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Forest and climate

 

It is a widely held and seldom challenged view that forests are beneficial for our climate. In most cases that is also correct. While they are growing, woodlands capture CO2 from the atmosphere and thereby alleviate climate warming. In some regions of the world, however, this simple equation does not apply. In boreal and alpine terrain, consideration must be given to another effect as well. There, forests also reduce the amount of sunlight reflected by the earth back into atmosphere (this reflectivity is sometimes referred to as albedo). As a result of this, the atmosphere warms, counteracting the cooling effects of locked-up CO2.

Steadily expanding forest area

In the Swiss Alps in particular, woodland has been on the increase for several decades. Especially at higher altitudes and in locations with difficult accessibility, more and more agricultural land is being abandoned. In these places, new forests subsequently gain a foothold and, over time, become established (Fig. 1). The goal of our research project was to investigate the effects of this forest expansion on the climate.

 

What is the impact of forest expansion on the Earth's radiation balance?

A lesser amount of CO2 in the atmosphere means that less longwave radiation (outgoing heat energy) is absorbed (Fig. 2) when it leaves the Earth’s surface. This negative radiation balance, or negative radiative forcing, effects a cooling of the atmosphere. Reduced reflectivity, on the other hand, enables the Earth's surface to absorb more sunlight, so that a positive balance is created. This creates a warming effect (Fig. 2). In order to assess the impact of Switzerland’s woodland expansion on the climate, we compared these two effects and calculated the estimated net radiation balance associated with an increase in forest area. For this purpose we used spatially explicit data sets for the whole of Switzerland relevant and applicable to forest-area changes (land use statistics), carbon capture, snowpack, solar radiation, and albedo. These data were used as input variables in simplified radiation models, which enabled us to calculate the radiation balance with high-precision, i.e. well defined resolution.

 

Albedo is critical at higher altitudes

Our investigation revealed that the radiation balance associated with forest expansion is negative in practically every region of Switzerland, but at higher altitudes tends towards zero. In some inneralpine regions, the balance is actually positive because the albedo effect outstrips the CO2 effect (Fig. 3). These results are explained as follows. At high altitudes, unforested land is covered with snow for long, continuous periods. These areas reflect a great deal of sunlight. In contrast, woodland remains dark even when blanketed in snow and absorbs a large percentage of the sunlight. The difference in the radiation balance between unforested and forested land is thus particularly large when snow covers the terrain for long, continuous periods. This disparity is further amplified by strong solar radiation. In addition, the woodlands at higher altitudes in the central part of the Alps (where extreme environmental conditions prevail) absorb less CO2 than, for example, forests in the Swiss Plateau or Prealps. For that reason the cooling effect is not as pronounced and can be counterbalanced more easily (cf. Fig. 3). Differences also exist between various stages of forest development. The onset of forest expansion in formerly open land initially exerts a strong influence on the albedo. After a still young (open) forest has become established, the rate of albedo decline diminishes considerably, while the forest continues to absorb large quantities of carbon dioxide. As a consequence, woodland expansion gives rise to a negative radiation balance, particularly after a young, open forest has become established.

Forest expansion in snowy regions could accelerate warming

Our results show that the extent to which sunlight is reflected (albedo) is another factor to consider when assessing how changes in woodland affect the climate. This applies in particular to higher altitudes exposed to large quantities of snow, where forest expansion can ultimately bring about a positive radiation balance and thus, a net warming effect. The large differences in the radiation balance within a small area and especially along altitudinal gradients, coupled with differences between various stages of forest development, call for a more differentiated perspective when evaluating the effects of forests on the climate.

Our study provides new insights into the radiation balance when associated with forest expansion. The Earth's radiation balance is one of the factors indicating whether the planet is undergoing net warming or cooling. At the same time, a change in the Earth's radiation balance accelerates an abundance of energy and material fluxes which can amplify or diminish the effect of the radiation balance on the Earth's temperature. In order to determine whether a positive radiation balance, as observed in snowy regions in the central part of the Alps, actually brings about increased warming, we therefore need to conduct further studies. In particular, the bringing together of regional and global climate models could help to close the gaps in our understanding of the relationship between the Earth's radiation balance and temperature.