Will more trees cause more rain?

JOHANNESBURG, 8 February 2013 (IRIN) - It is commonly believed that destroying trees will influence the climate of a region. But scientific evidence to support that deforestation and afforestation influence local climate - affecting temperature and rainfall - has only just started emerging.

A new study, led by Borbála Gálos of the Max Planck Institute for Meteorology, found that planting trees - or afforestation - in areas in Europe where there have previously been no trees can reduce the effect of climate change by cooling temperate regions. Using a computer-generated regional climate model, the study showed that afforestation in the northern part of central Europe and Ukraine could reduce temperatures by 0.3-0.5C and increase rainfall by 10 to 15 percent during summers by 2071-2090.

While the study was specific to the temperate regions, Gálos told IRIN that, in some regions, forests could be effectively used for climate-change mitigation. These studies gain more importance as drought-affected countries like Niger plan a massive afforestation campaign that will regenerate five million hectares of dry degraded land. Additionally, the UN Food and Agriculture Organization (FAO) recently published a policy guide to show that combining tree planting with crop or livestock production could not only stem climate change but also create incomes.

Senior scientist Gordon Bonan of the US-based National Center for Atmospheric Research, a leading authority on the influence of forests on climate change and a contributing author to Intergovernmental Panel on Climate Change (IPCC) assessments, spoke to IRIN about the status of the research in this area.

Q: What is the scientific history of linking climate and forests? There are studies that show conserving our existing forest cover is imperative for keeping carbon emissions low, but the impact of reforestation/afforestation on local climate has not been studied at length or at a regional scale, right?

A: Scientific interest in this problem goes back several centuries, with the European settlement of North America, India and Australia and subsequent widespread deforestation during land clearing. There was a common view that deforestation was altering climate - primarily temperature and rainfall - but the scientific tools to study this were not available until the past 30 years or so, with global and regional climate models and satellite observations.
 

"for more than 60 percent of the tropical land surface, air that has passed over extensive vegetation in the preceding few days produces at least twice as much rain as air that has passed over little vegetation"

The most prominent example has been studies of tropical deforestation in the Amazon. Most climate model simulations show that large-scale conversion of tropical rainforest to pastureland creates a warmer, drier climate. This model result is generally accepted. However, the observational evidence for this is lacking, primarily because the model simulations cut down all the rainforest while the observational record is based on much less extensive deforestation. So one cannot compare the observations with the models. Model simulations of less extensive deforestation show a smaller effect on climate.

There was a very interesting observational study [Spracklen et al 2012 Nature 489:282-286] that analysed satellite remote-sensing data of tropical precipitation and vegetation. They found that for more than 60 percent of the tropical land surface, air that has passed over extensive vegetation in the preceding few days produces at least twice as much rain as air that has passed over little vegetation.

Another region of extensive research is the Sahel of northern Africa: Most modelling studies show a warmer, drier climate because of loss of vegetation. Northern Africa was much wetter and supported lush vegetation 6,000 years ago. Studies find that the more productive vegetation at that time led to enhanced rainfall.

The newer research is focused on temperate latitudes in North America and Europe, to investigate how historical deforestation altered climate and how proposed reforestation or afforestation might affect climate.

This work is still uncertain, and one can find modelling studies showing cooling because of deforestation or warming because of deforestation. The response depends on the simulated change in surface albedo [the fraction of solar energy reflected back from the Earth]: Do forests reflect less solar radiation than cropland or grassland? If so, the greater absorption of solar radiation by forests heats the land surface. [Response also depends on] the hydrologic cycle: Do forests evaporate more water than cropland or grassland? If so, greater evaporation by forests cools the surface.

Q: When considering afforestation to mitigate climate change, what should one take into account? The growth period of the trees, the height, the area covered? What kind of difference it would make to temperatures?

A: Deforestation, afforestation, reforestation and other land-use practices are regional in scale. The affect that they have on temperature and precipitation is seen in regional climate. Greenhouse gas warming is a global phenomenon, well seen in the global temperature record and also in regional temperature records. When one compares the simulated regional effects of land use on climate with the simulated regional greenhouse gas effect, one can find that the land use signal is of similar magnitude.

There is much scientific uncertainty in how afforestation affects climate - again related to albedo and evaporation. Different types of trees or other vegetation do differ in growth rates, height, etc., all of which affect albedo and evaporation.

A big research question is the net effect of afforestation on climate. Forest ecosystems store carbon [reducing greenhouse gas warming], but do they warm or cool climate because of changes in albedo and evaporation? And how do these compare with the climate [influences] from carbon storage? These are all very active research areas.