Large plantations in deserts change atmospheric circulation, bringing more rain, scientists from Germany's University of Hohenheim claim. If their models are confirmed in practice, it will become much more realistic to use large-scale reforestation to slow global rising temperatures.
The idea of mass planting of forests to offset the carbon dioxide we are releasing into the atmosphere is so obvious it's been around as long as people have recognized the problem. Just this year a widely publicized paper ran the numbers and suggested there is enough land available to make it viable. Some countries, more for local than global reasons, have already started doing their bit.
Unfortunately, however, forests will face competition for the best land from the demands of agriculture to feed growing populations. This, Dr Oliver Branch and Professor Volker Wulfmeyer reason, will force carbon-storing plantations onto more marginal land, particularly deserts and their surrounds, raising the question of how the trees will get enough water. Near cities urban wastewater could help, but globally that is a small part of the answer. Underground water sources usually offer only a temporary solution.
Changes to land use can alter rainfall patterns, however, and the enormous precipitation over rainforests is in part because those forests are there. Studies on the topic, however, have usually focused on continental-scale effects. In the Proceedings of the National Academy of Sciences, Branch and Wulfmeyer modeled whether a plantation measuring 100 square kilometers (38 square miles) would change rainfall patterns within it and the immediate surrounds.
The pair chose two desert locations, one in Israel (extending slightly into Egypt), the other in Oman, and used a weather-forecasting system to model the effects of establishing a jojoba plantation in each. Although shorter than what we think of as forests, jojoba's capacity to draw carbon dioxide from the atmosphere in locations too hot and dry for most alternatives has seen it promoted as a climate stabilizer.
Such a plantation would change the local climate in many ways. Like most plants jojoba is darker than the area's soil, reducing the amount of light reflected. It releases aerosol particles that water vapor condenses around to form rain and its roughness interrupts the wind.
In both locations, the paper concludes, rainfall would increase significantly. Although this would be insufficient to allow these plantations to grow without irrigation, it could certainly reduce the amount required, significantly changing the economics of the idea in the process.
The effectiveness of the approach varies with local conditions, and might not be helpful everywhere. The authors argue this just means governments need to select the sites for mega-forestry projects carefully, and provide low-resolution maps of the parts of the world that would see the largest rainfall increases if planted in this way.