The Rainmakers

Guest Post by Willis Eschenbach

For more than a decade now I’ve been saying something without getting much agreement, which was:

“When you cut down the trees, you cut down the clouds”.

I based my saying on my own experience, first growing up in a ponderosa pine and fir forest, and later living in a redwood forest for half of the last thirty years. My theory was that the trees created the rain in several ways.

First, they “transpire”, meaning that they release water into the atmosphere. And not just a little water. Lots.

Next, they absorb sunlight and use it to drive chemical processes through photosynthesis. This means that the sunlight is NOT turned into heat, which leaves the area cooler.

Next, they shade the ground, again cooling the surface and the local area. This allows the surface to stay moist, increasing the amount of water available for the plants to transpire.

Finally, some of the plants rake the fog out of the air, collecting it on their surface. From there it drops to the ground, watering the forest.

However, I’ve never had a scrap of evidence to support my theory that if you cut down the trees, you cut down the clouds. So I was very happy to find the following article in Science magazine:

Clouds over the Amazon.

Trees in the Amazon make their own rain

By Ilima LoomisAug. 4, 2017 , 2:45 PM

The Amazon rainforest is home to strange weather. One peculiarity is that rains begin 2 to 3 months before seasonal winds start to bring in moist air from the ocean. Now, researchers say they have finally figured out where this early moisture comes from: the trees themselves.

The study provides concrete data for something scientists had theorized for a long time, says Michael Keller, a forest ecologist and research scientist for the U.S. Forest Service based in Pasadena, California, who was not involved with the work. The evidence the team provides, he says, is “the smoking gun.”

Previous research showed early accumulation of moisture in the atmosphere over the Amazon, but scientists weren’t sure why. “All you can see is the water vapor, but you don’t know where it comes from,” says Rong Fu, a climate scientist at the University of California, Los Angeles. Satellite data showed that the increase coincided with a “greening” of the rainforest, or an increase in fresh leaves, leading researchers to suspect the moisture might be water vapor released during photosynthesis. In a process called transpiration, plants release water vapor from small pores on the underside of their leaves.

Fu thought it was possible that plants were releasing enough moisture to build low-level clouds over the Amazon. But she needed to explicitly connect the moisture to the tropical forest.

So Fu and her colleagues observed water vapor over the Amazon with NASA’s Aura satellite, a spacecraft dedicated to studying the chemistry of Earth’s atmosphere. Moisture that evaporates from the ocean tends to be lighter than water vapor released into the atmosphere by plants. That’s because during evaporation, water molecules containing deuterium, a heavy isotope of hydrogen made of one proton and one neutron, get left behind in the ocean. By contrast, in transpiration, plants simply suck water out of the soil and push it into the air without changing its isotopic composition.

Aura found that the early moisture accumulating over the rainforest was high in deuterium—“too high to be explained by water vapor from the ocean,” Fu says. What’s more, the deuterium content was highest at the end of the Amazon’s dry season, during the “greening” period when photosynthesis was strongest.

In looking around while writing this post, I also find this very interesting file from the National Academies, which says in part:

Certain trees are highly adapted to harvest fog water—some, like the Californian redwood trees, satisfy the majority of their water needs in this way. The trees create a physical barrier that intercepts and precipitates fog that would otherwise rise and dissipate in the warm air. In doing so, the trees create a localized water cycle: The fog water collected on leaves drips down and nourishes grasses, shrubs, and other plants that in turn trap their own water. All this dripping water sinks into the ground, filling wells and giving rise to small streams that people can use.

In addition, the redwoods provide the lovely sound of dripping rain, that marvelous music of watery wealth, even when there is no rain falling … what’s not to like? Here’s a photo I just took of the source of our liquid musical accompaniment, with my house in the foreground …

Finally, that same article that discusses the redwood trees is mostly about collecting water from fog nets. This is something I’ve discussed before in the context of the “no-regrets” option for responding to possible future climate change. See my post “Harvesting Fog: The No-Regrets Option” for one example.

My point was that IF someone wants to fight the eeevil CO2, they should do things that bring us value whether or not CO2 is the culprit. In that post, I said that if you are concerned for example about future droughts, do something now to fix the effects of present droughts. That way, you’ve taken a step that will help regardless of the future. It obeys the doctor’s maxim, “First, do no harm” …

Overall? It’s a lovely summer’s day, I’m back from the gold mines, we’re winning the climate battle, and life is good.

Best to each of you, sunshine and following winds …

w.

My Usual Request: Misunderstandings start easily and can last forever. I politely request that commenters QUOTE THE EXACT WORDS YOU DISAGREE WITH, so we can all understand your objection.