Myth: Planting trees will cancel out your car's environmental impact

Fact: Although growing trees will absorb carbon dioxide, just planting more trees is not going to trap that carbon dioxide for all time. The land required to capture car emissions is substantial, so like car use itself, absorbing CO₂ in trees is only viable if not too many people are doing it. Natural events like bushfires can also release the stored carbon at any time. On the other hand, growing trees in plantations and harvesting them does not prevent the stored carbon being released in the future. Planting trees also does nothing about pollution, noise or other environmental problems generated by cars.

Guilt-Free Driving: A Lesson From History

In the fifteenth century in Europe, the Protestant Reformation overturned social and political structures, precipitated two centuries of religious warfare and laid some of the groundwork for the modern era. The Reformation began as a reaction to the corrupt habits of the Church at the time, foremost among which was the practice of selling 'indulgences'. For the right price, wealthy sinners could buy a place in Heaven and be absolved of their past and future sins.

What does medieval history have to do with the environmental impact of cars? Very little, were it not that the ancient practice of selling indulgences has found a modern equivalent in some schemes that promise to absolve people of environmental responsibilities in return for money. If you are at all concerned that your car is contributing to climate change, never fear: for just $40 a year (tax deductible!) you can arrange to have enough trees planted that the emissions will be cancelled out and you can drive to your heart's content.

Of course, the promoters of these schemes only claim to cancel out your car's greenhouse impact; they don't claim to undo your car's pollution or any of the other environmental and social impacts of car use. Nor do they account for the fossil fuel used in manufacturing the car, which is comparable to the amount of fuel used in the car's entire lifetime. And naturally they only claim to cover the greenhouse impact of those who participate in the scheme, so it only works as an effective mitigation measure if a majority of motorists can be convinced to participate. However, in combination with erroneous claims that cars are becoming more and more fuel efficient and that pollution can be reduced by building more roads, the idea that we can nullify our greenhouse contribution by doing nothing (except giving money to the scheme promoters) helps fuel the myth of 'sustainable automobility' and distracts from the search for effective long-term solutions.

Green Motoring Schemes: Telling Greenwash from Meaningful Action

One might still think that perhaps the scheme has some merit, as one of the very few ways in which ordinary people can help fight climate change in the face of hostile government attitudes. Unfortunately, when properly examined the idea of planting trees to absorb the greenhouse emissions from cars turns out to have fatal flaws.

To some extent these 'offset' schemes are just part of the 'green motoring' push that pervades the marketing of cars today. Car makers are naturally keen to tap into environmental consciousness among consumers by pitching their product as environmentally friendly. But as many observers have noted, there is no such thing as a car that's good for the environment: only cars that are less bad for the environment than others. Accordingly, car makers who advertise their cars as 'green' are increasingly likely to fall foul of rules that protect consumers against false advertising.

The crucial question to ask the promoters of tree planting, or any other 'green motoring' scheme, is: do people have any incentive to drive less through being part of the scheme? If not, then the only practical effect the scheme can have on personal awareness is to create the false impression that it doesn't matter how much you drive: the environmental problems of cars will be solved some other way. This actually encourages people to do nothing, and so is actively harmful (not to mention easily satirised). If the car industry itself wanted to deflect concerns about the environment while ensuring that car use keeps going up, this kind of scheme is tailor-made for the purpose.

Other common schemes of this sort include the offering of 'green car loans' or discounted insurance for energy-efficient cars. In this case, the people who take out the loans or buy the insurance are the ones who are most likely to buy more energy-efficient cars anyway, and those who aren't interested in energy efficiency have other places to go for competitive offers. So the availability of such schemes does not actually tilt the market in favour of more energy-efficient vehicles, though it doubtless increases market share for the individual institutions that offer them. And having bought or insured a more energy-efficient car on favourable terms, there is no incentive to drive it less and every reason to drive it more - a point we make on another page.

Having made these general points, we now take a closer look at the actual logistics of tree-planting schemes. Those who are interested in knowing still more details can find them in the Appendix.

