Carbon dioxide pollution from humans—mostly burning fossil fuels, cement-making and clearing forests—is the main driver of global warming. Not all of the CO2 that people emit ends up lingering in the atmosphere. Almost a quarter is absorbed by oceans, and 31% is taken up by land. The rest remains, and that running tally of atmospheric carbon dioxide has been fundamental to the discovery and monitoring of climate change. 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. 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 2.7°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.
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.
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, producing “rapid and sustained effects to limit human-caused climate change,” according to the UN Intergovernmental Panel on Climate Change. In such a scenario, some secondary effects of global warming would continue for a time, such as sea-level rise, which may be locked in for centuries or millennia.