Why won’t planting trees stop global warming?

We all like trees don’t we? Never hear a bad word said against them. They’re just there, solid and dependable and green. Soaking up evil carbon-dioxide, spitting out copious amounts of life-giving oxygen. Even the British Tory part got in on the act in 2006 when they adopted a tree as their new party logo.

Unfortunately, even though the idea that planting trees will save the planet from global warming has firmly–erm–planted itself in the public’s imagination, the notion is not backed up by the science.

Did he just say something bad about trees? Not really. To understand why planting trees won’t solve global warming, let’s unpack some of the basic science.

First of all, the amount of carbon in the Earth’s biosphere is constant (give or take satellite launches etc). It cannot be created or destroyed, only change form. This movement of carbon through its various forms is called the carbon-cycle, and it can be divided into two main parts: active and inert.

Trees absorbing carbon-dioxide and converting it into glucose and oxygen through the process of photosynthesis is one part of the active cycle, as carbon constantly moves through plants, organisms, water, and the atmosphere. In contrast, reserves of fossil fuels–underground lakes of natural gas and oil, and seams of coal–are inert; their carbon is locked away and plays no part in the active cycle unless we burn them. Note that movement between the inert pool and the active cycle is one-way on human timescales–no carbon will return to the inert pool unless it undergoes the millenial-long geological processes that brought it about in the first place.

From a global warming perspective, when carbon is in the guise of carbon-dioxide it adds to the greenhouse effect, which causes solar energy to be trapped within the Earth’s atmosphere leading to rises in global temperature.

So won’t planting trees cause carbon-dioxide to be taken out of the air? Yes. But what’s conveniently forgotten is that trees don’t only photosynthesize. Like all living organisms they respire too. And die. Two processes that produce carbon-dioxide. No carbon gets “locked-up” in the same way that fossil fuels sequester the element, only temporarily held in the structure of the tree. Even if planted trees are sustained for perpetuity, the amount of carbon coming out of the inert part of the cycle through the burning of fossil fuels dwarfs this small sink. And that gap is only going to get bigger.

Carbon offsetting schemes are so enticing because they offer us a quick monetary fix to preserve the lifestyle we’ve become accustomed to. Sadly, the only incontrovertible way to stop climate change is to drastically limit the quantity of fossil fuels being extracted.

The Carbon Neutral Myth provides much excellent in-depth reading if you’re interested in finding out more.

Image Credit: Wiki Commons

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4 thoughts on “Why won’t planting trees stop global warming?

  1. Of course trees take carbon dioxide out of the atmosphere. That’s not in doubt. All plants do.
    The question is, does it return to the atmosphere? How soon?
    The answer to those questions depends on what is done with the wood and the leaves.
    The carbon that’s in the ground in the form of coal and petroleum all originally came from plants, so it’s clear that, in the right circumstances, plants *can* take carbon out of the atmosphere for periods of millions of years.

  2. Yes, but “the right circumstances” needed for plant-life to take carbon out of the atmosphere for millions of years are not ones we can engineer on the timescale needed to avert potentially massive climate change.
    The point is carbon-dioxide levels in the atmosphere will inevitably rise over the course of the next century–tree-planting programs or not–unless we drastically reduce the amount fossil fuels that we are collectively burning. We can achieve this in two ways: change our lifestyles or find alternative sources of energy. To do either of these things we will need widespread political will.
    I’m not against tree-planting programs, but they’re the equivalent to securing your kite with a drawing pin when a hurricane is coming in.

  3. “Yes, but “the right circumstances” needed for plant-life to take carbon out of the atmosphere for millions of years are not ones we can engineer on the timescale needed to avert potentially massive climate change.”
    Aren’t they? This is a conjecture.

    However… it is worth pointing out that the process is a bit complicated, and the middle steps could be omitted.
    -we burn carbon to produce energy and put carbon dioxide into the air
    -plant trees to remove carbon dioxide in the air
    -the trees convert carbon dioxide to fixed carbon using sunlight as the energy source.

    The net result is sunlight turns into usable energy while carbon cycles into and out of the atmosphere. So, why not eliminate the middle steps and just turn the sunlight into energy in the first place?

  4. No conjecture whatsoever. Research into converting carbon-dioxide into synthetic fuels has been going on since WW2, but every effort comes up against the immutable fact that this process will *require* energy (hence part of the reason why it takes so long to happen naturally as the energy input is low). Conservation of energy is a physical law that you can’t work around with a technological fix. It is only by creating a non-greenhouse gas emitting energy source (i.e. renewables) to power this conversion, that this could this actually help lower carbon-dioxide levels. (And that’s assuming you don’t just burn it straight away). And on that front nothing is promising any kind of substantial delivery within the next 25 years.

    I think the idea of turning sunlight into energy has been floated already. That’s solar power! Unfortunately, it appears to be more expensive than burning fossil fuels in whatever guise it takes (space-based, rooftop, desert etc). So until we stop giving all the weight to the economic argument and let the environmental argument take precedence, looks like we’re going to keep adding CO2 to the atmosphere. Even discounting a runaway scenario for a long time, even modest rises in average global temperatures will cause more extreme weather that could devastate vulnerable communities.

    Tom Murphy, an associate professor of physics at the University of California, San Diego, has a great website with a lot of understanding underpinned by the real numbers.

    http://physics.ucsd.edu/do-the-math/

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