To Plant or not to Plant? Reforestation and Albedo

For thousands of years, humans have cleared land for agriculture, cutting down trees to make space for croplands. Deforestation is an ongoing process; in the tropics especially, trees are cut down or even burned at rapid pace.

Forests play an important role in climate, especially in the hydrological and carbon cycles. Trees control humidity (and thus temperature) through evapotranspiration, and it has been found that deforested areas in the Amazon area have become both drier and warmer. Moreover, a growing forest fixes carbon from the atmosphere - cutting down trees releases it. Reforestation has been suggested as an option to sequester carbon, and thereby reduce the carbon dioxide in the atmosphere and counter global warming.

So how does albedo fit into this? As explained in earlier posts, different surfaces have a different albedo. A good example of this can be found in the figure below, derived from an article by Bonan in 2008. As can be seen, the albedo of forests is generally lower than that of other biomes. This is due to the fact that forests are quite dark and shady, and they do not reflect much of the solar radiation.

Surface albedo of different forest biomes (Bonan 2008)

The question scientists have been asking is whether this 'warming' effect offsets the cooling benefits of a forest. It seems as though this varies per type of forest. Basically, we can identify three types; tropical, temperate and boreal. It was found that evapotranspiration is very important in tropical forests. Warmer air can contain more water vapour and so more heat can be removed in that way (this is called latent heat transport). As said above, cutting down trees in Amazonia led to a warmer and drier climate. Additionally, rain forests grow rapidly and are able to store much carbon.

In boreal forests, snowfall also plays a role. Research has shown quite clearly that snow on bare ground has a higher albedo than snow on trees (which makes sense; imagine a forest covered in snow compared to a field. The trees will still show dark patches, whereas the field is much more uniformly covered). In addition to that, the evaporative effect of boreal vegetation is much smaller, because the air is generally colder. Boreal forests also grow slower, so carbon storage benefits from reforestation would require more time to take effect.

For temperate forests, the effects of albedo and evaporative forcing are unclear - and it is not certain whether replanting those forests might actually help in negating global warming.

In a model run by Giddard et al. in 2005, it was found that changes in surface (so either forestation or deforestation) particularly impact the higher latitudes in the Northern Hemisphere (also see the figure below). They found that the albedo "side-effect" of reforestation would lead to a net-warming on a century timescale, offsetting any benefits of carbon sequestration.

Modelled changes in albedo (Gibbard et al. 2005)

As such, it has been concluded that reforestation of tropical areas has some use in counteracting global warming, temperate forests little to no effect, and replanting boreal forests actually exacerbates it. Nevertheless, though it is tempting to draw such a one-on-one relation between forests, albedo and climate, trees and forests have other effects on climate as well, on various spatial and temporal scales, not to mention the importance of forests for biodiversity and human culture. There is much research needed in modelling all interactions between forests and the climate system before it can conclusively be said whether (and where) reforestation can help mitigate global warming.

Sources: 

• Bonan, G.B. (2008), Forests and climate change: Forcings, feedbacks and the climate, Science, 320, 1444, doi:10.1126/science.1155121.
• Gibbard, S., K. Caldeira, G. Bala, T. J. Phillips, and M. Wickett (2005), Climate effects of global land cover change, Geophys. Res. Lett., 32, L23705, doi:10.1029/2005GL024550.