Do boreal forests warm or cool climate?

Previous climate model studies have concluded that afforestation in the boreal zone warms climate because the cooling from carbon sequestration is cancelled out by warming due to changes in land surface albedo. However, these studies ignore forest-aerosol interactions. We evaluate these interactions using a global chemical transport model with size-resolved aerosol microphysics to interpret the seasonal cycle of aerosol size distribution observed at boreal forest sites. Boreal forest monoterpene emissions increase regional (north of 60N) summertime cloud condensation nuclei (CCN) concentrations by 100 percent. This increase is due to secondary organic aerosol from forest monoterpenes growing small particles to CCN sizes. Our simulations show that 50 percent of these small particles are derived from boundary layer new particle formation (which in our model is driven by gas-phase sulfuric acid) with the remainder coming from primary particle sources and upper tropospheric nucleation. The sensitivity of CCN to changes in monoterpene emissions increases if we assume that organic species modify nucleation rates as well as particle growth rates. We use a 1D radiation model to estimate that the change in CCN due to boreal forest derived particles results in radiative forcing through the aerosol indirect effect of between -1.8 and -6.7 watts per square metre of forest. This forest-aerosol-cloud forcing is similar in magnitude to the land surface albedo and carbon cycle impacts of boreal forests and so has implications for the net radiative balance of the forest.