The truth about carbon offsetting
Green cliché or world saver? We look at whether planting trees can really save the planet or if the benefits of carbon offsetting are overstated
Ronald Reagan said several baffling things during his eight-year presidency, but none matches his 1981 claim that 'trees cause more pollution than automobiles'. It doesn’t take an environmental science degree to detect that there could be something fishy about that statement.
But over the years, and especially since the emergence of carbon offsetting, there is doubt about whether planting new trees is always environmentally beneficial. So what’s the truth – will planting a tree in your garden, or paying companies to plant saplings on your behalf, help tackle climate change?
Breathing in CO2
The main reason why tree planting is
seen as a good thing is that trees, like other plants, breathe in CO2
and breathe out oxygen. Given that CO2 is the main cause of global
warming, and that we need oxygen to survive, this can hardly be a bad
thing.
The fact that trees breathe in CO2 is not in question.
They need the carbon to grow – they’re basically made of the stuff –
and you can observe the effect in the Earth’s atmosphere. As the graph below shows, global CO2 levels actually drop a bit during the growing season
of the northern hemisphere, where most trees exist.
But,
as explained in our 60 Second Guide to Carbon Offsetting,
there are two downsides to using trees to soak up carbon.
First, trees can take ages to grow, so it’s not a very speedy solution.
Second,
trees eventually die and rot (or get burned), releasing much of their
stored carbon back into the air. So it’s not a long term solution
unless each tree that dies is replaced by another – hard to guarantee
over periods of decades or centuries.
Source: Woods Hole Research Center
Soaking up the sun
Just because trees soak up CO2 doesn’t
necessarily mean that they’re always good news in terms of global
warming. They may impact the climate in other ways, too.
For example,
in the past couple of years scientists have concluded that in temperate
and arctic areas, forests may actually have an overall warming effect. The
main reason for this is the warming effect of the dark colour of trees
- they reflect less than lighter surfaces such as snow or crops do.
It’s the same phenomenon as the extra heat you feel when wearing dark
clothes in summer.
By contrast, planting trees in tropical countries can help cool the air by trapping water and letting it slowly evaporate.
Methane emissions from trees
A major scientific debate was spawned in 2006 when Dr Frank Keppler of the Max Planck Institute for Nuclear Physics discovered that vegetation emits methane. Although short-lived, methane is a much more powerful greenhouse gas than CO2.
The scientists estimated that plants might account for 10–30 per cent of current methane emissions. That’s a huge overall impact, but the methane emitted from a single tree doesn’t outweigh the CO2 in it.
Protecting rather than planting
On balance, despite the controversies, planting a tree where there wasn’t one before is likely to help fight climate change – at least in the medium-term. But a far more important question is how we protect the trees that are already standing – and in particular the world’s tropical rainforests.
The reason rainforests are so crucial is only partly because they breathe in CO2 and cool local weather systems. More pressing is the simple fact that they hold around half the carbon present in vegetation around the world. When these forests are destroyed to clear farmland, huge volumes of carbon escape from the trees, undergrowth and soils to become CO2 in the air.
Though few people realise it, deforestation accounts for nearly a fifth of recent man-made carbon emissions – that’s more than America, China or the entire EU. No wonder, then, that the UK government’s Stern Review highlighted deforestation as one of the key areas for action in the fight against climate change.
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