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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 lawnmower – does it emit more carbon than the grass captures?
Eric Kvaalen
Les Essarts-le-Roi, France
The accompanying photo for the question when it was first published shows someone using a lawnmower with nothing to catch the grass clippings. That means they will land on the grass and eventually rot, returning their carbon to the atmosphere. So the grass in the final analysis doesn’t capture any carbon.
If you do use something to catch the clippings, then maybe, theoretically, you could do something with them so they don’t rot, but they probably will. If they decompose, then they will produce methane, which is worse than carbon dioxide from the point of view of climate change.
Another question is whether the petrol lawnmower puts more carbon dioxide into the air than the agriculture necessary to provide you with the extra calories you might need to consume if mowing your lawn with a push-mower!
Hillary Shaw
Newport, Shropshire, UK
Unless your garden soil level is slowly rising (carbon is being sequestered), your lawn superficially has a zero carbon budget, as its biomass stays roughly constant.
But the grass cuttings decay and release the carbon they absorbed. In fact, your lawn is a net carbon emitter because it takes energy to mow it: for petrol, the electric mower and also your human effort, which ostensibly also has a zero net carbon budget, but the food you ate to expend the calories used in mowing had to be grown and transported.
Your garden can be a net carbon sink (for a while) if you grow trees and shrubs, but these reach maximum size and then net-absorb no more carbon dioxide, and you may get complaints from the neighbours. But your garden will be more wildlife-friendly.
Sam Edge
Ringwood, Hampshire, UK
To the person who asks whether a petrol lawnmower emits more carbon than their lawn absorbs, the answer is always “yes”. Unless all the grass trimmings are collected, compressed down into blocks and buried very deeply so that they turn into peat, coal or oil and then stay buried, they will rot or otherwise be consumed by organisms. These processes release as much CO2 back into the atmosphere as the grass absorbed in the first place.
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The best thing for global warming might be to stop doing anything to gardens and to let them return to a wild state
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It gets worse. If the lawn is watered from the mains supply from time to time or chemical fertilisers or moss and weed inhibitors are applied, then the production, transport and supply also add to the lawn’s “carbon turf-print”. And if it is a manual lawnmower, then the extra energy needed to push it has to come from the operator’s food – unless they are an organic vegan smallholder who grows their entire diet, then that is probably going to have a terrible effect too, because of the hugely inefficient and carbon-belching Western agricultural system.
The best thing for global warming might be to stop doing anything whatsoever to gardens and to let them return to a wild state. Failing that, invest in photovoltaics on the roof and get an electric lawnmower!
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