Rise of Heat-Trapping Carbon Dioxide in the Atmosphere Unabated

Even though terrestrial plants and the global ocean absorb an amount of CO₂ equivalent to about half of the 40 billion tons of CO₂ pollution emitted by humans each year, the rate of CO₂ increase in the atmosphere has been steadily accelerating. In the 1960s, the annual growth averaged about 0.8 ppm per year. It doubled to 1.6 ppm per year in the 1980s and remained steady at 1.5 ppm per year in the 1990s. The average growth rate again surged to 2.0 ppm per year in the 2000s, and increased to 2.4 ppm per year during the last decade. “There is abundant and conclusive evidence that the acceleration is caused by increased emissions,” Tans said.

The Longest Unbroken Record of CO2 Measurements
Charles David Keeling of Scripps Oceanography, located at the University of California San Diego, began on-site CO₂ measurements at a NOAA’s weather building on Mauna Loa in 1958, initiating what has become the longest unbroken record of CO₂ measurements in the world. NOAA measurements began in 1974, and the two research institutions have made complementary, independent measurements ever since. 

The Mauna Loa observatory is a benchmark sampling location for CO₂. Perched on a barren volcano in the middle of the Pacific Ocean, the observatory is ideally situated for sampling well-mixed air - undisturbed by the influence of local pollution sources or vegetation - that represents the global background for the northern hemisphere. The Mauna Loa data, together with measurements from sampling stations around the world, are incorporated into NOAA’s Global Greenhouse Gas Reference Network, a foundational research dataset for international climate scientists. 

The Keeling Curve 
Keeling was the first to observe that even as CO₂  levels rose steadily from year to year, measurements also exhibited a seasonal fluctuation that peaked in May, just before plants in the northern hemisphere start to remove large amounts of CO₂ from the atmosphere during their growing season. In the northern fall, winter, and early spring, plants and soils give off CO₂, causing levels to rise through May. The continued increase in CO₂ and the seasonal cycle are the main features of what is known as the Keeling Curve. 

The two research institutions’ CO₂ measurements often vary by a small degree.  “We use independent instrumentation, calibration gases, and algorithms to compute the average, so small differences are to be expected,” Keeling said.

The two datasets, however, tell the same story.

“Well-understood physics tells us that the increasing levels of greenhouse gases are heating Earth’s surface, melting ice and accelerating sea-level rise,” Tans said. “If we do not stop greenhouse gases from rising further, especially CO₂, large regions of the planet will become uninhabitable.”