A Greenhouse effect has cooled the climate of Almerķa
Since the 1980s, Almerķa in southern Spain has developed the largest
concentration of greenhouses in the world, covering 26,000 hectares. The
greenhouses reflect so much sunlight back into the atmosphere that they are
actually cooling the province, Spanish researchers have found. While
temperatures in the rest of Spain have climbed at rates above the world average,
meteorological observatories located in the so-called sea of plastic have shown
them moving in the opposite direction, with a decline of 0.3 degrees per decade.
The strange phenomenon had not gone unnoticed in scientific circles, and now a
study has suggested a plausible explanation: the white colour of the plastic
reflects sunlight into the atmosphere as if it were a mirror, and it slows the
warming of the surface. In this way, the greenhouses at a local level offset the
rising temperatures associated with global warming.
The work, which has just been published in the Journal of
Geophysical Research, has been coordinated by a group of researchers led by Paul
Campra, a professor at the University of Almerķa (LAU) assisted by Monica
Garcia, Yolanda Canton and Alicia Palacios.
During the first phase of the study, temperatures of the two
major weather stations in the area, Las Palmerillas-Cajamar and La Mojonera,
were analysed and compared with those from other stations immediately adjacent
to the region. The result was that temperatures in the region have fallen by an
average of 0.9 degrees since 1980, when greenhouses expansion began, while in
Malaga, Granada and Murcia-San Javier the increase has varied between 1 and 3
degrees. Thus there has been a difference of more than two degrees. There have
been no significant changes in rainfall.
Temperatures at Almerķa airport increased during the 1980s,
but then decreased in the 90s, reaching a balance of virtually zero change.
"You can see how the greenhouses are increasing as the years pass,"
said Campra. The process of greenhouse expansion has now plateaued or is
The second phase of the study involved researchers analysing
changes in light reflectivity called the albedo effect, using data provided by
the Modis Terra satellite and NASA. The results were striking: since 1983, the
albedo in the Almerķa has increased, with 9% more energy reflected into the
atmosphere. The landscape has changed from semi-arid scrub to one covered almost
exclusively by white plastic, a colour that best reflects the light. Because of
changes in land use, the energy balance in the area has decreased by 20 watts
per square metre (30 watts in summer), an effect much greater than changes
caused by a global warming, an estimated increase of 1.66 watts.
Campra assumes that the study is controversial, but insists
that there is no other explanation "with demonstrable evidence." For
example, there has been speculation that temperatures may be affected by the
Mediterranean sea, but this is not possible because local sea temperatures have
risen 0.3 degrees per decade. Campra believes that irrigation water may have had
some influence but "drip feed irrigation or hydroponics predominate and
evaporation water acts as a powerful greenhouse gas."
There is rich irony in greenhouses raising doubts about
greenhouse gases as the main driver of climate change. If greenhouses are the
cause of local cooling in Almerķa, changing global land use, particularly the
increase in urbanisation, may have a far greater role in global warming than has
been previously realised.