May 2020 Had the Highest Monthly Atmospheric CO2 Reading Ever Recorded

NOAA Mauna Loa Observatory

NOAA”s Mauna Loa observatory, set high on the barren slopes of a volcano in the middle of the Pacific Ocean, is ideally situated to sample air that has not been influenced by local pollution sources or vegetation. Credit: Susan Cobb, NOAA

Rise of carbon dioxide unabated: Seasonal peak reaches 417 parts per million at Mauna Loa observatory

Atmospheric carbon dioxide measured at Mauna Loa Observatory reached a seasonal peak of 417.1 parts per million for 2020 in May, the highest monthly reading ever recorded, scientists from NOAA and Scripps Institution of Oceanography at the University of California San Diego announced on June 4, 2020.

This year’s peak value was 2.4 parts per million (ppm) higher than the 2019 peak of 414.7 ppm  recorded in May 2019. NOAA scientists reported a May average of 417.1 ppm. Scripps scientists reported an May average of 417.2 ppm. Monthly carbon dioxide (CO2) values at Mauna Loa first breached the 400 ppm threshold in 2014, and are now at levels not experienced by the atmosphere in several million years.

Mauna Loa CO2 Record 2016 2020

The carbon dioxide data on Mauna Loa constitute the longest record of direct measurements of carbon dioxide in the atmosphere. C. David Keeling of the Scripps Institution of Oceanography began measurements in 1958 at the NOAA weather station. NOAA started its own CO2 measurements in May of 1974, and they have run in parallel with those made by Scripps since then. Credit: NOAA and Scripps Institution of Oceanography.

“Progress in emissions reductions is not visible in the CO2 record,” said Pieter Tans, senior scientist with NOAA’s Global Monitoring Laboratory. ”We continue to commit our planet — for centuries or longer — to more global heating, sea level rise, and extreme weather events every year.” If humans were to suddenly stop emitting CO2, it would take thousands of years for our CO2 emissions so far to be absorbed into the deep ocean and atmospheric CO2 to return to pre-industrial levels.

No apparent response to economic impact of coronavirus 

The rate of increase during 2020 does not appear to reflect reduction in pollution emissions due to the sharp, worldwide economic slowdown in response to the coronavirus pandemic. The reason is that the drop in emissions would need to be large enough to stand out from natural CO2 variability, caused by how plants and soils respond to seasonal and annual variations of temperature, humidity, soil moisture, etc. These natural variations are large, and so far the emissions reductions associated with COVID19 do not stand out. If emissions reductions of 20 to 30 percent were sustained for six to 12 months, then the rate of increase of CO2 measured at Mauna Loa would be slowed. 

Mauna Loa Carbon Dioxide Record 1960 2020

The carbon dioxide data on Mauna Loa constitute the longest record of direct measurements of carbon dioxide in the atmosphere. C. David Keeling of the Scripps Institution of Oceanography began measurements in 1958 at the NOAA weather station. NOAA started its own CO2 measurements in May of 1974, and they have run in parallel with those made by Scripps since then. Credit: NOAA and Scripps Institution of Oceanography.

“People may be surprised to hear that the response to the coronavirus outbreak hasn’t done more to influence CO2 levels,” said geochemist Ralph Keeling, who runs the Scripps Oceanography program at Mauna Loa. “But the buildup of CO2 is a bit like trash in a landfill. As we keep emitting, it keeps piling up. The crisis has slowed emissions, but not enough to show up perceptibly at Mauna Loa. What will matter much more is the trajectory we take coming out of this situation.” 

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. 

Mauna Loa Atmospheric Baseline Observatory Sunset

The sun sets west of Mauna Loa on February 20, 2010, seen from NOAA’s Mauna Loa atmospheric baseline observatory, situated near the volcano’s peak. Credit: LTCDR Eric Johnson, NOAA Corps.

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.”

