s, atmospheric CO2 concentration sat at around 315 PPM,” the Washington Post reported. “On Saturday, the daily average was pegged at 421.21 PPM—the first time in human history that number has been so high.”

Climate activist Greta Thunberg took notice of NOAA’s most recent data on CO2 levels. She described the first-ever documented eclipse of 420 PPM of CO2 in Earth’s atmosphere as “truly groundbreaking.”

Exceeding 420 PPM of the heat-trapping gas “is a disconcerting milestone in the human-induced warming of the planet, around the halfway point on our path toward doubling preindustrial CO2 levels,” the Post noted, adding:

There is special significance in reaching and surpassing a concentration of 416 PPM. It means we’ve passed the midpoint between preindustrial CO2 levels, […]