Problem One: Not Enough Land

The first serious problem is that cancelling out the emissions from one car by absorbing them in native forest requires around 600 square metres of new forest each year - comparable to the size of a typical suburban house block. (See the Appendix for details.) Taking a person's driving lifetime as 50 years, the scheme will eventually have to re-forest 3 hectares of land for each participant.

There are around 10 million cars registered in Australia (about one for every two people), so leaving aside trucks and other commercial vehicles, accommodating the driving habits of Australians now alive would require 30 million hectares of new forest. For comparison, the area of the entire state of Victoria is 23 million hectares. Truck emissions for the next 50 years would require an additional 15 million hectares or so, and then one must consider the needs of all those Australians yet to be born.

This all assumes that the land has an average timber yield equal to the world average, when in fact it is likely to be substantially less.

In principle, one can work out the amount of land actually available for re-forestation by taking all the land in Australia and excluding the desert, urban land, lakes and dams, land with trees already on it, and land that would have been reforested by volunteers anyway (a far more satisfying way to help the environment than just handing over money!). A Federal Government agency carried out this process in 1996 and found that, of 18.4 million hectares potentially available for tree planting, only around 9.6 million hectares are actually viable (meaning sufficiently fertile and not currently used for high-intensity agriculture). At 3 hectares per person, this is just enough to support one in six Australians over their lifetime, with none left over for future generations.

And while for the time being there is land more-or-less freely on offer for tree-planting, not all of those 9.6 million hectares come for free. Prices for rural land start at $1000 per hectare, or $60 for a 600-square-metre plot, increasing the annual fee from $40 to $100.

In other words, carbon-sinking schemes only remain viable as long as they cater to a minority of the population, and are prepared to charge an increasing marginal cost. The more honest of the scheme promoters admit themselves that theirs is only a short-term solution. At best, then, such schemes will compensate for a small part of our car emissions, and the bulk will have to be dealt with by other means, such as shifting car journeys to more environmentally friendly modes.

Problem Two: Forests Don't Last Forever

The second problem is more subtle. All the calculations presented are for undisturbed forest that once established, exists in a permanent state of equilibrium with new trees replacing those that die and rot. They make no allowance for events such as bushfires that destroy this equilibrium. Though trees often grow back after fires, they usually do not do so with their full timber content, and in old-growth eucalypt forests one often comes across trees with hollowed-out trunks caused by bushfire.

With probability one, every piece of Australian forest will eventually be subjected to bushfire, and so schemes to lock up CO₂ in trees need to account for long-term bushfire losses, either by planting a greater area of land at the outset or by ensuring that forests are restored to their original yield following a bushfire.

Similarly, they must also allow for the fact that natural events (including bushfires) can over time reduce a forest to a scrubland or a grassland. Nature is never in perfect equilibrium, and the carbon content of a forest left to its own devices is never guaranteed to be constant over time.

To some extent, one can avoid the vagaries of Nature by managing the forest like a plantation, periodically harvesting mature trees, planting new trees in their place, and keeping a running account of the forest's timber content. This is in fact what one is required to do in order for a forest to participate in the carbon credit scheme now operating in New South Wales. This of course requires a good deal of management overhead, more than can be provided for $40 a year per participant.

The Evidence Emerges

The above problems with tree-planting schemes were somewhat theoretical while these schemes were still in their infancy. Like cars themselves, offset schemes show most promise when the ideas are new and haven't yet been taken up by enough people for the 'scaling' problems to emerge. But as the first schemes pass their tenth anniversary, the evidence is starting to come in, and confirms what the PTUA and other critics have long suspected.

But the Problems Don't Stop There

It is also often thought that timber harvested from a plantation and used in buildings or furniture can still be counted as 'sunk' carbon emissions, so the replanted land can then be used to cancel out entirely new emissions. However, in the long run all such timber will end its life by releasing its carbon back to the atmosphere as CO₂. Good-quality timber in buildings is recycled, but the bulk winds up in landfill whenever a building is demolished or renovated. Natural decay of timber in landfill or in situ produces either CO₂ or methane, a more potent greenhouse gas. CO₂ is also released when timber burns or is eaten by termites. And in any case, only about 25 per cent of harvested timber winds up as building materials or furniture anyway: the rest is made into short-lived products such as paper, or is wasted during the harvesting and milling process.

