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Last Word is Â鶹´«Ã½â€™s long-running series in which readers give scientific answers to each other’s questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com
I use a petrol lawn mower – does it emit more carbon than the grass captures? (continued)
Ned Gayner
Charing, Kent, UK
We have half an acre of wild-flower meadow on the shoulder of the North Downs that I mow once a year in the early spring.
When we first moved here, I used a heavy duty brush-cutter to mow the meadow, but that had a couple of problems: I started to develop early symptoms of the condition known as vibration white finger and I was also a bit unhappy to be killing reptiles.
So I bought a scythe, reasoning that a design that had been continuously refined for more than 5000 years had to be intrinsically easy to use. What I didn’t anticipate was that a job that used to take upwards of two weeks could be easily completed in two days. And the bonus? No more dead snakes and lizards. It seems they know about scythes, but not about machine tools.
So: gas or grass? Neither!
Chris Daniel
Glan Conwy, Colwyn Bay, UK
A lawn, like any other biological system, is part of Earth’s carbon cycle. In the carbon capture or sequestration phase of the cycle, the grass absorbs carbon dioxide from the atmosphere during photosynthesis along with water and sunlight and converts it to energy for growth. Carbon is also absorbed into the soil.
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I was unhappy to be killing reptiles when mowing my wild-flower meadow, so I bought a scythe. No more dead snakes
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In the carbon release phase of the cycle, some of the carbohydrates in the cut grass are returned to the atmosphere immediately in a process called respiration.
If the grass clippings are left on the lawn, the organic matter is broken down by soil microbes, , and enabling further carbon sequestration and growth. This cycle continues indefinitely.
However, fossil fuels such as oil, gas or coal come from a different carbon “account”, having been underground for hundreds of millions of years. A petrol mower emits CO2 that hasn’t been part of the current carbon cycle, so no matter how efficient its motor is, it .
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