Inside Central American rainforest. Photo by Rhett A. Butler.
Thanks to the world’s voracious appetite for crops like coffee, palm oil, rice, rubber, soy and tea, large-scale agriculture is one of the main drivers of deforestation around the globe.
The conversion of forests to cropland can drive local temperatures up or down by as much as a few degrees, according to a new report. Ironically, the authors write that these temperature fluctuations can lead to less productivity from the very same agricultural operations the forests were cleared to make way for.
The connection between forests and the global climate has been studied extensively, and there have already been numerous studies showing the impacts of deforestation on agriculture. But this study, published in Nature Communications, shows how we can arrive at a quantitative estimate of the impact of forests in a precise geographical location anywhere in the world.
“We know the importance of forest on climate, but our knowledge on this topic mostly comes from global climate models and field data, which have certain drawbacks,” Yan Li, a researcher with Peking University who is currently a visiting climate scientist at the University of Maryland, said in a statement to mongabay.com. Li led the team that wrote the report.
Plants transport water from the ground through their roots and up through leaves, where some of it evaporates into the air in a process known as transpiration. Photo taken in Uganda by Rhett A. Butler.
Climate models require vast amounts of data and computational resources, so they’re not particularly well-suited to making precise, local forecasts. Fieldwork, on the other hand, can provide that level of location-specific detail, but it’s expensive and time-intensive to do, Li says. Local data therefore tends to only be available for limited areas and is difficult to scale up.
Yet as deforestation continues apace and shifts in local climate are becoming more and more difficult to ignore, it’s more urgent than ever to understand the relationship between forest loss and local temperature changes.
According to a press release issued with the report, which cites data from the Food and Agriculture Organization of the United Nations, the world lost 130 million hectares (321.2 million acres) of forest — an area roughly the size of France —in just the past decade. The authors caution that as more forests are cleared, food production is increasingly exposed to the risks associated with changing temperatures, like higher incidence of heat waves and freezes.
Using data from NASA’s Moderate Resolution Imaging Spectroradiometer, Li and team were able to closely examine two key mechanisms of cooling and warming at work in forests: albedo and evapotranspiration. Albedo is the measure of the amount of the sun’s energy reflected back into space; evapotranspiration is surface evaporation combined with absorption of water by forests and its transpiration through leaves as water vapor. Forests are usually darker than, say, agricultural land, so they tend to have lower albedo values, meaning that they capture more heat energy. However, tropical forests offset this with high evapotranspiration rates, effectively releasing that captured energy back into the air. Negative albedo causes warming and evapotranspiration causes cooling, and by understanding which is the dominant effect in any given forest, Li says his team could determine with high accuracy whether the clearing of a particular forest would lead to a rise or fall in local temperatures.
The researchers found that, generally speaking, tropical forests have a strong cooling effect year-round due to their high evapotranspiration rates, while boreal and temperate forests show seasonal variation. Boreal forests have a strong warming effect in winter as their dark evergreen leaves act as a heat sink and moderate cooling in summer (their net effect is warming over the course of the year); temperate forests have a moderate cooling effect in summer and moderate warming in winter (with a net cooling effect annually).
Forests tend to be darker than the cropland they’re cleared to make room for, which increases regional albedo and reflects more of sun’s energy. Photo of Amazon rainforest and soy fields in Brazil by Rhett A. Butler.
“Our results indicated that deforestation in most parts of the world can increase daytime temperature and decrease nighttime temperature, and this can result in an increased diurnal temperature range (the difference of maximum and minimum temperature over a day),” Li said. “This is not a good thing for many crops because forests act as a buffer to mediate extreme temperatures that crops are susceptible to.”
When forests are cleared, this buffer effect disappears and extreme weather events like heat waves and freezes can occur more frequently, which has an obvious impact on crop yield. The report does not seek to quantify that impact, but to provide a means by which policy makers and local farmers can make better decisions around land use conversion.
“Our results could let them know whether the conversion of forest to pasture/grass/crop at their location could increase or decrease local temperature,” Li said. “If they care about the climate consequence, they can also change their strategy to reduce the negative impact. For example, if they want to mitigate the warming when trees are lost, they could try to avoid cutting down trees but switch to shrub land instead, or increase irrigation in crops.”
By providing a more holistic view of how forest cover changes affect local climate, the report’s findings could also be used to improve global climate models, according to Nicholas Magliocca, a computational research fellow at the University of Maryland who was not involved in the study
“It’s difficult to get measurements that are both accurate at a fine scale and have a large enough coverage that they can inform global climate models,” Magliocca said. “This analysis offers an important empirical benchmark against which global climate models can be validated to accurately represent the temperature-mediating effects of forests.”
Li, Y., Zhao, M., Motesharrei, S., Mu, Q., Kalnay, E., & Li, S. (2015). Local cooling and warming effects of forests based on satellite observations. Nature communications, 6.