Guest essay by Eric Worrall
h/t Howard “Cork” Hayden – According to New York Times all the commercial greenhouse growers who artificially elevate CO2 in greenhouses to more than double natural atmospheric levels are making a terrible mistake, because increased CO2 does not produce better crop yields.
‘Global Greening’ Sounds Good. In the Long Run, It’s Terrible.
Rising carbon dioxide levels are making the world greener. But that’s nothing to celebrate.
By Carl Zimmer
July 30, 2018
“Global greening” sounds lovely, doesn’t it?
Plants need carbon dioxide to grow, and we are now emitting 40 billion tons of it into the atmosphere each year. A number of small studies have suggested that humans actually are contributing to an increase in photosynthesis across the globe.
Elliott Campbell, an environmental scientist at the University of California, Santa Cruz, and his colleagues last year published a study that put a number to it. Their conclusion: plants are now converting 31 percent more carbon dioxide into organic matter than they were before the Industrial Revolution.
While photosynthesis does pull carbon dioxide out of the atmosphere, much of that gas goes right back into the air. The reason: At night, the chemical reactions in plants essentially run backward. In a process known as respiration, plants pump out carbon dioxide instead of pulling it in.
“Part of the story is that photosynthesis is going up, and part of the story is that so is respiration,” said Dr. Campbell.
While the increase in photosynthesis is greater than that of respiration, the ultimate benefit to crops has been small — and it doesn’t explain our modern agricultural revolution.
“The driving factor has to be the fertilizers, the seed varieties, the irrigation,” Dr. Campbell said.
That “small” benefit is lucrative enough that commercial greenhouses burn tons of natural gas every year, discard the heat, and feed their crops with the CO2 produced by burning the gas, using devices like the Johnson CO2 generator.
Even the Canadian Government advises CO2 be elevated to 800-1000ppm in greenhouses, to increase photosynthesis by up to 50%.
To be fair to Dr. Campbell current atmospheric levels of around 400ppm are far lower than what the Canadian government advises for improved crop yields. So perhaps the natural effect is currently too small, we need to push a lot more CO2 into the atmosphere to realise the full benefits.