Related stories

There are a few different levels of understanding about the water cycle. There are the diagrams hung in elementary schools, showing water evaporating up from the ocean into clouds, then falling back down to the ground. A step beyond, there’s a general understanding that takes into account evaporation from trees, wind patterns and the like.

Then, there are the highly technical approaches that look at a wild array of minutia and contingencies: nighttime sap flow, isoprene emissions, ice-nucleating particles released from decaying leaves, even the phases of the moon.

And yet, even with all of these details researched and logged and plugged into different models and algorithms, it’s hard to trust a given morning’s forecast. Cloud behavior and rainfall still remains a mystery – let alone how they will react with climate change.

“The models just don’t do this,” says Center for International Forestry Research (CIFOR) Associate Douglas Sheil, who also serves as faculty of environmental sciences and natural resource management at the Norwegian University of Life Sciences. “And the problem is that we’re using these models to predict climate change, when they can’t even predict current rainfall patterns correctly.”

Taking progress on this matter into his own hands, Sheil has recently published a paper that seeks to show why such mysteries still exist and point out areas where research is needed in order to solve them. In simple, readable language, he summarizes the most prevalent water cycle theories and hypothesizes the enormous change that could happen, should we finally agree upon one that works – perhaps even his own.


Like calculating numerical probabilities for dice rolled many times versus guessing the value of a single toss, climate patterns can be more consistently predicted than the weather. We know, more or less, that after winter comes spring; we don’t know exactly how hot next weekend will be. However, when it comes to how rain fits in, Sheil is of the belief that there isn’t an agreed-upon water cycle theory that helps predict either short- or long-term events – and he isn’t alone.

The water cycle was a popular research topic in the early 1900s, with scientists already noticing how deforestation led to changes in rainfall and weather. A dip in interest occurred in the middle part of the century, led by coastal populations brushing off freak weather events as having simply blown in off the ocean.

However, as the weather has gotten more erratic and global social issues starker, the past 10-15 years have seen scientists revisit the water cycle in spades, recognizing it as something we must understand. Between 1992 and 2015, for instance, the world’s freshwater resources divided by the global population declined by 25%, and reliable access has declined too – apparent in catastrophic events like the Ethiopian drought of 2015.

As the water cycle comes back into focus, it’s bringing forests along with it this time. “Everyone knows forests are important for rainfall. That’s not news,” says Sheil. “But I think what is news is the extent to which they matter, and that the sharpness and clarity of those links are becoming so much better understood – just in the past few years.

Approximately 117,600 cubic kilometers of water falls to the earth as precipitation each year. Sixty-one percent of this comes from land, and more than half of that comes from plant transpiration. A different recent study found that land cover changes have already caused a 5-6% reduction in global atmospheric moisture. “The sum emission of water vapor from forests (transpiration and evaporation) typically surpasses that from other vegetation and open water,” states Sheil’s article.

A number of theories take into account the role of forests in the water cycle, beginning with the temperature-gradient theory, developed by Isaac Newton’s contemporary Edmund Halley in 1686. Linking winds and global circulation to the temperature patterns of air cooling over sea and warming over land, this theory and others like it remain preeminent, and research continues on the effect of forests within this framework.

However, as Sheil explains in his paper, such a temperature-driven theory can’t explain various features of the weather, such as abrupt shifts in monsoon climates, why rainfall is maintained in certain areas but not in others, and why places like the Amazon Basin and Indonesia attract so much rain.

In 2008, Sheil became interested in and joined the research endeavors around a novel approach to the water cycle that physicists Anastassia Makarieva and Victor Gorshkov had developed, named the ‘biotic pump’. Rather than focusing on temperature alone, the pump takes into account how water condensation and evaporation influences air pressure, which creates rainfall patterns. When moisture condenses or freezes – forming clouds or rain drops – the air tends to rise and atmospheric pressure below it drops, drawing the surrounding air in and up so that the moisture in this air also tends to condense or freeze, fueling the process.

Through this approach, the biotic pump can better describe the role of forests in the water cycle. Emitting moisture as they do, trees initially raise air pressure in the atmosphere around them. This is reversed once rain starts to fall.

“A lot of people assume that a new theory is really radical and sophisticated, but the basic idea behind this is just school physics,” he says. This caveat, though, might undermine the fact that the pump has since proven itself repeatedly by predicting various relationships in climate and weather.

“My colleagues and I conducted a study that we published as an article, predicting and showing that air pressure will often increase before rain in the Amazon. In the standard temperature theory you’d just expect low pressure prior to rain, but that is not what we see. The pump theory was able to make this unique prediction of a physical relationship. So, making predictions that others wouldn’t predict and then showing that they work – that’s science.”


Aside from giving guidance on when to carry an umbrella, a better understanding of the water cycle can potentially equip scientists with the ability to institute massive continental change.

This begins with understanding the water cycle fundamentals underpinning all the theories, that being: water is drawn up from the soil and elsewhere, emitted into the atmosphere, and falls back to earth as rain several times before it makes it out to sea.

“Trees re-emit water that then falls somewhere else,” says Sheil. “A little patch of trees may use more water than the surrounding landscape, but the larger landscape overall will likely have higher rainfall. These things happen at different scales.”

Zoom out, and this applies across regions as well, with rainfall losses in one place serving as gains in another.

“I highlight that farmers in China are very dependent on what land users in Europe and Africa are doing, because that’s where most of their rainfall is coming from.”

On a very generous scale of geography and time, this potentially means that with an understanding of the water cycle guiding strategic planting of trees, it could be possible to turn dry landscapes wet, and vice versa. Such a new understanding would inform where to plant forests that channel rainfall to dry areas, and then once those dry areas have enough natural irrigation, forests can be planted that will re-wet themselves.

“A potential way to re-green the Sahara… It sounds bizarre and crazy, but it could actually be practical if you get the resources and time to do it. That’s pretty exciting. I think starting in coastal areas and then moving inland over the long-term, these things could be possible.”

But the first step to doing so, he says, is to stop debating and start researching, to prove whether the biotic pump is true, or develop something else that is. The point of Sheil’s research is to stoke the research world into finally landing upon an understanding of the water cycle that works, so that it can be used to bring water to the places that need it most.

“If the biotic pump is true, we shouldn’t be arguing about it. We shouldn’t be wasting another 10 years, while it actually tells us what’s at risk and could bring solutions to people who are suffering from water shortages. This has scary aspects – like high-rainfall landscapes switching rapidly to dryland due to forest loss – but, amazingly positive aspects too.”

For more information on this topic, please contact Douglas Sheil at
Copyright policy:
We want you to share Forests News content, which is licensed under Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0). This means you are free to redistribute our material for non-commercial purposes. All we ask is that you give Forests News appropriate credit and link to the original Forests News content, indicate if changes were made, and distribute your contributions under the same Creative Commons license. You must notify Forests News if you repost, reprint or reuse our materials by contacting