Rainforests & Rainfall

Rainforests & Rainfall Photo Credit: Clipart.com

Deforestation can lead to droughts hundreds of miles away.


Rain-making rainforests. I’m Bob Hirshon and this is Science Update.

Cutting down rainforests in one region can deprive farms in other regions of rainfall. This according to British researchers reporting in the journal Nature. University of Leeds environmental scientist Dominick Spracklen says they used newly available NASA satellite data, which tracks both forest cover and rainfall in the tropics.

And we use our model to predict where air has come from over the past ten days, at any location in the tropics.

They found that air masses that had passed over large areas of forest provided more rainfall than air masses that had not. And forests in the Amazon and the Congo support rainfall in several key agricultural areas.

Spracklen says that other countries should follow Brazil’s lead in curbing deforestation, since depleting forests within their borders could affect not only their own food supply, but their neighbors’ as well. I’m Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

It's long been believed that leafy plants like trees contribute to rainfall. That's because they absorb water from the soil, and secrete water droplets through their leaves (a sweat-like process called transpiration). Presumably, those water droplets end up in the atmosphere and eventually fall back as rain. If not for the plants, the same amount of water would mostly remain trapped in the soil. 

Using what we know about transpiration and atmospheric patterns, computer modeling studies have projected that removing large amounts of forests would result in decreased rainfall elsewhere. What's new about this study is that it incorporated numerical data from the past. Specifically, a NASA project called the Tropical Rainfall Measuring Mission (TRMM) has been closely tracking rainfall in tropical areas.

By combining past measurements from TRMM with changes in rainforest cover (also tracked by satellite data), along with advanced knowledge of wind patterns, the researchers were able to design a new kind of computer model that can calculate exactly where air masses have traveled over the last ten days. The model analyzes the amount of forest in that ten-day path, and compares it to how much rainfall that air mass produced after passing over it.

The researchers found that the more forest an air mass passes over, the more rain it releases later—sometimes in areas hundreds or thousands of miles away. That's significant because tropical areas include not only shrinking rainforests, but farmland that's critical to the global economy. The researchers estimate that on our current course, rainfall in the Amazon basin could decline by up to 21 percent by the middle of the century—enough to take a serious toll on crops. And it stands to reason that temperate forests affect rainfall as well. 

The findings provide yet another reason why preserving rainforests is important. Not only could deforestation affect crop supplies, it could also set off international conflicts, in which a country with failing farms blames a neighboring country for destroying its own forests.

Now try and answer these questions:

  1. What was already suspected about the relationship between rainforests and rainfall?
  2. How did this study add to that knowledge? What was different about it?
  3. Why is it especially significant that deforestation can impact rainfall hundreds or thousands of miles away?
You may want to check out the September 28, 2012, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: New developments in wind power, a solar cell made from spinach, transparent soil clarifies plant research, extending the growing season, and how rainforests prevent drought in farmland.

For more on rain and computer modeling, see the Science Updates Mountain RainFlowers & Rainfall, and Earth Simulator.

Going Further

For Educators

The Science of Weather is a collection of resources from Science NetLinks focused on different types of weather and the natural forces that cause them.

NASA's Tropical Rainfall Measuring Mission is a joint mission between NASA and the Japan Aerospace Exploration Agency (JAXA) designed to monitor and study tropical rainfall.

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