COVER/Lance Olsen

How Deforestation Changes the Climate

"Knowing what we do about the extent of deforestation over the past generation and about the way the hydrologic cycle works, it would be surprising if climate were not changing." --Lester Brown

Poetically lovely though a tree may be, it's also a multi-purpose tool including two kinds of umbrella, a mighty pump, and a storage closet.

Getting rid of lots of trees can be enough to redesign not only the scenery of a state, bioregion, or even a whole nation, but the loss of trees can also redesign regional climate, weather and the lives of children and adults for many decades to come. The basics are simple enough for kids to understand, and fun to teach them.

Everyone knows that we can use a tree as an umbrella when it rains. We've known it long enough and well enough that we've also learned how risky it can be to take shelter under a tree during a lightning storm, because lightning can target trees, including the ones under which people have run for shelter.

Aside from the risk of lightning, though, ducking the rain by running under a tree is sound advice when we are caught in rain that comes without lightning, because trees do act as umbrellas.

As it turns out, some species are better umbrellas than others --Russian scientists have reported that that a fir tree, for example, can catch about half the rain that falls on it, and any rain caught in the fir's many needles doesn't fall on the person standing under the limbs. That's all well and good for people caught outside in rain, but a major part of this living umbrella's story starts when the rain stops falling.

After a rain moves on past us, the water caught in a tree drips slowly off its leaves or needles. The net effect is the same as if the rain itself kept falling. To demonstrate the point, stand under a tree loaded with water, then shake the tree. What do you get? Drenched.

The lesson learned here is that a tree in the woods is a self-watering entity, because it captures water that might otherwise be free to rush away in a quick return to the oceans. Multiply this over an entire forest, and the result can amount to many millions of gallons of water kept in the forest, by the forest's trees.

A tree becomes a second kind of umbrella after the rainclouds pass, the captured raindrops have stopped dripping, and the sun comes out from behind the clouds. After a tree's leaves and needles act as a rain umbrella, or water-catcher, a tree then does duty as a sun umbrella, or water-keeper. The keyword here is shade, because the cooler temperatures of shaded places stop the moisture under trees from evaporating as rapidly as it would in direct sunlight. We see the same thing with snow, because the snow that falls into the shade of a forest melts and runs away a lot more slowly than the snow lying in a sunny clearing or clearcut. Give a kid a trowel, have her start digging, and let her see for herself that soils under trees will generally be more moist than the soils of nearby clearcut slopes.

Whether we see a tree as a rain umbrella or a sun umbrella, its effect is magnified many times over when we ponder the practical impact of an entire forest. A forested region is a massive reservoir of water, and it's one that leaks away its captured booty slowly enough to feed mountain springs and streams all through a hot Montana summer. Forested regions are so very good at hanging onto water that scientists have lately been thinking that deforestation around the globe accounts for a "significant" portion of the planet's rising sea levels, because falling rain can rush back to the sea much more quickly when there's less forest left to catch it, and delay its escape.

Because a forested region lets water go slowly, it can feed water to everything and everyone (trout and beaver, fisherman and farmer) downstream, and for months on end. But there's selfishness here, too: the water a forest captures also satisfies the thirst of the trees themselves.

At this stage of the game, a tree shows itself as a pump. It starts when the roots of trees grab the water saved by its needles and its shade, and pull that water into the body of the trees just as effectively as a kid might sip a nice cold glass of iced tea. The water goes up the tree's trunk, into the limbs, progressing into its leaves and needles, and through little openings called stomata, where water vapor is expelled back to the atmosphere from which it came. Your sweat on a hot day probably illustrates the basic point about as well as any other real-world analogy.

By pumping underground water back to the atmosphere, each tree irrigates our skies. Multiply this thousands, hundreds of thousands, or even millions of times over, and a whole forest ends up pushing a pretty serious river of water back to the atmosphere. We see some of this airborne river's fallout as morning dew, but it also supplies atmospheric moisture for subsequent rains; a lot of this moisture goes up to recharge rainclouds that will be carried away by winds that take them to nearby or distant areas before their load of rain falls once again.

All this is well understood enough that, for many years now, scientists have regarded intact forests as rainmakers.

So far, so good, but no self-respecting Swiss army knife would settle for being nothing more than a dual-purpose umbrella and a powerful pump; a tree is also a storage closet. Even here, it does at least double duty.

From time to time, science has reported that trees store the pollutants they get from air and water, but lately the big story is their impressive storage of carbon. This is where the story of trees and forest really heats up.

Over the past 20 years, scientists have turned increasing attention to the vast amounts of carbon that is stored in trees. For any tree that had been storing carbon for, say, 700 years, a lot of carbon is set loose when any such tree is toppled by the saw. Knowing this, some scientists have said that deforestation has potential to loose more carbon dioxide into the atmosphere than we liberate in our burning of fossil fuels.

Science has been sorting out the facts of the matter ever since, and, while the estimates have been refined, the gist of the tale holds true.

Just two years ago, for example, Sir John Houghton, England's chief scientist on the Intergovernmental Panel of Climate Change, said that the two major causes of global atmospheric warming are fossil fuel consumption and, you guessed it, deforestation.

There's scarcely a person in America who didn't see carbon in the form of smoke when trees burned in the summer of 2000 - even the people who weren't living near the fires saw them and their smoke on national TV reports.

With another round of drought seeming likely in the summer of 2002, that basic lesson is probably going to get some repetition. But few people know -- yet -- that cutting down a tree releases its carbon as effectively as burning it does. Here again, the basics are simple.

After a tree is cut, it begins to decompose. Then, in the process of rot, it lets go of its carbon in the form of carbon dioxide, the same gas that we get on burning fossil fuels. Now, cutting one tree is of course no big deal, but multiply this simple loss over western Montana, for example, or the entire Pacific Northwest, and we see lots of carbon gone from its storage closets to the skies, where it now warms the planet. The basic lesson is that drought and climate change can begin very close to home.

One of ecology's standard maxims is that "You can't do just one thing." That old dictum sure holds true when it comes to cutting trees. Because a tree is a Swiss army knife, cutting it down takes a paragon of multiple-use out of survival's tool chest.

Lance Olsen is project director of the Ambience Project, Institute of the Rockies, 802 East Main, Missoula MT 59802; phone (406)549-1179 or email This originally appeared in

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