Science to Live By: Carbon Dioxide: The Essential Pollutant (Part Six)


© J. Dirk Nies, Ph.D.

“I’m blowin’ the day to take a walk in the sun
And fall on my face on somebody’s new-mowed lawn.”

Lyrics from “Daydream” by John Sebastian of the Lovin’ Spoonful

“Mauna Loa Carbon Dioxide Apr2013” by Narayanese, Sémhur, and the NOAA. Licensed under CC BY-SA 3.0 via Wikimedia Commons.
“Mauna Loa Carbon Dioxide Apr2013” by Narayanese, Sémhur, and the NOAA. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

Spring is busting out all over. Pastures around Crozet are verdant and the familiar muffled roar of lawnmowers once again is in the air.

When John Sebastian’s song “Daydream” hit No. 2 on the pop charts in 1966, the level of carbon dioxide nourishing new-mowed lawns was 320 parts per million (ppm). Since then, levels of atmospheric carbon dioxide have risen 25 percent. This spring, pastures and lawns now are bathed in 400 ppm CO2. Has this increase in carbon dioxide improved the vigor of grasses growing during the spring season?  Yes, according to a recently published study in the journal Climate Research.

But before we take a look at this study, there is someone I want to introduce to you, an American who has quietly changed our understanding of the natural world and our impact on it. His name is Charles David Keeling. Dr. Keeling (1928-2005) was a highly honored chemist, accomplished classical pianist and avid outdoorsman. His research led to the eponymous ‘Keeling Curve,’ a graph so important that it has been given a special place of honor on a wall at the National Academy of Sciences in Washington, D.C.

As a young, inventive, post- doctoral fellow at the California Institute of Technology in the mid-1950s, Dr. Keeling developed the world’s first electronic instrument capable of directly and very precisely measuring trace quantities of carbon dioxide in the air. Upon completing his fellowship at Caltech, Dr. Keeling joined the Scripps Institution of Oceanography in La Jolla, California. There he received funding to install his cutting-edge CO2 analyzer at a governmental geophysical observatory situated at a pristine site 11,135 feet above sea level near the top of Mauna Loa, Hawaii.

Dr. Keeling began taking measurements of CO2 at this remote location in 1958. Carbon dioxide has been monitored regularly there ever since, creating the world’s longest, continuous record of atmospheric CO2 at one location.

By 1961, Dr. Keeling had amassed enough measurements in Hawaii and other sites to report two important discoveries: CO2 levels waxed and waned with the seasons; and they increased from year to year. His first discovery disclosed a natural, annual pattern. His second discovery revealed our on-going human impact on the atmosphere.

Concentrations of carbon dioxide build up in Earth’s atmosphere during late fall, winter and early spring and then drop steadily during the late spring, summer and early fall. This regular, annual swing of CO2 results from the fact that there is more landmass in the Northern Hemisphere than in the Southern Hemisphere. On a global scale, terrestrial photosynthetic activity is more robust during May through October than November through April. When photosynthetic activity and new plant growth are high, CO2 is sequestered from the air, converted into biomass, and atmospheric concentrations drop. When photosynthetic activity and plant growth are low, natural sources of CO2 to the atmosphere (which are always on-going) predominate and CO2 levels rise. Dr. Keeling was the first to measure this yearly phenomenon.

Superimposed on this natural, annual, rhythmic pattern, Dr. Keeling also observed overall levels of CO2 increasing with each passing year. He attributed this increase to our combustion of fossil fuels, which is emitting CO2 into the atmosphere faster than plants, soil and the oceans can absorb it.

The plot of these natural and artificial changes in atmospheric CO2 levels over time has been dubbed the ‘Keeling Curve.’

As years have gone by and scientists have monitored CO2 levels at numerous places around the globe, we now know that not only is CO2 relentlessly accumulating in the atmosphere–on May 9, 2013, the daily mean concentration of carbon dioxide measured at Mauna Loa surpassed 400 ppm for the first time–but the rate of accumulation also is increasing. Had global emissions of CO2 from combustion of fossil fuels held steady from 1958 onward, atmosphere levels would be lower than they are today.  From the initial slope of the ‘Keeling Curve,’ I estimate CO2 levels would be around 30 ppm lower (370 ppm instead of 400 ppm).

Now let us return to the Climate Research study published in 2014. Dr. Paul Newton and colleagues at AgResearch looked at the impact increasing atmospheric CO2 has had on the growth of pasture.  Using data collected in New Zealand over a 44-year period–the longest grazed, irrigated pasture study in the world–they we are able to detect and to quantify a spring-time CO2 fertilization effect.  From 1960 to 2004, average CO2 levels rose by 55 ppm and net herbage accumulation rose by 11.4 percent. In other words, when everything else was accounted for (water, soil, fertilizer, temperature, sunlight, number and type of grazing animals, variety of grass, etc.), they found these fields had become significantly more productive over time. Specifically, they calculated the CO2 fertilization effect promoted a 1 percent increase in forage yield for every 5 ppm increase of CO2 in the air.

CO2 levels have risen an additional 25 ppm since 2004, the last year of the pasture study. Applying their findings to today, this translates into an additional 5 percent increase in growth of grasses during the spring.  This is good news for farmers growing hay for livestock, but perhaps not so welcome news for homeowners with a large yard to mow.


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