As CO2 Passes 400 PPM, What Goes Up Might Not Come Down

Reaching the symbolic 400 ppm threshold underscores governments' inaction and worsening global warming impacts.

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Carbon dioxide concentrations in the atmosphere once again exceeded 400 parts per million Monday, but this time they may never fall back down, according to scientists.

While not a tipping point that signals climate catastrophe, the 400 ppm mark is an important symbolic threshold in the fight against climate change. It represents a 43 percent jump in greenhouse gases since pre-industrial times and underscores governments’ inaction and worsening global warming impacts. 

Scientists at the Mauna Loa Observatory in Hawaii, the world’s longest-running CO2 monitoring station, predicted in October that because of extra warming from El Nino, 2015 could be the last year CO2 concentrations in the Northern Hemisphere stay in the 300 ppm range. They strengthened that prediction on Monday after a routine calibration of data raised measurements taken since April by 0.4 ppm. “The adjustment increases the likelihood that [CO2] concentrations will remain above 400 ppm permanently after 2015,” they wrote.

Mauna Loa’s CO2 record is known as the Keeling Curve, named after the climate scientist Dave Keeling who started the monitoring project in 1958. For 57 years, the station has taken hourly atmospheric CO2 readings from atop a Hawaiian volcano two miles up in the air.

Annual CO2 concentrations ebb and flow depending on the season, with the lowest levels typically recorded in late August or early September when vegetation growth in the Northern Hemisphere is at its peak. Concentrations are at their highest in late winter when there is little or no growth. But the Mauna Loa record overall shows that CO2 in the atmosphere has been increasing for more than half a century—a trend that’s mirrored by the rise in the burning of fossil fuels like coal, oil and natural gas. The graph is considered key evidence that human activity is the main driver of climate change.

The recent Keeling Curve announcement follows another earlier this week by the World Meteorological Organization—which also tracks CO2 concentrations at various stations across the globe—that average monthly carbon dioxide levels exceeded 400 ppm in early 2015. The Met Office and Climatic Research Unit at the University of East Anglia in Britain also reported Monday that the globe’s average temperature has warmed 1 degree Celsius (or 1.8 degrees Fahrenheit) since pre-industrial levels—marking the crossing of another global warming milestone.

Scientists widely predict that 2015 will be the world’s warmest year on record.