The Albedo effect

 

Albedo is the property of a surface to reflect heat back. Snow covered surfaces reflect a lot of heat back into the atmosphere whereas vegetation, especially dark "rough" forest canopies, don't reflect as much. They tend to absorb the heat from sunlight and so exert a warming effect on the earth while snowfields reflect heat back to the sky and exert a cooling effect. It is a well known phenomenon though difficult to quantify with any certainty.

It has been studied quite extensively by UK scientists at the Hadley Centre and Meteorological Office. A new paper by Bala in the USA has modelled changes in albedo following changes in existing vegetation induced by the fertilising effect of increased levels of carbon dioxide in the atmosphere. What is of more interest to us is the possible effect of changing landcover by planting new woodlands. The Hadley and Meteorological Office work addressed this. The conclusions were that that you get an overall warming effect of planting forests in the coldest regions (eg: eastern Siberia and eastern Canada). In slightly less cold regions (central and western Russia) the effect is neutral. In milder regions (western Europe and the USA) the overall effect of reforestation / afforestation is still an overall cooling, just less than you would have expected if you only considered carbon sequestration. In the coldest regions, the albedo effect dominates for two reasons: carbon sequestration is smaller due to slower growth; and the effect on annual mean albedo is larger because snow is lying for more of the year.

The American paper has emphasised the many good things about planting trees. We certainly endorse this and we are also convinced that that the weight of current evidence suggests that there is also – for the UK - a positive benefit in terms of carbon absorbed. As a mitigation measure for climate change, carbon sequestration by forests has clear limitations and should not affect our absolute priority to tackle the problem at source by reducing emissions from burning fossil fuels. Nonetheless, afforestation and reforestation in appropriate circumstances are a valuable addition to our armoury.

Carbon sequestration by forests is a topic of immense research interest and we are sure that scientists will continue to elucidate the scale and importance of the albedo effect and other factors. We look forward to this so that we can be sure about the effects of woodland creation and management on global climate.