 

5 Comments on "May 2020 Had the Highest Monthly Atmospheric CO2 Reading Ever Recorded"

  1. Clyde Spencer | June 5, 2020 at 2:44 pm | Reply

    Something seems amiss. The seasonal variations in CO2 strongly suggest that there is little lag in the response of the atmosphere to increases and decreases in CO2. The steady increase of the cumulative CO2 is attributed to the human-produced component of the Carbon Cycle. That shows up as (Currently, about 2.5 PPMv per year, or about 0.21 PPMv per month.) a long-term, approximately linear trend. According to the Guardian [ https://www.theguardian.com/environment/2020/may/19/lockdowns-trigger-dramatic-fall-global-carbon-emissions ] world-wide emissions fell by an average of 17% in April. [And an unspecified amount in February, March, and May.] Now, let’s assume that only half of anthropogenic emissions make it into the atmosphere; or, put another way, only half of that 17% decline has an effect on the cumulative concentration. That means that of the expected 0.21 PPMv increase expected for every month, there might only be (100% – (17%/2)) x 0.21 = 91.5% x 0.21 = 0.71 PPMv increase. Instead of a small, but measurable reduction, Scripps is reporting an even larger increase for May of this year. While the expected change is in the last significant figure of the measurements, I find it surprising that instead of seeing a decline, or staying the same as last year, there is a claimed increase with sufficient precision to tease out a decline. Would anyone care to explain to me, without resorting to authority, just how this is happening?

  2. Clyde, you are calculating a 91.5% reduction of the annual increase of 0.21 ppmv CO2, which amounts to 0.192 ppmv. Nonetheless, your question is valid, as to why the monthly increase is not reduced whereas the measurements show a greater the monthly increase. The reason is that atmospheric CO2 concentrations are controlled by other factors as well, such as ocean temperature and soil respiration rates which are a function of temperature and soil humidity. In fact, almost 25% of the entire atmospheric CO2 reservoir is cycled annually through the terrestrial biosphere and ocean surface waters. The natural factors which affect the rates of these exchanges dominate over anthropogenic contributions. Therefore the increases seen in May have more to do with terrestrial soil respiration rates than the reduction of emissions over this period. Anthropogenic contributions are not insignificant, but do not dominate on these short time scales.

  3. Clyde Spencer | June 6, 2020 at 8:13 am | Reply

    Ian
    Thank you for the response. No, the 0.21 number is the average MONTHLY increase (2.5/12), supposedly driven by anthropogenic contributions. Yes, the changes leading up to the peak in May are primarily driven by terrestrial soil respiration rates. However, the seasonal changes are against a constant background change. I didn’t claim that the anthropogenic contributions dominated at seasonal time scales. I only questioned why we are not seeing the “not insignificant” anthropogenic changes. I don’t feel that my question has been answered adequately.

  4. Should at least be a dip – also it’s one of the longest coldest springs on record in my neck of the woods – next to oncology and psychology climatology it’s one of the least trustworthy sciences

  5. Clyde Spencer | June 7, 2020 at 7:57 am | Reply

    Note: In my original comment, the 0.71 should be 0.19; that means April and subsequent months should be at least 0.02 PPMv lower than expected, based on the historic trend line. My apologies for the typo’.

    Note also that because the -0.02 is for one month only, it is a lower-bound on what should be observed in May because economic activity started to decline in January and has not recovered yet in June. That is, we should probably have seen a decrease of about 0.1 PPMv, compared to what was expected, instead of an increase of that amount. Note also that it is a conservative estimate because I’ve only used half of the estimated decline in anthropogenic CO2 emissions.

    The article claims, “The rate of increase during 2020 does not appear to reflect reduction in pollution emissions …. The reason is that the drop in emissions would need to be large enough to stand out from natural CO2 variability, …. These natural variations are large, and so far the emissions reductions associated with COVID19 do not stand out.” It is correct that they do not “stand out,” but they could probably be teased out with some analysis. Otherwise, measuring CO2 to 2-significant figures to the right of the decimal point is an exercise in futility.

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