The only way in which timber can be harvested and remain as a carbon sink is to bury it deep in the ground and hope it turns back into oil or coal in a few million years. However, one still has to allow for the fact that by removing the timber from the growth site one has stripped some of the nutrients from the soil, so these have to be replenished somehow, most likely with oil-derived fertiliser produced by an energy-intensive process that releases yet more CO₂ into the environment.

Last but not least, research within the last decade also points to a possible third problem with planting trees to offset emissions. Once a tree becomes mature and saturated with carbon, it may actually start releasing that carbon back into the atmosphere, turning into a net CO₂ emitter, according to studies by the Intergovernmental Panel on Climate Change and the Hadley Centre for Climate Prediction and Research in the UK.

It has long been known that trees, in addition to capturing carbon dioxide, also 'breathe' much like we do, exhaling carbon dioxide back into the atmosphere. However, the balance between these processes is only now beginning to be understood. Recent research suggests that higher temperatures reduce the ability of plants to absorb CO₂, both by slowing down photosynthesis and by increasing the rate of respiration.

Drier climates seem to have a similar effect. It is now known that for a period of several months during the 2002 drought, Australia's forests as a whole became net emitters of carbon dioxide. While this was a one-off event, it shows that if climate change makes droughts more prevalent, we cannot rely on even our existing forests to soak up carbon dioxide as much as they have in the past.

Conclusion: Plant Trees, But Don't Count On Them To Offset CO₂

All the above helps to explain why, according to a New Scientist article in March 2007,

(It appears the preferred approach of carbon-offset companies is now to invest in renewable energy projects. This of course raises the question whether this investment would be occurring anyway. But at least all indications are that properly designed renewable energy projects do displace CO₂ emissions from burning fossil fuels.)

To sum up: while there are plenty of very good reasons to plant trees, mitigating the environmental effects of car use is a long, long way down the list. The best strategy is still to avoid producing so much CO₂ in the first place: choose the smallest car that's practical for the task (ideally second-hand, as manufacturing uses a lot of energy) and use it only as much as absolutely necessary, choosing alternatives wherever possible. A car trip avoided through use of public transport, walking or cycling is worth any amount of 'damage control' whether through tree planting or otherwise - not least because the more people do so, the greater the overall benefit.

Technical Appendix: Land required to absorb CO₂ emissions from one car

The typical argument used to establish the '17 trees per car' figure for absorbing CO₂ emissions runs as follows. Burning a litre of petrol releases 2.3kg of CO₂, so a typical car with a fuel efficiency of 12 litres per 100km and driven 16,000km in a year will emit 4,416kg of CO₂ each year. If a single tree in a bio-diverse forest absorbs 268kg of CO₂ over its lifetime, then 17 such trees will (eventually) absorb 17 times 268 or 4,556kg of CO₂. So it is argued that if one plants another 17 trees every year, they will eventually absorb all the car-related CO₂ emissions.

The source for this argument is Working Paper 23 from the Federal Bureau of Transport and Regional Economics (BTRE) which provided the impetus for many carbon-sinking schemes now in operation. This same working paper is the source for our figures on total land available for planting trees in Australia.

Importantly, this argument considers only the direct emissions from the combustion of petrol in a car, and excludes many indirect sources of CO₂ emissions. A litre of petrol releases 2.3kg of CO₂ when burned, but also releases about 0.5kg of CO₂ when it's produced from crude oil in the refinery. Extracting and transporting the crude oil also consumes energy and adds to the indirect emissions. For completeness, one should also consider the energy used to manufacture the car one is driving. All in all, the average car accounts for a lot more than 4.4 tonnes of CO₂ emissions each year, but we will use this figure anyway for the sake of argument.

