The invigorating scent of pine assaulting the nostrils is one of the pleasures of any walk through pine forests but an international team of scientists has established there’s a lot more to pine-scented vapors than olfactory indulgence.
New research by German, Finnish and US scientists points to the astringent scents given off by coniferous forests having a part to play in moderating climate. The gas emitted by pine forests creates particles in the atmosphere above the forest that can reflect sunlight back into space and promote cloud formation — two factors that are important climate feed-backs — serving to moderate climate.
The study published Feb. 27 in Nature explains how the medium responsible for the unmistakable smell of pine forests — ultra-low volatile organic compounds or VOCs — forms protective aerosols above boreal forests, the expansive coniferous wildernesses, consisting mostly of pine, spruce and larch that still carpet much of the Northern Hemisphere.
The effect of such aerosols in the atmosphere generally is well-known. Volcanic eruption is another natural process that causes aerosols. Coincidentally, a separate study last week found that aerosols caused by the reaction of sulfur dioxide in the atmosphere, after being ejected from volcanoes, is a significant contributory factor to the pause seen in average land temperatures during this millennium to date.
Although the broad effects of aerosols — reflecting sunlight and cloud formation — are well documented, less is known about how much such aerosols might limit climate change.
One of the contributions to the Intergovernmental Panel on Climate Change fifth assessment report (2013) took the view that, “Clouds and aerosols continue to contribute the largest uncertainty to estimates and interpretations of the Earth’s changing energy budget.”
In the case of pine forests, up till now scientists haven’t been able to predict either how aerosols form or the mechanism which transforms that sweet smell of pine into reflective aerosols.
But the new study sheds light on how some types of atmospheric aerosols form above the vast pine forest of North America, Northern Europe and Russia. Not only does it establish the chemistry behind the formation of these pine forest aerosols but it estimates the forest itself may be the dominant source of aerosols above boreal forests.
The researchers’ findings are likely to strengthen the arguments of those campaigning against the felling of large swathes of boreal forest in North America, the Boreal Forest Conservation Framework being one example, as well as providing impetus for forest re-generation projects such as the Scotland-based Trees for Life that’s engaged in a project to re-establish the ancient Caledonian Forest.
Outlining the new findings, Joel Thornton, a University of Washington associate professor of atmospheric sciences and second author on the paper, said, “In many forested regions, you can go and observe particles apparently form from thin air. They're not emitted from anything, they just appear."
For years, scientists have known that gases from pine forests trees can form particles that can grow one-hundredfold in size in just a day — expanding from just one nanometer (one billionth of a meter) to 100 nanometers. When the particles grow to that size, they’re big enough to condense water vapor and in so doing, promote cloud formation.
The researchers took measurements from pine forests in Finland. Subsequently, they replicated the same particle formation in the lab — an air chamber at Germany's Jülich Research Centre. Advances in mass spectrometry enabled them to track one-in-a-trillion molecules and observe what happened.
This demonstrated that when a pine-scented molecule combines with ozone in the surrounding air, some of the resulting free radicals grab oxygen with unprecedented speed.
"The radical is so desperate to become a regular molecule again that it reacts with itself. The new oxygen breaks off a hydrogen from a neighboring carbon to keep for itself, and then more oxygen comes in to where the hydrogen was broken off," Thornton said.
What followed was almost a runaway reaction. Contrary to current chemistry which predicted that three to five oxygen molecules could be added per day during oxidation, instead researchers found the free radical adding 10 to 12 oxygen molecules in one go.
If molecules were animate, the process could be described as the new, bigger molecule “pining” to be in a solid or liquid state, rather than gas, and condensing onto small particles of just 3 nanometers. So many of these molecules are produced, they clump together and grow to a size big enough to influence climate.
It’s mostly boreal forests, typically the large tracts of forest at higher latitudes in the Northern Hemisphere, which emit the largest amount of these compounds. Other forests emit similar vapors, said Thornton, who believes the rapid oxidation may apply to a broad range of atmospheric compounds.
As temperatures rise, forests emit exponentially more of these pine-scented vapors, say the researchers. Understanding how these vapors react may help predict how forested regions will respond to global warming, and what role they’ll play in the planet's response.
"It's thought that as the Earth warms there will be more of these vapors emitted, and some fraction of them will be converted to particles which can potentially shade the Earth's surface," Thornton said. "How effective that is at temperature regulation is still very much an open question."
But the findings shouldn’t be seen as a panacea for climate change. Although the research points to the possibility that forests, by becoming more fragrant, are attempting to cool themselves, just as humans can suffer from heat stroke or lack of water, sustained excess heat or insufficient water and forests, too, get stressed and will eventually die.