Image from thebittenword.com
The residents of Almeria, Spain, could be forgiven for not thinking global warming a great threat to their fair city. While their countrymen have had to endure an annual temperature increase of 0.5°C since the early 1980s, the citizens of this small city have, instead, experienced a period of cooling. According to a new study published in the Journal of Geophysical Research, local temperatures fell 0.3°C per year between 1983 and 2006 -- an unexpected trend they attribute to the presence of a significant concentration of greenhouses, reports Anna Armstrong for Nature Geoscience (sub. required).
In recent years, Almeria has become a major provider of produce to regions of Europe that receive little natural sunlight. Once renowned for being the prime setting for a number of "spaghetti westerns," the city, located in the southeastern region of Spain, has since become home to the world's largest number of greenhouses.
Image from LWY
Greenhouses: more than just for farming
Greenhouses are buildings that have roofs and walls made either of glass or plastic and are used to grow a variety of plants. lncoming solar radiation helps heat up the plants and soil inside the structure, facilitating growth; the roof and walls help retain air that is warmed up during the process, creating what is known as the "greenhouse effect" (which is where the term "greenhouse gas" comes from). This occurs because the glass or plastic used in the greenhouse acts as a selective transmission medium for several spectral frequencies, effectively trapping certain wavelengths of light within the structure and warming the surrounding air.
Strong albedo effect observed with greenhouses
Pablo Campra of the University of Almaria and several of his colleagues were interested in gauging the climatic impact of this large aggregation of greenhouses. They had already found that greenhouses in the coastal areas of Almeria reflected significantly more radiation back into space compared with plants in surrounding regions (this is the "albedo effect" I've mentioned in the past); the effect is greatest during the summer when farms whitewash the greenhouses to prevent the plants from being exposed to excessive sunlight.
By studying past satellite records of surface reflectivity and local temperature increases, they calculated that the greenhouses accounted for a whopping reduction in solar radiation of 19.8 Watts per square meter -- equivalent to an annual 0.3°C decrease between 1983 and 2006.
Here is the conclusion Campra and his colleagues draw from their findings:
Our results show that, at local and meso-scale, greenhouse farming is very likely the most powerful driver of climate change in the area of study, probably due to the dramatic increase in surface albedo of the highly reflective plastic cover over a widespread agricultural area, which largely offsets positive forcing (+2 W m2) very probably induced by global increase in greenhouse gases [Forster et al., 2007]. The main general implication of these findings is to highlight the importance of human development of high albedo surfaces in the strategies of mitigation and adaptation to global warming at local scale. However control stations
records outside the GH area show that little or no effects on surface temperature extend far from the high albedo area, so the forcing caused by greenhouse development seems to be very localized.
Via: Nature Geoscience: Climate Science: The other greenhouse effect (sub. required)
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