Want clean water? Save your forests!

Cities can ensure they have a cheap and abundant supply of clean water by protecting and regenerating forests in their surrounding watersheds, according to a major analysis just undertaken in Malaysia.

Forests are a vital source of clean water -- and important for lots of other reasons too.

Forests are a vital source of clean water -- and important for lots of other reasons too.

Jeffrey Vincent from Duke University, USA and his colleagues have just published the largest cost-benefit analysis ever conducted in the tropics, and they find that pristine and even selectively logged forests are very cost-effective ways to produce clean, drinkable water. 

Vincent and colleagues ran their analyses under a wide range of scenarios.  They used as a baseline the costs of standard water-treatment plants, which are often required to make polluted water safe to drink.

The authors found that the relative advantages of forests depend on local circumstances, with the financial benefits being greater in some situations than others. 

Also, if one factors in profits that can be made by exploiting the forests -- such as by converting them to agriculture -- then the numbers could change. 

The problem, of course, is that waters that drain off of agricultural lands are often polluted by fertilizers, pesticides, and organic wastes, making expensive water-treatment necessary.

The authors argue that, beyond water purification, intact forests have many other financial and non-financial values. 

For instance, they store large stocks of carbon, and thereby reduce greenhouse gas emissions. 

They also can harbor enormous biodiversity while helping to attract substantial income from ecotourism.  Remarkably, it's currently estimated that protected areas across the world attract some 8 billion visits annually, underscoring their financial value for local communities.

Furthermore, via the process of evapotranspiration, forests emit enormous quantities of water vapor.  For example, one-third to one-half all the rainfall that falls on a tropical forest is quickly recycled back to the atmosphere, as water vapor.

This water vapor (plus natural organic aerosols emitted from forests) help to form clouds, which in turn reflect solar radiation back into outer space, thereby reducing global warming. 

Forests are natural cloud-makers.

Forests are natural cloud-makers.

Such clouds also help to produce life-giving rainfall during the dry season -- when forests are most drought-stressed and prone to fires.

Finally, forests are very good at reducing destructive floods.  They tend to act like giant biological sponges, trapping water and releasing it slowly, thereby reducing downstream flooding.  Especially in areas where forests are denuded, flooding can cause billions of dollars in damage and costs thousands of lives each year.

The conclusion: It's increasingly becoming apparent that it's smart to conserve pristine and selectively logged forests -- even when one uses just hard, cold economic logic. 


Every tree matters: Even a little deforestation alters climate

Thinking about knocking down a few trees in the backyard?  Think again.  Felling even a handful of trees can change the local climate, according to a new study

Think twice before cutting...

Think twice before cutting...

It's been known for some time that clearing forests can have regional-scale impacts on climate by reducing evapotranspiration (the emission of water vapor by plants, which cools the land) and changing albedo (how much solar radiation gets reflected away from the ground surface). 

But now it appears these effects happen at surprisingly small scales.  Especially in warmer parts of the world, clearing even a football field-sized area is enough to provoke significant heating of the immediate area.

That's an important insight.  Folks living in tropical and subtropical areas often complain that deforested lands are unpleasantly warm, less productive for farming, and more prone to harboring diseases

So, spread the word: Cutting down trees doesn't just have a global impact, by increasing carbon emissions; or a regional impact, by changing evapotranspiration and albedo. 

Killing trees also has a sizable local impact, meaning it directly affects the quality of life of those living nearby.  

Do forests function like 'biotic pumps' for rainfall?

One of the more striking and controversial hypotheses to emerge in the last decade is the notion that intact tracts of forest, stretching from coastal to inland areas, may help to suck oceanic moisture far inland--functioning like a giant 'biotic pump'. 

This idea might sound slightly prosaic, but its potential implications are so profound and its putative mechanism so controversial that it has created fierce divides within the climatology and environmental-science communities.

No forest, no rain?

No forest, no rain?

The biotic-pump idea was first proposed in 2006 by a pair of Russian biophysicists, Anastasia Makareiva and Victor Gorshkov, in a highly theoretical analysis that even mathematicians found daunting.  The controversy has raged ever since then.

Everyone knows that forests (especially rainforests) emit a lot of water vapor, and Makareiva and Gorshkov argued that when this vapor condensed into rain it created a large suction.  That suction pulled cloudy, rain-bearing water from coastal areas inland, they argued, and thereby was crucial for maintaining rainfall in inland areas.  

The really striking implication of the Makareiva -Gorshkov hypothesis is this: If you break up the forest, you lose the suction, and the biotic pump stops working.  This, of course, would be an enormously important implication for forest conservation, if true.  Imagine millions of farmers clamoring for more forest in their region because their crops were drying up!

But the idea remains intensely controversial, because many physicists simply don't buy the idea of condensing water vapor creating a big suction.  For instance, just one website focused on this issue (and there are quite a number) had hundreds of individual comments, many very impassioned in nature.  This only happens in science if a debate is truly volcanic in nature.

This is a debate to keep your eye on--huge implications, and huge controversy about the biotic-pump idea itself.