There is a time in late March when you enter your backyard greenhouse and notice the difference. Outside, the air bites. The glass becomes blurry due to condensation, the warmth remains motionless, and a subtle smell of wet dirt permeates the room. It seems like a small act of resistance to the weather. Scientists believe there may be more to it than meets the eye.
Climate scientists, agronomists, and gardeners have consistently reached the same startling conclusion about backyard greenhouses over the past few months: these structures, which adore tomato and orchid growers, are just a small part of a much larger phenomenon that scientists are just beginning to understand. Most people were surprised by the responses.
Pablo Campra, an agronomist at the University of Almeria in southern Spain, has spent years studying greenhouse agriculture in the province. Approximately 100,000 acres of plastic-covered growing land make this the largest in Europe. When they analyzed temperature data from weather stations positioned throughout the greenhouses, his team discovered something that kept them warm. The greenhouse zone has cooled by an average of 1.3 degrees Fahrenheit over the past 23 years, while the surrounding areas have warmed by more than 1.8 degrees. Reflective plastic roofs, which reflect sunlight back into the atmosphere rather than some expensive intervention, are responsible for that swing of more than three degrees.
Most backyard greenhouse owners may not have considered their pastime to be climate-relevant. In all honesty, it probably isn’t a single garden structure. In 2024, University of Copenhagen geographer Xiaoye Tong oversaw a satellite mapping project that made it harder to overlook the extent of what people have subtly created. Globally, there are about 3.2 million acres of permanent greenhouses, which is about the size of Connecticut, according to his team. Compared to forty years ago, this is more than twice the previous estimate. China accounts for more than 60% of that total. In addition, Tong informed researchers that temporary plastic crop covers would increase the figure to the size of New York State. A massive reflective surface on the ground is altering local temperatures in unexpected ways.
According to researchers, this is equivalent to unintentional geoengineering. Chinese scientist Xuehua Fan has described the spread of agricultural plastic film as a possible geoengineering project because the effect is real rather than intended. Cold Aston, an English village with a shorter growing season than most, uses greenhouses as a gardening tool. Climate change is mostly undetectable and sits in the background.
Invisibility, however, is reciprocal. As the research progresses, it’s hard to ignore the tension at its core. In addition to lowering water tables and producing tens of thousands of tons of plastic waste annually, greenhouses also leave microplastic particles embedded in agricultural soil at densities that can decrease plant growth by up to a fifth, as well as lowering water tables and producing tens of thousands of tons of plastic waste annually. In Ethiopia’s Rift Valley, Lake Ziway has lost about twenty inches of depth due to greenhouse operators extracting water at rates that smallholder farmers cannot match. A dead sperm whale washed up near Almeria in 2013. Its stomach contained 37 pounds of plastic sheeting from surrounding greenhouses.
In England, gardeners who have kept meticulous records for several decades describe something more personal. There has been a change in spring’s arrival. Even though the lock-down spring of 2020 was almost perfect—warm nights after warm days, a gardener’s paradise—a greenhouse keeper in Cold Aston observed that plants had to cover themselves with fleece for two weeks in 2019 because of unusually hot days and nearly freezing nights. A gardener noticed that climate change isn’t providing steady warmth. We are experiencing unpredictability. The allium leaf miner, which was first observed in Wolverhampton in 2002, has steadily moved northward as winters get milder. According to a warming map, this tiny, burrowing insect is expanding its range in exactly the same direction.
The greenhouse roof cooling effect may not be significant or long-lasting enough to compensate for the environmental costs of industrial-scale operations. Fernando Aguilar, a specialist in remote sensing at the University of Almeria, is straightforward about it. Local cooling requires a high concentration of reflective surfaces, but this concentration strains water supplies and floods soils with plastic residue. In his opinion, intensive greenhouse farming should not be marketed as a climate solution on its own.
However, the discomfort of the trade-offs does not change the physics. Almeria’s greenhouses reduce global warming by about 45%, according to Campra’s calculations, which take into account both local and global cooling effects. Reflective surfaces, such as painted rooftops or greenhouse roofs, clearly cool the area around them. Tong is unwavering on this point. There is a functioning system. The question is whether the cost is worth it.
The person wearing rubber boots and checking on their seedlings on a chilly morning isn’t particularly concerned about any of this. To shield young plants from spring gales that are becoming more severe every decade, broad beans and leeks are started in March in the greenhouse. Because of the current climate’s unpredictability, a greenhouse is more crucial than ever. An increasingly unpredictable sky surrounds this tiny haven. As a whole, those tiny shelters are growing into something much bigger than anyone could have imagined.