Why this number

Carbon dioxide pollution from humans—mostly burning fossil fuels and clearing forests—is the main driver of global warming. Not all of the CO2 that people emit ends up lingering in the atmosphere. About a quarter is absorbed by oceans, and another quarter is taken up by plants. The rest remains, and that running tally of atmospheric carbon dioxide has been fundamental to the discovery and monitoring of climate change. The number above means CO2 now makes up 0.0418% of the planet’s overall atmospheric volume, more than 49% higher than before industrialization. Geologic evidence suggests that the earth’s CO2 was last at this level approximately 3 million years ago—an era when the world was 2°C or 3°C warmer and the oceans may have been more than 65 feet higher.

Scientists don’t usually think about CO2 as a percent of the earth’s atmosphere. It’s better to envision a sample of atmospheric gas divided into a million equal parts. Carbon dioxide now makes up about 418 parts per million (ppm) of that air. The CO2 level is the world’s thermostat. The higher it goes, the warmer things will get. At this rate the planet could warm by a dangerous 3°C above pre-industrial levels by 2100.

The atmospheric CO2 level has an annual cycle. It rises during fall and winter in the Northern Hemisphere, where most plants and trees live, and drops during spring and summer, when those organisms draw down carbon-dioxide during photosynthesis. That’s why the counter here will move up and down by season, even as the trendline points upwards year over year.

How we know

The Scripps Institution of Oceanography pioneered CO2-monitoring in the 1950s. The U.S. National Oceanic and Atmospheric Administration has been tracking it since 1974. The Bloomberg Carbon Clock provides a real-time estimate of the current monthly global average, based on historical data from these sources.

What progress looks like

The carbon clock will be a lagging indicator of progress. If new greenhouse-gas emissions ended tomorrow, CO2 in the atmosphere would begin washing into the ocean, rolling the parts-per-million count backwards outside of the seasonal up-down cycle. This would start a very long process. Getting it back to pre-industrial levels would take thousands of years—unless we deploy technologies that can suck up vast quantities of CO2 from the air.