April 29, 2013
Are Plants Natural Air Conditioners?
Brett Smith for redOrbit.com - Your Universe Online
It has long been understood that plants are able to mitigate the forces of climate change by absorbing carbon dioxide from the air. However, a new study adds another dimension to the role played by plants in curbing global warming by showing that they also“¯emit gases that assist in cloud formation and atmospheric cooling.
According to the study, which was recently published in the journal“¯Nature Geoscience, an international team of scientists has shown that plants actually respond to higher temperatures by increasing the concentration of temperature-lowering aerosols. These aerosols are responsible for offsetting about one percent of warming worldwide and up to 30 percent locally over vast stretches of forests in the Northern Hemisphere.
"Plants, by reacting to changes in temperature, also moderate these changes," said lead author Pauli Paasonen, a researcher with the University of Helsinki and the International Institute for Applied Systems Analysis (IIASA).
Aerosol particles are usually known as a man-made pollutant, but so-called biogenic aerosols that originate from plants have been less understood. According to the research team, these plant-generated biogenic aerosols begin with gases that, after becoming oxidized in the atmosphere, convert atmospheric aerosols into bigger particles that reflect sunlight and set the stage for cloud formation.
The biggest revelation of the new study was showing how warmer temperatures induce plants to release more of these reactive vapors, increasing the concentrations of particles that facilitate cloud formation.
"Everyone knows the scent of the forest," said study co-author Ari Asmi, a University of Helsinki researcher. "That scent is made up of these gases."
The team based their findings on observations of eleven forested sites in Europe, North America and southern Africa. The sites were located in both remote areas with low levels of atmospheric pollution as well as densely populated, more polluted locales.
The researchers noted the natural cooling effect is smaller over hotter tropical forests such as the Congo basin and more polluted regions. They also found that the region´s so-called ℠boundary layer´ is a key factor — referring to the layer of air closest to the Earth where gases and particles mix effectively.
Paasonen says that the boundary layer changes with weather, making calculations of its effects more difficult.
"One of the reasons that this phenomenon was not discovered earlier was because these estimates for boundary layer height are very difficult to do,” he said. “Only recently have the reanalysis estimates been improved to where they can be taken as representative of reality."
They researchers did not explain why plants emit more vapors at higher temperatures, speculating that it is a side-effect of trees' natural air conditioning. However, the team did note that warmer temperatures also bring with them the dangers of more prevalent wildfires and increased insect activity.
The Finnish scientist added that his study´s findings should not be viewed as a way to solve the challenges presented by climate change.
"This does not save us from climate warming," Paasonen explained. "Aerosol effects on climate are one of the main uncertainties in climate models. Understanding this mechanism could help us reduce those uncertainties and make the models better."
A recent UN report appeared to agree with Paasonen´s assessment of the dangers of climate change, saying that it is 90 percent certain human activities, not natural variations, are to blame for the warming trends of the past decades.