Do high latitude forests warm or cool climate?
Reported in the Guardian on 31 October 2008: Chemical released by trees can help cool planet, scientists find
Planting forests is a popular way of reducing greenhouse gas concentrations and, apparently, slowing global warming. But forests have lots of other impacts on climate which make things more complicated. In fact, previous studies have suggested that trees at high latitudes (for example, in Europe) warm, rather than cool, the climate. These studies predict that dark forest vegetation absorbs more sunlight than farmland resulting in more warming than the cooling that is due to storage of carbon in forest vegetation. However, these studies did not consider that forests also produce particles. Particles are produced in the air above trees from natural emissions of terpene compounds (the compounds that give pine trees their distinctive smell). These particles reflect sunlight back to space and make clouds brighter, which acts to cool the climate. In a study published in the Philosophical Transactions of the Royal Society, Dominick Spracklen and Ken Carslaw have shown that these forest-derived particles could exert a considerable cooling effect that may offset the warming.
They used the Global Model of Aerosol Processes (GLOMAP) to simulate the effect of replacing boreal forest with Arctic grassland. They found that the concentration of cloud condensation nuclei (the particles that form cloud drops) reduced from about 230 to 120 particles per cubic centimeter when forest was replaced with grass. The reason is that forest emissions of terpenes react in the atmosphere to produce organic compounds that condense on the many small particles that form over forests. These particles are then able to grow large enough to be active cloud condensation nuclei, altering the properties of clouds and cooling the climate.
Understanding these processes is very important as it will help decide whether we should plant forests in Europe to help slow climate change.
Boreal forests, aerosols and the impacts on clouds and climate by Spracklen, D.V., Bonn, B., and Carslaw, K.S., Phil. Trans. A., 10.1098/rsta.2008.0201, 2008.