Many tree-planting programmes pride themselves on being environmentally sustainable and avoiding the kind of monoculture plantations found in commercial forestry, that achieve high timber yield at the expense of biodiversity and soil fertility. Such plantations fail to act as carbon sinks in the long term because replenishing the soil after each harvest requires a continual input of fertiliser, which is derived from fossil fuel by an energy-intense process. Nonetheless, figures like that above for CO₂ absorption by trees are typically calculated by taking figures for monoculture plantations, and reducing them by an arbitrary 'fudge factor' that is supposed to account for the difference in productivity between monoculture plantations and native forest.

In the timber industry, the 'productivity' of a forest is measured by area, not by number of trees, because it is found that particular species of tree produce the same quantity of timber per hectare regardless of the number of trees planted. For this reason foresters often 'thin' plantations as they grow so that more timber can be extracted. This Irish Government site explains the process, and gives an example of a typical plantation with a 'yield class' of 20 cubic metres per hectare per year. BTRE Working Paper 23 gives estimates of the same order of magnitude, concluding that a rotating plantation of Radiata pine stores the equivalent of 219 tonnes of carbon per hectare. (This figure also takes into account the carbon in wood products derived from the plantation.)

These figures drop significantly when the intention is to plant native forest rather than monoculture plantations. The average yield of all the world's forests is about 2.1 cubic metres per hectare per year, and this varies a lot between regions, as these industry sites from India and Canada show. This annual yield covers the time between a new forest being planted and the forest reaching the equilibrium state, where any subsequent new growth is matched by decay of existing material. Typically, it takes around 35 years for new forest to reach equilibrium.

Likewise in Australia, one must distinguish between dense, wet-climate rainforest ecosystems and the bulk of arable land that is available for planting new forests. The former can indeed store impressive amounts of carbon, as an ANU study found in 2008. But they are the exception to the rule (and in any case are already standing, storing carbon that has been around since prehistoric times). In Australia's dry climate the yield from forests is likely to be below the world average, but assuming it to be equal to the average for argument's sake, the timber content at equilibrium for native forest will be around 35 times 2.1, or 73.5 cubic metres per hectare.

We now need to work out just how much CO₂ is absorbed in a cubic metre of native forest timber. Photosynthesis in plants, described very simply, splits CO₂ into carbon and oxygen, returns the oxygen to the atmosphere, and stores the carbon in cellulose (wood fibre). One tonne of CO₂ contains about 0.27 tonnes of carbon, while one tonne of cellulose contains about 0.4 tonnes of carbon. It follows that the carbon in one tonne of cellulose corresponds to 1.5 tonnes of absorbed CO₂.

To convert cubic metres of forest timber to tonnes of absorbed CO₂, one needs to know the density of the timber and its actual cellulose content. Very dense hardwood dried in an oven contains nearly 1 tonne of cellulose per cubic metre, but most live timber yields less than this, both because it is less dense and because a large percentage of its weight is water. (According to BTRE Working Paper 23, Radiata pine contains just 0.44 tonnes of cellulose per cubic metre.) Thus a conservative 'ballpark' figure is that each cubic metre of new timber absorbs 1 tonne of CO₂, rather than 1.5.

So one concludes that a hectare of established native forest, with 73.5 cubic metres of timber, has absorbed roughly 73.5 tonnes of CO₂. To absorb the 4.4 tonnes a typical car emits in a year requires one hectare (10,000 square metres), divided by 73.5, times 4.4, or 600 square metres of forest. Whether this corresponds to 17 trees naturally depends on the kind of tree, but the crucial factor is the eventual volume of timber, not the number of trees planted.

© 2007 Public Transport Users Association Inc. (PTUA), Victoria, Australia. ABN 83 801 487 611.
General copying and distribution on a non-commercial basis is permitted subject to proper acknowlegement.
Authorised by Tony Morton, 247 Flinders Lane, Melbourne, for the PTUA

Common Urban Myths About Transport